Chapter 29: Functions

29.2. Perl Functions in Alphabetical Order

Many of the following function names are annotated with, um, annotations. Here are their meanings:

Uses $_ ($ARG) as a default variable.

Sets $! ($OS_ERROR) on syscall errors.

Raises exceptions; use eval to trap $@ ($EVAL_ERROR).

Sets $? ($CHILD_ERROR) when child process exits.

Taints returned data.

Taints returned data under some system, locale, or handle settings.

Raises an exception if given an argument of inappropriate type.

Raises an exception if modifying a read-only target.

Raises an exception if fed tainted data.

Raises an exception if unimplemented on current platform.

Functions that return tainted data when fed tainted data are not marked, since that's most of them. In particular, if you use any function on %ENV or @ARGV, you'll get tainted data.

Functions marked with raise an exception when they require, but do not receive, an argument of a particular type (such as filehandles for I/O operations, references for blessing, etc.).

Functions marked with sometimes need to alter their arguments. If they can't modify the argument because it's marked read-only, they'll raise an exception. Examples of read-only variables are the special variables containing data captured during a pattern match and variables that are really aliases to constants.

Functions marked with may not be implemented on all platforms. Although many of these are named after functions in the Unix C library, don't assume that just because you aren't running Unix, you can't call any of them. Many are emulated, even those you might never expect to see--such as fork on Win32 systems, which works as of the 5.6 release of Perl. For more information about the portability and behavior of system-specific functions, see the perlport manpage, plus any platform-specific documentation that came with your Perl port.

Functions that raise other miscellaneous exceptions are marked with , including math functions that throw range errors, such as sqrt(-1).

29.2.1. abs    

abs VALUE
abs

$diff = abs($first - $second);

my $diff = abs($first - $second);

29.2.2. accept     

accept SOCKET, PROTOSOCKET

unless ($peer = accept(SOCK, PROTOSOCK)) {
    die "Can't accept a connection: $!\n";
}

See accept(2). See also the example in the section "Sockets" in Chapter 16, "Interprocess Communication".

29.2.3. alarm    

alarm EXPR
alarm

Only one timer may be active at once. Each call disables the previous timer, and an EXPR of 0 may be supplied to cancel the previous timer without starting a new one. The return value is the amount of time remaining on the previous timer.

print "Answer me within one minute, or die: ";
alarm(60);             # kill program in one minute
$answer = <STDIN>;
$timeleft = alarm(0);  # clear alarm
print "You had $timeleft seconds remaining\n";

For alarms of finer granularity than one second, you might be able to use the syscall function to access setitimer(2) if your system supports it. The CPAN module, Timer::HiRes, also provides functions for this purpose.

29.2.4. atan2    

atan2 Y, X

$pi = atan2(1,1) * 4;

sub tan { sin($_[0]) / cos($_[0]) }

29.2.5. bind    

bind SOCKET, NAME

use Socket;
$port_number = 80;      # pretend we want to be a web server
$sockaddr = sockaddr_in($port_number, INADDR_ANY);
bind SOCK, $sockaddr or die "Can't bind $port_number: $!\n";

29.2.6. binmode    

binmode FILEHANDLE, DISCIPLINES
binmode FILEHANDLE

The binmode function should be called after the open but before any I/O is done on the filehandle. The only way to reset the mode on a filehandle is to reopen the file, since the various disciplines may have treasured up various bits and pieces of data in various buffers. This restriction may be relaxed in the future.

In the olden days, binmode was used primarily on operating systems whose run-time libraries distinguished text from binary files. On those systems, the purpose of binmode was to turn off the default text semantics. However, with the advent of Unicode, all programs on all systems must take some cognizance of the distinction, even on Unix and Mac systems. These days there is only one kind of binary file (as far as Perl is concerned), but there are many kinds of text files, which Perl would also like to treat in a single way. So Perl has a single internal format for Unicode text, UTF-8. Since there are many kinds of text files, text files often need to be translated upon input into UTF-8, and upon output back into some legacy character set, or some other representation of Unicode. You can use disciplines to tell Perl how exactly (or inexactly) to do these translations.[2]

[2]More precisely, you will be able to use disciplines for this, but we're still implementing them as of this writing.

For example, a discipline of ":text" will tell Perl to do generic text processing without telling Perl which kind of text processing to do. But disciplines like ":utf8" and ":latin1" tell Perl which text format to read and write. On the other hand, the ":raw" discipline tells Perl to keep its cotton-pickin' hands off the data. For more on how disciplines work (or will work), see the open function. The rest of this discussion describes what binmode does without the DISCIPLINES argument, that is, the historical meaning of binmode, which is equivalent to:

binmode FILEHANDLE, ":raw";

In the absence of a DISCIPLINES argument, binmode has no effect under Unix or Mac OS, both of which use \n to end each line and represent that as a single character. (It may, however, be a different character: Unix uses \cJ and older Macs use \cM. Doesn't matter.)

The following example shows how a Perl script might read a GIF image from a file and print it to the standard output. On systems that would otherwise alter the literal data into something other than its exact physical representation, you must prepare both handles. While you could use a ":raw" discipline directly in the GIF open, you can't do that so easily with pre-opened filehandles like STDOUT:

binmode STDOUT;
open(GIF, "vim-power.gif") or die "Can't open vim-power.gif: $!\n";
binmode GIF;
while (read(GIF, $buf, 1024)) {
    print STDOUT $buf;
}

29.2.7. bless    

bless REF, CLASSNAME
bless REF

$pet = Beast->new(TYPE => "cougar", NAME => "Clyde");

# then in Beast.pm:
sub new {
    my $class  = shift;
    my %attrs = @_;
    my $self   = { %attrs };
    return bless($self, $class);
}

Make sure that CLASSNAME is not false; blessing into false packages is not supported and may result in unpredictable behavior.

It is not a bug that there is no corresponding curse operator. (But there is a sin operator.) See also Chapter 12, "Objects", for more about the blessing (and blessings) of objects.

29.2.8. caller    

caller EXPR
caller

($package, $filename, $line) = caller;

sub careful {
    my ($package, $filename) = caller;
    unless ($package  eq __PACKAGE__ && $filename eq __FILE__) {
        die "You weren't supposed to call me, $package!\n";
    }
    print "called me safely\n";
}

sub safecall {
    careful();
}

$i = 0;
while (($package, $filename, $line, $subroutine,
        $hasargs, $wantarray, $evaltext, $is_require,
        $hints, $bitmask) = caller($i++) )
{
    ...
}

In a fit of even deeper magic, caller also sets the array @DB::args to the arguments passed in the given stack frame--but only when called from within the DB package. See Chapter 20, "The Perl Debugger".

29.2.9. chdir    

chdir EXPR
chdir

chdir "$prefix/lib" or die "Can't cd to $prefix/lib: $!\n";

29.2.10. chmod    

chmod LIST

$cnt = chmod 0755, 'file1', 'file2';

Here's a more typical usage:

chmod(0755, @executables) == @executables
        or die "couldn't chmod some of @executables: $!";

@cannot = grep {not chmod 0755, $_} 'file1', 'file2', 'file3';
die "$0: could not chmod @cannot\n" if @cannot;

When using nonliteral mode data, you may need to convert an octal string to a number using the oct function. That's because Perl doesn't automatically assume a string contains an octal number just because it happens to have a leading "0".

$DEF_MODE = 0644;   # Can't use quotes here!
PROMPT: {
    print "New mode? ";
    $strmode = <STDIN>;
        exit unless defined $strmode;   # test for eof
    if ($strmode =~ /^\s*$/) {          # test for blank line
        $mode = $DEF_MODE;
    }
    elsif ($strmode !~ /^\d+$/) {
        print "Want numeric mode, not $strmode\n";
        redo PROMPT;
    }
    else {
        $mode = oct($strmode);          # converts "755" to 0755
    }
    chmod $mode, @files;
}

You can also import the symbolic S_I* constants from the Fcntl module:

use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;

29.2.11. chomp    

chomp VARIABLE
chomp LIST
chomp

Unlike chop, chomp returns the number of characters deleted. If $/ is "" (in paragraph mode), chomp removes all trailing newlines from the selected string (or strings, if chomping a LIST). You cannot chomp a literal, only a variable.

For example:

while (<PASSWD>) {
    chomp;   # avoid \n on last field
    @array = split /:/;
    ...
}

29.2.12. chop    

chop VARIABLE
chop LIST
chop

You cannot chop a literal, only a variable.

If you chop a LIST of variables, each string in the list is chopped:

@lines = `cat myfile`;
chop @lines;

chop($cwd = `pwd`);
chop($answer = <STDIN>);

$answer = chop($tmp = <STDIN>);  # WRONG

$answer = substr <STDIN>, 0, -1;

chop($answer = <STDIN>);

$last_char = chop($var);
$last_char = substr($var, -1, 1, "");   # same thing

substr($caravan, -5) = "";

$tail = substr($caravan, -5, 5, "");

29.2.13. chown    

chown LIST

chown($uidnum, $gidnum, 'file1', 'file2') == 2
        or die "can't chown file1 or file2: $!";

chown($uidnum, $gidnum, @filenames) == @filenames
        or die "can't chown @filenames: $!";

sub chown_by_name {
    my($user, @files) = @_;
    chown((getpwnam($user))[2,3], @files) == @files
            or die "can't chown @files: $!";
}

chown_by_name("fred", glob("*.c"));

On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On POSIX systems, you can detect which rule applies like this:

use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
# only try if we're the superuser or on a permissive system
if ($> == 0 || !sysconf(_PC_CHOWN_RESTRICTED) ) {
    chown($uidnum, -1, $filename)
        or die "can't chown $filename to $uidnum: $!";
}

29.2.14. chr    

chr NUMBER
chr

This function returns the character represented by that NUMBER in the character set. For example, chr(65) is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face. For the reverse of chr, use ord.

If you'd rather specify your characters by name than by number (for example, "\N{WHITE SMILING FACE}" for a Unicode smiley), see charnames in Chapter 31, "Pragmatic Modules".

29.2.15. chroot    

chroot FILENAME
chroot

chroot((getpwnam('ftp'))[7])
    or die "Can't do anonymous ftp: $!\n";

29.2.16. close    

close FILEHANDLE
close

FILEHANDLE may be an expression whose value can be used as an indirect filehandle (either the real filehandle name or a reference to anything that can be interpreted as a filehandle object).

If the filehandle came from a piped open, close will return false if any underlying syscall fails or if the program at the other end of the pipe exited with nonzero status. In the latter case, the close forces $! ($OS_ERROR) to zero. So if a close on a pipe returns a nonzero status, check $! to determine whether the problem was with the pipe itself (nonzero value) or with the program at the other end (zero value). In either event, $? ($CHILD_ERROR) contains the wait status value (see its interpretation under system) of the command associated with the other end of the pipe. For example:

open(OUTPUT, '| sort -rn | lpr -p') # pipe to sort and lpr
    or die "Can't start sortlpr pipe: $!";
print OUTPUT @lines;                # print stuff to output
close OUTPUT                        # wait for sort to finish
    or warn $! ? "Syserr closing sortlpr pipe: $!"
               : "Wait status $? from sortlpr pipe";

open(NETSTAT, "netstat -rn |")
    or die "can't run netstat: $!";
open(STDIN, "<&NETSTAT")
    or die "can't dup to stdin: $!";

If you somehow manage to reap an exited pipe child on your own, the close will fail. This could happen if you had a $SIG{CHLD} handler of your own that got triggered when the pipe child exited, or if you intentionally called waitpid on the process ID returned from the open call.

29.2.17. closedir    

closedir DIRHANDLE

29.2.18. connect    

connect SOCKET, NAME

use Socket;

my ($remote, $port) = ("www.perl.com", 80);
my $destaddr = sockaddr_in($port, inet_aton($remote));
connect SOCK, $destaddr
    or die "Can't connect to $remote at port $port: $!";

29.2.19. cos     

cos EXPR
cos

# Here's the lazy way of getting degrees-to-radians.

$pi = atan2(1,1) * 4;
$piover180 = $pi/180;

# Print table.
for ($deg = 0; $deg <= 90; $deg++) {
    printf "%3d %7.5f\n", $deg, cos($deg * $piover180);
}

sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

29.2.20. crypt    

crypt PLAINTEXT, SALT

[3] Only people with honorable intentions are allowed to do this.

crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) way to decrypt an encrypted password apart from exhaustive, brute-force guessing.

When verifying an existing encrypted string, you should use the encrypted text as the SALT (like crypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt, and with more exotic implementations, too.

When choosing a new SALT, you minimally need to create a random two character string whose characters come from the set [./0-9A-Za-z] (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). Older implementations of crypt only needed the first two characters of the SALT, but code that only gives the first two characters is now considered nonportable. See your local crypt(3) manpage for interesting details.

Here's an example that makes sure that whoever runs this program knows their own password:

$pwd = (getpwuid ($<))[1];    # Assumes we're on Unix.

system "stty -echo";   # or look into Term::ReadKey on CPAN
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";

if (crypt($word, $pwd) ne $pwd) {
    die "Sorry...\n";
} else {
    print "ok\n";
}

Shadow password files are slightly more secure than traditional password files, and you might have to be a superuser to access them. Because few programs should run under such powerful privileges, you might have the program maintain its own independent authentication system by storing the crypt strings in a different file than /etc/passwd or /etc/shadow.

The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.

29.2.21. dbmclose    

dbmclose HASH

29.2.22. dbmopen    

dbmopen HASH, DBNAME, MODE

The dbmopen function is really just a call to tie with the proper arguments, but is provided for backward compatibility with ancient versions of Perl. You can control which DBM library you use by using the tie interface directly or by loading the appropriate module before you call dbmopen. Here's an example that works on some systems for versions of DB_File similar to the version in your Netscape browser:

use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.dat", undef)
    or die "Can't open netscape history file: $!";

while (($url, $when) = each %NS_Hist) {
    next unless defined($when);
    chop ($url, $when);        # kill trailing null bytes
    printf "Visited %s at %s.\n", $url,
        scalar(localtime(unpack("V",$when)));
}

Functions such as keys and values may return huge list values when used on large DBM files. You may prefer to use the each function to iterate over large DBM files so that you don't load the whole thing in memory at once.

Hashes bound to DBM files have the same limitations as the type of DBM package you're using, including restrictions on how much data you can put into a bucket. If you stick to short keys and values, it's rarely a problem. See also the DB_File module in Chapter 32, "Standard Modules".

Another thing you should bear in mind is that many existing DBM databases contain null-terminated keys and values because they were set up with C programs in mind. The Netscape history file and the old sendmail aliases file are examples. Just use "$key\0" when pulling out a value, and remove the null from the value.

$alias = $aliases{"postmaster\0"};
chop $alias;   # kill the null

29.2.23. defined    

defined EXPR
defined

Many operations return undef under exceptional conditions, such as at end-of-file, when using an uninitialized variable's value, an operating system error, etc. Since undef is just one kind of false value, a simple Boolean test does not distinguish between undef, numeric zero, the null string, and the one-character string, "0"--all of which are equally false. The defined function allows you to distinguish between an undefined null string and a defined null string when you're using operators that might return a real null string.

Here is a fragment that tests a scalar value from a hash:

print if defined $switch{D};

In the next example we exploit the convention that some operations return the undefined value when you run out of data:

print "$val\n" while defined($val = pop(@ary));

setpwent();
while (defined($name = getpwent())) {
    print "<<$name>>\n";
}
endpwent();

die "Can't readlink $sym: $!"
    unless defined($value = readlink $sym);

indir("funcname", @arglist);
sub indir {
    my $subname = shift;
    no strict 'refs';  # so we can use subname indirectly
    if (defined &$subname) {
        &$subname(@_);    # or $subname->(@_);
    }
    else {
        warn "Ignoring call to invalid function $subname";
    }
}

if (@an_array) { print "has array elements\n" }
if (%a_hash)   { print "has hash members\n"   }

29.2.24. delete    

delete EXPR

Deleting from the %ENV hash modifies the environment. Deleting from a hash that is bound to a (writable) DBM file deletes the entry from that DBM file.

Historically, you could only delete from a hash, but with Perl version 5.6 you may also delete from an array. Deleting from an array causes the element at the specified position to revert to a completely uninitialized state, but it doesn't close up the gap, since that would change the positions of all the subsequent entries. Use a splice for that. (However, if you delete the final element in an array, the array size will shrink by one (or more, depending on the position of the next largest existing element (if any))).

EXPR can be arbitrarily complicated, provided that the final operation is a hash or array lookup:

# set up array of array of hash
$dungeon[$x][$y] = \%properties;

# delete one property from hash
delete $dungeon[$x][$y]{"OCCUPIED"};

# delete three properties all at once from hash
delete @{ $dungeon[$x][$y] }{ "OCCUPIED", "DAMP", "LIGHTED" };

# delete reference to %properties from array
delete $dungeon[$x][$y];

foreach $key (keys %hash) {
    delete $hash{$key};
}

delete @hash{keys %hash};

%hash = ();         # completely empty %hash
undef %hash;        # forget %hash ever existed

foreach $index (0 .. $#array) {
    delete $array[$index];
}

delete @array[0 .. $#array];

@array = ();         # completely empty @array
undef @array;        # forget @array ever existed

29.2.25. die    

die LIST
die

Within an eval, the function sets the $@ variable to the error message that would have otherwise been produced, then aborts the eval, which returns undef. The die function can thus be used to raise named exceptions that can be caught at a higher level in the program. See eval later in this chapter.

If LIST is a single object reference, that object is assumed to be an exception object and is returned unmodified as the exception in $@.

If LIST is empty and $@ already contains a string value (typically from a previous eval) that value is reused after appending "\t...propagated". This is useful for propagating (reraising) exceptions:

eval { ... };
die unless $@ =~ /Expected exception/;

If LIST is empty and $@ is empty, then the string "Died" is used.

If the final value of LIST does not end in a newline (and you're not passing an exception object), the current script filename, line number, and input line number (if any) are appended to the message, as well as a newline. Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at scriptname line 123" is appended. Suppose you are running script canasta; consider the difference between the following two ways of dying:

die "/usr/games is no good";
die "/usr/games is no good, stopped";

/usr/games is no good at canasta line 123.
/usr/games is no good, stopped at canasta line 123.

die '"', __FILE__, '", line ', __LINE__, ", phooey on you!\n";

"canasta", line 38, phooey on you!

die "Can't cd to spool: $!\n"   unless chdir '/usr/spool/news';

chdir '/usr/spool/news'         or die "Can't cd to spool: $!\n"

See also exit, warn, %SIG, and the Carp module.

29.2.26. do (block)    

do BLOCK

29.2.27. do (file)    

do FILE

do 'stat.pl';

scalar eval `cat stat.pl`;   # `type stat.pl` on Windows

If do can't read the file, it returns undef and sets $! to the error. If do can read the file but can't compile it, it returns undef and sets an error message in $@. If the file is successfully compiled, do returns the value of the last expression evaluated.

Inclusion of library modules (which have a mandatory .pm suffix) is better done with the use and require operators, which also do error checking and raise an exception if there's a problem. They also offer other benefits: they avoid duplicate loading, help with object-oriented programming, and provide hints to the compiler on function prototypes.

But doFILE is still useful for such things as reading program configuration files. Manual error checking can be done this way:

# read in config files: system first, then user
for $file ("/usr/share/proggie/defaults.rc",
           "$ENV{HOME}/.someprogrc")
{
    unless ($return = do $file) {
        warn "couldn't parse $file: $@" if $@;
        warn "couldn't do $file: $!"    unless defined $return;
        warn "couldn't run $file"       unless $return;
    }
}

29.2.28. do (subroutine)    

do SUBROUTINE(LIST)

29.2.29. dump    

dump LABEL
dump

This function is now largely obsolete, partly because it's difficult in the extreme to convert a core file into an executable in the general case, and because various compiler backends for generating portable bytecode and compilable C code have superseded it.

If you're looking to use dump to speed up your program, check out the discussion of efficiency matters in Chapter 24, "Common Practices", as well the Perl native-code generator in Chapter 18, "Compiling". You might also consider autoloading or selfloading, which at least make your program appear to run faster.

29.2.30. each    

each HASH

while (($key,$value) = each %ENV) {
    print "$key=$value\n";
}

There is a single iterator for each hash, shared by all each, keys, and values function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating keys %hash or values %hash. If you add or delete elements of a hash while you're iterating over it, the resulting behavior is not well-defined: entries might get skipped or duplicated.

See also keys, values, and sort.

29.2.31. eof    

eof FILEHANDLE
eof()
eof

while (<>) {
    if (eof()) {
        print "-" x 30, "\n";
    }
    print;
}

# reset line numbering on each input file
while (<>) {
    next if /^\s*#/;        # skip comments
    print "$.\t$_";
} continue {
    close ARGV if eof;      # Not eof()!
}

while (<>) {
    print if /pattern/ .. eof;
}

Warning: The eof function reads a byte and then pushes it back on the input stream with ungetc(3), so it is not useful in an interactive context. In fact, experienced Perl programmers rarely use eof, since the various input operators already behave politely in while-loop conditionals. See the example in the description of foreach in Chapter 4, "Statements and Declarations".

29.2.32. eval    

eval BLOCK
eval EXPR
eval

For either form of eval, the value returned from an eval is the value of the last expression evaluated, just as with subroutines. Similarly, you may use the return operator to return a value from the middle of the eval. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.

If there is a trappable error (including any produced by the die operator), eval returns undef and puts the error message (or object) in $@. If there is no error, $@ is guaranteed to be set to the null string, so you can test it reliably afterward for errors. A simple Boolean test suffices:

eval { ... };    # trap run-time errors
if ($@) { ... }    # handle error

The evalEXPR form can trap syntax errors because it parses the code at run time. (If the parse is unsuccessful, it places the parse error in $@, as usual.) Otherwise, it executes the value of EXPR as though it were a little Perl program. The code is executed in the context of the current Perl program, which means that it can see any enclosing lexicals from a surrounding scope, and that any non-local variable settings remain in effect after the eval is complete, as do any subroutine or format definitions. The code of the eval is treated as a block, so any locally scoped variables declared within the eval last only until the eval is done. (See my and local.) As with any code in a block, a final semicolon is not required.

Here is a simple Perl shell. It prompts the user to enter a string of arbitrary Perl code, compiles and executes that string, and prints whatever error occurred:

print "\nEnter some Perl code: ";

while (<STDIN>) {
    eval;
    print $@;
    print "\nEnter some more Perl code: ";
}

#!/usr/bin/perl
# rename - change filenames
$op = shift;
for (@ARGV) {
    $was = $_;
    eval $op;
    die if $@;
    # next line calls the built-in function, not the script by the same name
    rename($was,$_) unless $was eq $_;
}

$ rename 's/\.orig$//'                *.orig
$ rename 'y/A-Z/a-z/ unless /^Make/'  *
$ rename '$_ .= ".bad"'               *.f

Because evalBLOCK is syntax-checked at compile time, any syntax error is reported earlier. Therefore, if your code is invariant and both evalEXPR and evalBLOCK will suit your purposes equally well, the BLOCK form is preferred. For example:

# make divide-by-zero nonfatal
eval { $answer = $a / $b; };    warn $@ if $@;

# same thing, but less efficient if run multiple times
eval '$answer = $a / $b';       warn $@ if $@;

# a compile-time syntax error (not trapped)
eval { $answer = };             # WRONG

# a run-time syntax error
eval '$answer =';               # sets $@

The block of evalBLOCK does not count as a loop, so the loop control statements next, last, or redo cannot be used to leave or restart the block.

29.2.33. exec    

exec PATHNAME LIST
exec LIST

If there is only one scalar argument, the argument is checked for shell metacharacters. If metacharacters are found, the entire argument is passed to the system's standard command interpreter (/bin/sh under Unix). If there are no metacharacters, the argument is split into words and executed directly, since in the interests of efficiency this bypasses all the overhead of shell processing. It also gives you more control of error recovery should the program not exist.

If there is more than one argument in LIST, or if LIST is an array with more than one value, the system shell will never be used. This also bypasses any shell processing of the command. The presence or absence of metacharacters in the arguments doesn't affect this list-triggered behavior, which makes it the preferred form in security-conscious programs that do not wish to expose themselves to potential shell escapes.

This example causes the currently running Perl program to replace itself with the echo program, which then prints out the current argument list:

exec 'echo', 'Your arguments are: ', @ARGV;

exec "sort $outfile | uniq"
    or die "Can't do sort/uniq: $!\n";

When you ask the operating system to execute a new program within an existing process (as Perl's exec function does), you tell the system the location of the program to execute, but you also tell the new program (through its first argument) the name under which the program was invoked. Customarily, the name you tell it is just a copy of the location of the program, but it doesn't necessarily have to be, since there are two separate arguments at the level of the C language. When it is not a copy, you have the odd result that the new program thinks it's running under a name that may be totally different from the actual pathname where the program resides. Often this doesn't matter to the program in question, but some programs do care and adopt a different persona depending on what they think their name is. For example, the vi editor looks to see whether it was called as "vi" or as "view". If invoked as "view", it automatically enables read-only mode, just as though it was called with the -R command-line option.

This is where exec's optional PATHNAME parameter comes into play. Syntactically, it goes in the indirect-object slot like the filehandle for print or printf. Therefore, it doesn't take a comma after it, because it's not exactly part of the argument list. (In a sense, Perl takes the opposite approach from the operating system in that it assumes the first argument is the important one, and lets you modify the pathname if it differs.) For example:

$editor = "/usr/bin/vi";
exec $editor "view", @files      # trigger read-only mode
    or die "Couldn't execute $editor: $!\n";

exec { "/usr/bin/vi" } "view" @files      # trigger read-only mode
    or die "Couldn't execute $editor: $!\n";

@args = ("echo surprise");  # just one element in list
exec @args                  # still subject to shell escapes
     or die "exec: $!";     #   because @args == 1

exec { $args[0] } @args   # safe even with one-argument list
    or die "can't exec @args: $!";

Because the exec function is most often used shortly after a fork, it is assumed that anything that normally happens when a Perl process terminates should be skipped. Upon an exec, Perl will not call your END blocks, nor will it call any DESTROY methods associated with any objects. Otherwise, your child process would end up doing the cleanup you expected the parent process to do. (We wish that were the case in real life.)

Because it's such a common mistake to use exec instead of system, Perl warns you if there is a following statement that isn't die, warn, or exit when run with the popular -w command-line option, or if you've used the use warnings qw(exec syntax) pragma. If you really want to follow an exec with some other statement, you can use either of these styles to avoid the warning:

exec ('foo')   or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";

See also system.

29.2.34. exists    

exists EXPR

print "True\n"      if         $hash{$key};
print "Defined\n"   if defined $hash{$key};
print "Exists\n"    if exists  $hash{$key};

print "True\n"      if         $array[$index];
print "Defined\n"   if defined $array[$index];
print "Exists\n"    if exists  $array[$index];

EXPR can be arbitrarily complicated, provided that the final operation is a hash key or array index lookup:

if (exists $hash{A}{B}{$key}) { ... }

undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref;   # prints HASH(0x80d3d5c)

if ($ref                        and
    exists $ref->[$x]           and
    exists $ref->[$x][$y]       and
    exists $ref->[$x][$y]{$key} and
    exists $ref->[$x][$y]{$key}[2] ) { ... }

sub flub;
print "Exists\n"     if exists  &flub;
print "Defined\n"    if defined &flub;

29.2.35. exit    

exit EXPR
exit

$ans = <STDIN>;
exit if $ans =~ /^[Xx]/;

We said that the exit function exits immediately, but that was a bald-faced lie. It exits as soon as possible, but first it calls any defined END routines for at-exit handling. These routines cannot abort the exit, although they can change the eventual exit value by setting the $? variable. Likewise, any class that defines a DESTROY method will invoke that method on behalf of all its objects before the real program exits. If you really need to bypass exit processing, you can call the POSIX module's _exit function to avoid all END and destructor processing. And if POSIX isn't available, you can exec "/bin/false" or some such.

29.2.36. exp    

exp EXPR
exp

use Math::Complex;
print -exp(1) ** (i * pi);   # prints 1

29.2.37. fcntl    

fcntl FILEHANDLE, FUNCTION, SCALAR

use Fcntl;

SCALAR will be read or written (or both) depending on the FUNCTION. A pointer to the string value of SCALAR will be passed as the third argument of the actual fcntl call. (If SCALAR has no string value but does have a numeric value, that value will be passed directly rather than passing a pointer to the string value.) See the Fcntl module for a description of the more common permissible values for FUNCTION.

The fcntl function will raise an exception if used on a system that doesn't implement fcntl(2). On systems that do implement it, you can do such things as modify the close-on-exec flags (if you don't want to play with the $^F ($SYSTEM_FD_MAX) variable), modify the nonblocking I/O flags, emulate the lockf(3) function, and arrange to receive the SIGIO signal when I/O is pending.

Here's an example of setting a filehandle named REMOTE to be nonblocking at the system level. This makes any input operation return immediately if nothing is available when reading from a pipe, socket, or serial line that would otherwise block. It also works to cause output operations that normally would block to return a failure status instead. (For those, you'll likely have to negotiate $| as well.)

use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

$flags = fcntl(REMOTE, F_GETFL, 0)
            or die "Can't get flags for the socket: $!\n";

$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
            or die "Can't set flags for the socket: $!\n";

Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:

$retval = fcntl(...) || -1;
printf "fcntl actually returned %d\n", $retval;

29.2.38. fileno    

fileno FILEHANDLE

So, for example, the fileno function is useful for constructing bitmaps for select and for passing to certain obscure system calls if syscall(2) is implemented. It's also useful for double-checking that the open function gave you the file descriptor you wanted and for determining whether two filehandles use the same system file descriptor.

if (fileno(THIS) == fileno(THAT)) {
    print "THIS and THAT are dups\n";
}

One caution: don't count on the association of a Perl filehandle and a numeric file descriptor throughout the life of the program. If a file has been closed and reopened, the file descriptor may change. Perl takes a bit of trouble to try to ensure that certain file descriptors won't be lost if an open on them fails, but it only does this for file descriptors that don't exceed the current value of the special $^F ($SYSTEM_FD_MAX) variable (by default, 2). Although filehandles STDIN, STDOUT, and STDERR start out with file descriptors of 0, 1, and 2 (the Unix standard convention), even they can change if you start closing and opening them with wild abandon. You can't get into trouble with 0, 1, and 2 as long as you always reopen immediately after closing. The basic rule on Unix systems is to pick the lowest available descriptor, and that'll be the one you just closed.

29.2.39. flock    

flock FILEHANDLE, OPERATION

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly ORed with LOCK_NB. These constants are traditionally valued 1, 2, 8, and 4, but you can use the symbolic names if you import them from the Fcntl module, either individually or as a group using the :flock tag.

LOCK_SH requests a shared lock, so it's typically used for reading. LOCK_EX requests an exclusive lock, so it's typically used for writing. LOCK_UN releases a previously requested lock; closing the file also releases any locks. If the LOCK_NB bit is used with LOCK_SH or LOCK_EX, flock returns immediately rather than waiting for an unavailable lock. Check the return status to see whether you got the lock you asked for. If you don't use LOCK_NB, you might wait indefinitely for the lock to be granted.

Another nonobvious but traditional aspect of flock is that its locks are merely advisory. Discretionary locks are more flexible but offer fewer guarantees than mandatory ones. This means that files locked with flock may be modified by programs that do not also use flock. Cars that stop for red lights get on well with each other, but not with cars that don't stop for red lights. Drive defensively.

Some implementations of flock cannot lock things over the network. While you could in theory use the more system-specific fcntl for that, the jury (having sequestered itself on the case for a decade or so) is still out on whether this is (or even can be) reliable.

Here's a mailbox appender for Unix systems that use flock(2) to lock mailboxes:

use Fcntl qw/:flock/;       # import LOCK_* constants
sub mylock {
    flock(MBOX, LOCK_EX)
        or die "can't lock mailbox: $!";
    # in case someone appended while we were waiting
    # and our stdio buffer is out of sync
    seek(MBOX, 0, 2)
        or die "can't seek to the end of mailbox: $!";
}

open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
    or die "can't open mailbox: $!";

mylock();
print MBOX $msg, "\n\n";
close MBOX
    or die "can't close mailbox: $!";

29.2.40. fork    

fork

In versions of Perl prior to 5.6, unflushed buffers remain unflushed in both processes, which means you may need to set $| on one or more filehandles earlier in the program to avoid duplicate output.

A nearly bulletproof way to launch a child process while checking for "cannot fork" errors would be:

use Errno qw(EAGAIN);
FORK: {
    if ($pid = fork) {
        # parent here
        # child process pid is available in $pid
    }
        elsif (defined $pid) { # $pid is zero here if defined
        # child here
        # parent process pid is available with getppid
    }
        elsif ($! == EAGAIN) {
        # EAGAIN is the supposedly recoverable fork error
        sleep 5;
        redo FORK;
    }
        else {
        # weird fork error
        die "Can't fork: $!\n";
    }
}

If you fork without ever waiting on your children, you will accumulate zombies (exited processes whose parents haven't waited on them yet). On some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE"; on most, you must wait for your moribund children. See the wait function for examples of doing this, or see the "Signals" section of Chapter 16, "Interprocess Communication" for more on SIGCHLD.

If a forked child inherits system file descriptors like STDIN and STDOUT that are connected to a remote pipe or socket, you may have to reopen these in the child to /dev/null. That's because even when the parent process exits, the child will live on with its copies of those filehandles. The remote server (such as, say, a CGI script or a background job launched from a remote shell) will appear to hang because it's still waiting for all copies to be closed. Reopening the system filehandles to something else fixes this.

On most systems supporting fork(2), great care has gone into making it extremely efficient (for example, using copy-on-write technology on data pages), making it the dominant paradigm for multitasking over the last few decades. The fork function is unlikely to be implemented efficiently, or perhaps at all, on systems that don't resemble Unix. For example, Perl 5.6 emulates a proper fork even on Microsoft systems, but no assurances can be made on performance at this point. You might have more luck there with the Win32::Process module.

29.2.41. format    

format NAME =
    picture line
    value list
    ...
.

my $str = "widget";               # Lexically scoped variable.

format Nice_Output =
Test: @<<<<<<<< @||||| @>>>>>
      $str,     $%,    '$' . int($num)
.

local $~ = "Nice_Output";         # Select our format.
local $num = $cost * $quantity;   # Dynamically scoped variable.

write;

The "Format Variables" section in Chapter 7, "Formats" contains numerous details and examples of their use. Chapter 28, "Special Names" describes the internal format-specific variables, and the English and IO::Handle modules provide easier access to them.

29.2.42. formline    

formline PICTURE, LIST

Be careful if you put double quotes around the picture, since an @ character may be taken to mean the beginning of an array name. See "Formats" in Chapter 6, "Subroutines" for example uses.

29.2.43. getc    

getc FILEHANDLE
getc

This function is somewhat slow, but occasionally useful for single-character (byte, really) input from the keyboard--provided you manage to get your keyboard input unbuffered. This function requests unbuffered input from the standard I/O library. Unfortunately, the standard I/O library is not so standard as to provide a portable way to tell the underlying operating system to supply unbuffered keyboard input to the standard I/O system. To do that, you have to be slightly more clever, and in an operating-system-dependent fashion. Under Unix you might say this:

if ($BSD_STYLE) {
  system "stty cbreak </dev/tty >/dev/tty 2>&1";
} else {
  system "stty", "-icanon", "eol", "\001";
}

$key = getc;

if ($BSD_STYLE) {
  system "stty -cbreak </dev/tty >/dev/tty 2>&1";
} else {
  system "stty", "icanon", "eol", "^@"; # ASCII NUL
}
print "\n";

The POSIX module provides a more portable version of this using the POSIX::getattr function. See also the Term::ReadKey module from your nearest CPAN site for a more portable and flexible approach.

29.2.44. getgrent    

getgrent
setgrent
endgrent

($name, $passwd, $gid, $members)

while (($name, $passwd, $gid) = getgrent) {
    $gid{$name} = $gid;
}

29.2.45. getgrgid    

getgrgid GID

($name, $passwd, $gid, $members)

In scalar context, getgrgid returns only the group name. The User::grent module supports a by-name interface to this function. See getgrgid(3).

29.2.46. getgrnam    

getgrnam NAME

($name, $passwd, $gid, $members)

In scalar context, getgrnam returns only the numeric group ID. The User::grent module supports a by-name interface to this function. See getgrnam(3).

29.2.47. gethostbyaddr    

gethostbyaddr ADDR, ADDRTYPE

($name, $aliases, $addrtype, $length, @addrs) =
    gethostbyaddr($packed_binary_address, $addrtype);

($a, $b, $c, $d) = unpack('C4', $addrs[0]);

$dots = sprintf "%vd", $addrs[0];

use Socket;
$printable_address = inet_ntoa($addrs[0]);

To produce an ADDR from a dot vector, say this:

use Socket;
$ipaddr = inet_aton("127.0.0.1");       # localhost
$claimed_hostname = gethostbyaddr($ipaddr, AF_INET);

$ipaddr = v127.0.0.1;

29.2.48. gethostbyname    

gethostbyname NAME

($name, $aliases, $addrtype, $length, @addrs) =
    gethostbyname($remote_hostname);

($a, $b, $c, $d) = unpack('C4', $addrs[0]);

$dots = sprintf "%vd", $addrs[0];

use Socket;
$ipaddr = gethostbyname($remote_host);
printf "%s has address %s\n",
    $remote_host, inet_ntoa($ipaddr);

29.2.49. gethostent    

gethostent
sethostent STAYOPEN
endhostent

($name, $aliases, $addrtype, $length, @addrs)

($a, $b, $c, $d) = unpack('C4', $addrs[0]);

The Net::hostent module supports a by-name interface to this function.

29.2.50. getlogin    

getlogin

$login = getlogin() || (getpwuid($<))[0] || "Intruder!!";

29.2.51. getnetbyaddr    

getnetbyaddr ADDR, ADDRTYPE

use Socket;
($name, $aliases, $addrtype, $net) = getnetbyaddr(127, AF_INET);

29.2.52. getnetbyname    

getnetbyname NAME

($name, $aliases, $addrtype, $net) = getnetbyname("loopback");

29.2.53. getnetent    

getnetent
setnetent STAYOPEN
endnetent

($name, $aliases, $addrtype, $net) = getnetent();

The concept of network names seems rather quaint these days; most IP addresses are on unnamed (and unnameable) subnets.

29.2.54. getpeername    

getpeername SOCKET

use Socket;
$hersockaddr      = getpeername SOCK;
($port, $heraddr) = sockaddr_in($hersockaddr);
$herhostname      = gethostbyaddr($heraddr, AF_INET);
$herstraddr       = inet_ntoa($heraddr);

29.2.55. getpgrp    

getpgrp PID

29.2.56. getppid    

getppid

29.2.57. getpriority    

getpriority WHICH, WHO

The BSD::Resource module from CPAN provides a more convenient interface, including the PRIO_PROCESS, PRIO_PGRP, and PRIO_USER symbolic constants to supply for the WHICH argument. Although these are traditionally set to 0, 1, and 2 respectively, you really never know what may happen within the dark confines of C's #include files.

A value of 0 for WHO means the current process, process group, or user, so to get the priority of the current process, use:

$curprio = getpriority(0, 0);

29.2.58. getprotobyname    

getprotobyname NAME

($name, $aliases, $protocol_number) = getprotobyname("tcp");

29.2.59. getprotobynumber    

getprotobynumber NUMBER

($name, $aliases, $protocol_number) = getprotobynumber(6);

29.2.60. getprotoent    

getprotoent
setprotoent STAYOPEN
endprotoent

($name, $aliases, $protocol_number) = getprotoent();

29.2.61. getpwent    

getpwent
setpwent
endpwent

($name,$passwd,$uid,$gid,$quota,$comment,$gcos,$dir,$shell) = getpwent();

while (($name, $passwd, $uid) = getpwent()) {
    $uid{$name} = $uid;
}

29.2.62. getpwnam    

getpwnam NAME

($name,$passwd,$uid,$gid,$quota,$comment,$gcos,$dir,$shell) = getpwnam("daemon");

For repeated lookups, consider caching the data in a hash using getpwent.

In scalar context, getpwnam returns only the numeric user ID. The User::pwent module supports a by-name interface to this function. See getpwnam(3) and passwd(5).

29.2.63. getpwuid    

getpwuid UID

($name,$passwd,$uid,$gid,$quota,$comment,$gcos,$dir,$shell) = getpwuid(2);

In scalar context, getpwuid returns the username. The User::pwent module supports a by-name interface to this function. See getpwnam(3) and passwd(5).

29.2.64. getservbyname    

getservbyname NAME, PROTO

($name, $aliases, $port_number, $protocol_name) = getservbyname("www", "tcp");

29.2.65. getservbyport    

getservbyport PORT, PROTO

($name, $aliases, $port_number, $protocol_name) = getservbyport(80, "tcp");

29.2.66. getservent    

getservent
setservent STAYOPEN
endservent

($name, $aliases, $port_number, $protocol_name) = getservent();

29.2.67. getsockname    

getsockname SOCKET

use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
$myname = gethostbyaddr($myaddr,AF_INET);
printf "I am %s [%vd]\n", $myname, $myaddr;

29.2.68. getsockopt    

getsockopt SOCKET, LEVEL, OPTNAME

29.2.69. glob    

glob EXPR
glob

For historical reasons, the algorithm matches the csh(1)'s style of expansion, not the Bourne shell's. Versions of Perl before the 5.6 release used an external process, but 5.6 and later perform globs internally. Files whose first character is a dot (".") are ignored unless this character is explicitly matched. An asterisk ("*") matches any sequence of any character (including none). A question mark ("?") matches any one character. A square bracket sequence ("[...]") specifies a simple character class, like "[chy0-9]". Character classes may be negated with a circumflex, as in "*.[^oa]", which matches any non-dot files whose names contain a period followed by one character which is neither an "a" nor an "o" at the end of the name. A tilde ("~") expands to a home directory, as in "~/.*rc" for all the current user's "rc" files, or "~jane/Mail/*" for all of Jane's mail files. Braces may be used for alternation, as in "~/.{mail,ex,csh,twm,}rc" to get those particular rc files.

If you want to glob filenames that might contain whitespace, you'll need to use the File::Glob module directly, since glob grandfathers the use of whitespace to separate multiple patterns such as <*.c *.h>. For details, see File::Glob in Chapter 32, "Standard Modules". Calling glob (or the <*> operator) automatically uses that module, so if the module mysteriously vaporizes from your library, an exception is raised.

When you call open, Perl does not expand wildcards, including tildes. You need to glob the result first.

open(MAILRC, "~/.mailrc")                # WRONG: tilde is a shell thing
    or die "can't open ~/.mailrc: $!";

open(MAILRC, (glob("~/.mailrc"))[0])     # expand tilde first
    or die "can't open ~/.mailrc: $!";

See also the "Filename globbing operator" section of Chapter 2, "Bits and Pieces".

29.2.70. gmtime    

gmtime EXPR
gmtime

#  0    1    2     3     4    5     6     7     8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = gmtime;

All list elements are numeric and come straight out of a struct tm (that's a C programming structure--don't sweat it). In particular this means that $mon has the range 0..11 with January as month 0, and $wday has the range 0..6 with Sunday as day 0. You can remember which ones are zero-based because those are the ones you're always using as subscripts into zero-based arrays containing month and day names.

For example, to get the current month in London, you might say:

$london_month = (qw(Jan Feb Mar Apr May Jun
                    Jul Aug Sep Oct Nov Dec))[(gmtime)[4]];

In scalar context, gmtime returns a ctime(3)-like string based on the GMT time value. The Time::gmtime module supports a by-name interface to this function. See also POSIX::strftime() for a more fine-grained approach to formatting times.

This scalar value is not locale dependent but is instead a Perl built-in. Also see the Time::Local module and the strftime(3) and mktime(3) functions available via the POSIX module. To get somewhat similar but locale-dependent date strings, set up your locale environment variables appropriately (please see the perllocale manpage), and try:

use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

29.2.71. goto    

goto LABEL
goto EXPR
goto &NAME

[4]This means that if it doesn't find the label in the current routine, it looks back through the routines that called the current routine for the label, thus making it nearly impossible to maintain your program.

Going to even greater heights of orthogonality (and depths of idiocy), Perl allows gotoEXPR, which expects EXPR to evaluate to a label name, whose location is guaranteed to be unresolvable until run time since the label is unknown when the statement is compiled. This allows for computed gotos per FORTRAN, but isn't necessarily recommended[5] if you're optimizing for maintainability:

goto +("FOO", "BAR", "GLARCH")[$i];

[5] Understatement is reputed to be funny, so we thought we'd try one here.

29.2.72. grep    

grep EXPR, LIST
grep BLOCK LIST

If @all_lines contains lines of code, this example weeds out comment lines:

@code_lines = grep !/^\s*#/, @all_lines;

@list = qw(barney fred dino wilma);
@greplist = grep { s/^[bfd]// } @list;

See also map. The following two statements are functionally equivalent:

@out = grep { EXPR } @in;
@out = map { EXPR ? $_ : () } @in

29.2.73. hex    

hex EXPR
hex

$number = hex("ffff12c0");

sprintf "%lx", $number;         # (That's an ell, not a one.)

29.2.74. import    

import CLASSNAME LIST
import CLASSNAME

29.2.75. index    

index STR, SUBSTR, OFFSET
index STR, SUBSTR

$pos = -1;
while (($pos = index($string, $lookfor, $pos)) > -1) {
    print "Found at $pos\n";
    $pos++;
}

29.2.76. int    

int EXPR
int

$average_age = 939/16;      # yields 58.6875 (58 in C)
$average_age = int 939/16;  # yields 58

$n = sprintf("%.0f", $f);   # round (not trunc) to nearest integer

29.2.77. ioctl    

ioctl FILEHANDLE, FUNCTION, SCALAR

require "sys/ioctl.ph";     # perhaps /usr/local/lib/perl/sys/ioctl.ph

require 'sys/ioctl.ph';

$size = pack("L", 0);
ioctl(FH, FIONREAD(), $size)
    or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);

The return value of ioctl (and fcntl) is as follows:

Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:

$retval = ioctl(...) || -1;
printf "ioctl actually returned %d\n", $retval;

Calls to ioctl should not be considered portable. If, say, you're merely turning off echo once for the whole script, it's more portable to say:

system "stty -echo";   # Works on most Unix boxen.

For still better portability, you might look at the Term::ReadKey module from CPAN.

29.2.78. join    

join EXPR, LIST

$rec = join ':', $login,$passwd,$uid,$gid,$gcos,$home,$shell;

$string = join "", @array;

29.2.79. keys    

keys HASH

@keys   = keys   %ENV;    # keys are in the same order as
@values = values %ENV;    # values, as this demonstrates
while (@keys) {
    print pop(@keys), '=', pop(@values), "\n";
}

foreach $key (sort keys %ENV) {
    print $key, '=', $ENV{$key}, "\n";
}

foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
    printf "%4d %s\n", $hash{$key}, $key;
}

In scalar context, keys returns the number of elements of the hash (and resets the each iterator). However, to get this information for tied hashes, including DBM files, Perl must walk the entire hash, so it's not efficient then. Calling keys in a void context helps with that.

Used as an lvalue, keys increases the number of hash buckets allocated for the given hash. (This is similar to pre-extending an array by assigning a larger number to $#array.) Pre-extending your hash can gain a measure of efficiency if you happen to know the hash is going to get big, and how big it's going to get. If you say:

keys %hash = 1000;

See also each, values, and sort.

29.2.80. kill    

kill SIGNAL, LIST

$cnt = kill 1, $child1, $child2;
kill 9, @goners;
kill 'STOP', getppid       # Can *so* suspend my login shell...
        unless getppid == 1;    # (But don't taunt init(8).)

use Errno qw(ESRCH EPERM);
if (kill 0 => $minion) {
    print "$minion is alive!\n";
} elsif ($! == EPERM) {             # changed UID
    print "$minion has escaped my control!\n";
} elsif ($! == ESRCH) {
    print "$minion is deceased.\n";  # or zombied
} else {
    warn "Odd; I couldn't check on the status of $minion: $!\n";
}

See the section "Signals" in Chapter 16, "Interprocess Communication".

29.2.81. last    

last LABEL
last

LINE: while (<MAILMSG>) {
    last LINE if /^$/; # exit when done with header
    # rest of loop here
}

A block by itself is semantically identical to a loop that executes once. Thus last can be used to effect an early exit out of such a block.

See also Chapter 4, "Statements and Declarations" for illustrations of how last, next, redo, and continue work.

29.2.82. lc    

lc EXPR
lc

29.2.83. lcfirst    

lcfirst EXPR
lcfirst

29.2.84. length    

length EXPR
length

Do not try to use length to find the size of an array or hash. Use scalar @array for the size of an array, and scalar keys %hash for the number of key/value pairs in a hash. (The scalar is typically omitted when redundant.)

To find the length of a string in bytes rather than characters, say:

$blen = do { use bytes; length $string; };

$blen = bytes::length($string);    # must use bytes first

29.2.85. link    

link OLDFILE, NEWFILE

29.2.86. listen    

listen SOCKET, QUEUESIZE

use Socket;
listen(PROTOSOCK, SOMAXCONN)
    or die "cannot set listen queue on PROTOSOCK: $!";

29.2.87. local    

local EXPR

This operator works by saving the current values of the specified variables on a hidden stack and restoring them upon exiting the block, subroutine, eval, or file. After the local is executed, but before the scope is exited, any subroutines and executed formats will see the local, inner value, instead of the previous, outer value because the variable is still a global variable, despite having a localized value. The technical term for this is "dynamic scoping". See the section "Scoped Declarations" in Chapter 4, "Statements and Declarations".

The EXPR may be assigned to if desired, which allows you to initialize your variables as you localize them. If no initializer is given, all scalars are initialized to undef, and all arrays and hashes to (). As with ordinary assignment, if you use parentheses around the variables on the left (or if the variable is an array or hash), the expression on the right is evaluated in list context. Otherwise, the expression on the right is evaluated in scalar context.

In any event, the expression on the right is evaluated before the localization, but the initialization happens after localization, so you can initialize a localized variable with its nonlocalized value. For instance, this code demonstrates how to make a temporary modification to a global array:

if ($sw eq '-v') {
    # init local array with global array
    local @ARGV = @ARGV;
    unshift @ARGV, 'echo';
    system @ARGV;
}
# @ARGV restored

# temporarily add a couple of entries to the %digits hash
if ($base12) {
    # (NOTE: We're not claiming this is efficient!)
    local(%digits) = (%digits, T => 10, E => 11);
    parse_num();
}

if ($protected) {
    local $SIG{INT} = 'IGNORE';
    precious();     # no interrupts during this function
}                   # previous handler (if any) restored

local *MOTD;                # protect any global MOTD handle
my $fh = do { local *FH };  # create new indirect filehandle

But in general, you usually want to use my instead of local, because local isn't really what most people think of as "local", or even "lo-cal". See my.

29.2.88. localtime    

localtime EXPR
localtime

#  0    1    2     3     4    5     6     7     8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime;

All list elements are numeric and come straight out of a struct tm. (That's a bit of C programming lingo--don't worry about it.) In particular, this means that $mon has the range 0..11 with January as month 0, and $wday has the range 0..6 with Sunday as day 0. You can remember which ones are zero-based because those are the ones you're always using as subscripts into zero-based arrays containing month and day names.

For example, to get the name of the current day of the week:

$thisday = (Sun,Mon,Tue,Wed,Thu,Fri,Sat)[(localtime)[6]];

The Perl library module Time::Local contains a subroutine, timelocal, that can convert in the opposite direction.

In scalar context, localtime returns a ctime(3)-like string. For example, the date(1) command can be (almost)[6] emulated with:

perl -le 'print scalar localtime'

[6]date(1) prints the timezone, whereas scalar localtime does not.

29.2.89. lock    

lock THING

29.2.90. log    

log EXPR
log

sub log10 {
    my $n = shift;
    return log($n)/log(10);
}

29.2.91. lstat    

lstat EXPR
lstat

29.2.92. m//    

/PATTERN/
m/PATTERN/

29.2.93. map    

map BLOCK LIST
map EXPR, LIST

@words = map { split ' ' } @lines;

@chars = map chr, @nums;

%hash = map { genkey($_) => $_ } @array;

%hash = ();
foreach $_ (@array) {
    $hash{genkey($_)} = $_;
}

29.2.94. mkdir    

mkdir FILENAME, MASK
mkdir FILENAME

If MASK is omitted, a mask of 0777 is assumed, which is almost always what you want anyway. In general, creating directories with permissive MASKs (like 0777) and letting the user modify that with their umask is better than supplying a restrictive MASK and giving the user no way to be more permissive. The exception to this rule is when the file or directory should be kept private (mail files, for instance). See umask.

If the mkdir(2) syscall is not built into your C library, Perl emulates it by calling the mkdir(1) program for each directory. If you are creating a long list of directories on such a system, it'll be more efficient to call the mkdir program yourself with the list of directories than it is to start zillions of subprocesses.

29.2.95. msgctl    

msgctl ID, CMD, ARG

This function is available only on machines supporting System V IPC, which turns out to be far fewer than those supporting sockets.

29.2.96. msgget    

msgget KEY, FLAGS

This function is available only on machines supporting System V IPC.

29.2.97. msgrcv    

msgrcv ID, VAR, SIZE, TYPE, FLAGS

This function is available only on machines supporting System V IPC.

29.2.98. msgsnd    

msgsnd ID, MSG, FLAGS

$msg = pack "L a*", $type, $text_of_message;

This function is available only on machines supporting System V IPC.

29.2.99. my    

my TYPE EXPR : ATTRIBUTES
my EXPR : ATTRIBUTES
my TYPE EXPR
my EXPR

The variable name cannot be package qualified, because package variables are all globally accessible through their corresponding symbol table, and lexical variables are unrelated to any symbol table. Unlike local, then, this operator has nothing to do with global variables, other than hiding any other variable of the same name from view within its scope (that is, where the private variable exists). A global variable can always be accessed through its package-qualified form, however, or through a symbolic reference.

A private variable's scope does not start until the statement after its declaration. The variable's scope extends into any enclosed blocks thereafter, up to the end of the scope of the variable itself.

However, this means that any subroutines you call from within the scope of a private variable cannot see the private variable unless the block that defines the subroutine itself is also textually enclosed within the scope of that variable. That sounds complicated, but it's not once you get the hang of it. The technical term for this is lexical scoping, so we often call these lexical variables. In C culture, they're sometimes called "auto" variables, since they're automatically allocated and deallocated at scope entry and exit.

The EXPR may be assigned to if desired, which allows you to initialize your lexical variables. (If no initializer is given, all scalars are initialized to the undefined value and all arrays and hashes to the empty list.) As with ordinary assignment, if you use parentheses around the variables on the left (or if the variable is an array or hash), the expression on the right is evaluated in list context. Otherwise, the expression on the right is evaluated in scalar context. For example, you can name your formal subroutine parameters with a list assignment, like this:

my ($friends, $romans, $countrymen) = @_;

my $country = @_;  # right or wrong?

sub simple_as {
    my $self = shift;   # scalar assignment
    my ($a,$b,$c) = @_; # list assignment
    ...
}

The TYPE and ATTRIBUTES are optional, which is just as well, since they're both considered experimental. Here's what a declaration that uses them might look like:

my Dog $spot :ears(short) :tail(long);

my Dog $lassie = new Collie;

Interestingly, up through version 5.6.0, the only time Perl pays attention to the TYPE declaration is when the corresponding class has declared fields with the use fields pragma. Together, these declarations allow the pseudohash implementation of a class to "show through" to code outside the class, so that hash lookups can be optimized by the compiler into array lookups. In a sense, the pseudohash is the interface to such a class, so our theory remains intact, if a bit battered. For more on pseudohashes, see the section "Pseudohashes" in Chapter 8, "References".

In the future, other types of classes may interpret the TYPE differently. The TYPE declaration should be considered a generic type interface that might someday be instantiated in various ways depending on the class. In fact, the TYPE might not even be an official class name. We're reserving the lowercase type names for Perl, because one of the ways we'd like to extend the type interface is to allow optional low-level type declarations such as int, num, str, and ref. These declarations will not be for the purpose of strong typing; rather, they'll be hints to the compiler telling it to optimize the storage of the variable with the assumption that the variable will be used mostly as declared. The semantics of scalars will stay pretty much the same--you'll still be able to add two str scalars, or print an int scalar, just as though they were the ordinary polymorphic scalars you're familiar with. But with an int declaration Perl might decide to store only the integer value and forget about caching the resulting string as it currently does. Loops with int loop variables might run faster, particularly in code compiled down to C. In particular, arrays of numbers could be stored much more compactly. As a limiting case, the built-in vec function might even become obsolete when we can write declarations such as:

my bit @bitstring;

The ATTRIBUTES declaration is even more experimental. We haven't done much more than reserve the syntax and prototype the internal interface; see the use attributes pragma in Chapter 31, "Pragmatic Modules" for more on that. The first attribute we'll implement is likely to be constant:

my num $PI : constant = atan2(1,1) * 4;

See also local, our, and the section "Scoped Declarations" in Chapter 4, "Statements and Declarations".

29.2.100. new    

new CLASSNAME LIST
new CLASSNAME

29.2.101. next    

next LABEL
next

LINE: while (<STDIN>) {
    next LINE if /^#/;     # discard comments
    ...
}

A block by itself is semantically identical to a loop that executes once. Thus, next will exit such a block early (via the continue block, if there is one).

next cannot be used to exit a block that returns a value, such as eval {}, sub {}, or do {}, and should not be used to exit a grep or map operation. With warnings enabled, Perl will warn you if you next out of a loop not in your current lexical scope, such as a loop in a calling subroutine. See the section "Loop Statements" in Chapter 4, "Statements and Declarations".

29.2.102. no    

no MODULE LIST

29.2.103. oct    

oct EXPR
oct

$val = oct $val if $val =~ /^0/;

$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;

29.2.104. open    

open FILEHANDLE, MODE, LIST
open FILEHANDLE, EXPR
open FILEHANDLE

If only two arguments are present, the mode and filename/command are assumed to be combined in the second argument. (And if you don't specify a mode in the second argument, just a filename, then the file is opened read-only to be on the safe side.)

With only one argument, the package scalar variable of the same name as the FILEHANDLE must contain the filename and optional mode:

$LOG = ">logfile";        # $LOG must not be declared my!
   open LOG or die "Can't open logfile: $!";

The open function returns true when it succeeds and undef otherwise. If the open starts up a pipe to a child process, the return value will be the process ID of that new process. As with any syscall, always check the return value of open to make sure it worked. But this isn't C or Java, so don't use an if statement when the or operator will do. You can also use ||, but if you do, use parentheses on the open. If you choose to omit parentheses on the function call to turn it into a list operator, be careful to use "or die" after the list rather than "|| die", because the precedence of || is higher than list operators like open, and the || will bind to your last argument, not the whole open:

open LOG, ">logfile" || die "Can't create logfile: $!";  # WRONG
open LOG, ">logfile" or die "Can't create logfile: $!";  # ok

open LOG, ">logfile"
        or die "Can't create logfile: $!";

{
        my $fh;                   # (uninitialized)
        open($fh, ">logfile")     # $fh is autovivified
            or die "Can't create logfile: $!";
        ...                       # do stuff with $fh
}                                 # $fh closed here

open my $fh, ">logfile" or die ...

open(LOG, ">", "logfile")  or die "Can't create logfile: $!";

The modes that indicate how to open a file are shell-like redirection symbols. A list of these symbols is provided in Table 29-1. (To access a file with combinations of open modes not covered by this table, see the low-level sysopen function.)

Table 29.1. Modes for open

If the mode is "<" or nothing, an existing file is opened for input. If the mode is ">", the file is opened for output, which truncates existing files and creates nonexistent ones. If the mode is ">>", the file is created if needed and opened for appending, and all output is automatically placed at the end of the file. If a new file is created because you used a mode of ">" or ">>" and the file did not previously exist, access permissions will depend on the process's current umask under the rules described for that function.

Here are common examples:

open(INFO,      "datafile")  || die("can't open datafile: $!");
open(INFO,    "< datafile")  || die("can't open datafile: $!");
open(RESULTS, "> runstats")  || die("can't open runstats: $!");
open(LOG,    ">> logfile ")  || die("can't open logfile:  $!");

open INFO,      "datafile"   or die "can't open datafile: $!";
open INFO,    "< datafile"   or die "can't open datafile: $!";
open RESULTS, "> runstats"   or die "can't open runstats: $!";
open LOG,    ">> logfile "   or die "can't open logfile:  $!";

open(INPUT,  "-" ) or die;     # re-open standard input for reading
open(INPUT,  "<-") or die;     # same thing, but explicit
open(OUTPUT, ">-") or die;     # re-open standard output for writing

You may also place a "+" in front of any of these three modes to request simultaneous read and write. However, whether the file is clobbered or created and whether it must already exist is still governed by your choice of less-than or greater-than signs. This means that "+<" is almost always preferred for read/write updates, as the dubious "+>" mode would first clobber the file before you could ever read anything from it. (Use that mode only if you want to reread only what you just wrote.)

open(DBASE, "+< database")
    or die "can't open existing database in update mode: $!";

If the leading character in EXPR is a pipe symbol, open fires up a new process and connects a write-only filehandle to the command. This way you can write into that handle and what you write will show up on that command's standard input. For example:

open(PRINTER, "| lpr -Plp1")    or die "can't fork: $!";
print PRINTER "stuff\n";
close(PRINTER)                  or die "lpr/close failed: $?/$!";

open(NET, "netstat -i -n |")    or die "can't fork: $!";
while (<NET>) { ... }
close(NET)                      or die "can't close netstat: $!/$?";

Any pipe command containing shell metacharacters such as wildcards or I/O redirections is passed to your system's canonical shell (/bin/sh on Unix), so those shell-specific constructs can be processed first. If no metacharacters are found, Perl launches the new process itself without calling the shell.

You may also use the three-argument form to start up pipes. Using that style, the equivalent of the previous pipe opens would be:

open(PRINTER, "|-", "lpr -Plp1")    or die "can't fork: $!";
open(NET, "-|", "netstat -i -n")    or die "can't fork: $!";

open(PRINTER, "|-", "lpr", "-Plp1")    or die "can't fork: $!";
open(NET, "-|", "netstat", "-i", "-n") or die "can't fork: $!";

defined($pid = open(FROM_CHILD, "-|"))
        or die "can't fork: $!";

if ($pid) {
        @parent_lines = <FROM_CHILD>;  # parent code
}
else {
        print STDOUT @child_lines;     # child code
}

open FH,            "| tr   'a-z'  'A-Z'";        # pipe to shell command
open FH, "|-",        'tr', 'a-z', 'A-Z';         # pipe to bare command
open FH, "|-" or exec 'tr', 'a-z', 'A-Z' or die;  # pipe to child

open FH,              "cat    -n   'file' |";     # pipe from shell command
open FH, "-|",        'cat', '-n', 'file';        # pipe from bare command
open FH, "-|" or exec 'cat', '-n', 'file' or die; # pipe from child

[7]Or you can think of it as leaving the command off of the three-argument forms above.

When starting a command with open, you must choose either input or output: "cmd|" for reading or "|cmd" for writing. You may not use open to start a command that pipes both in and out, as the (currently) illegal notation, "|cmd|", might appear to indicate. However, the standard IPC::Open2 and IPC::Open3 library routines give you a close equivalent. For details on double-ended pipes, see the section "Bidirectional Communication" in Chapter 16, "Interprocess Communication".

You may also, in the Bourne shell tradition, specify an EXPR beginning with >&, in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duplicated using the dup2(2) syscall.[8] You may use & after >, >>, <, +>, +>>, and +<. (The specified mode should match the mode of the original filehandle.)

[8]This doesn't (currently) work with I/O objects on typeglob references by filehandle autovivification, but you can always use fileno to fetch the file descriptor and dup that.

One reason you might want to do this would be if you already had a filehandle open and wanted to make another handle that's really a duplicate of the first one.

open(SAVEOUT, ">&SAVEERR") or die "couldn't dup SAVEERR: $!";
open(MHCONTEXT, "<&4")     or die "couldn't dup fd4: $!";

somefunction("&main::LOGFILE");

#!/usr/bin/perl
open SAVEOUT, ">&STDOUT";
open SAVEERR, ">&STDERR";

open STDOUT, ">foo.out" or die "Can't redirect stdout";
open STDERR, ">&STDOUT" or die "Can't dup stdout";

select STDERR; $| = 1;         # enable autoflush
select STDOUT; $| = 1;         # enable autoflush

print STDOUT "stdout 1\n";     # these I/O streams propagate to
print STDERR "stderr 1\n";     # subprocesses too

system("some command");        # uses new stdout/stderr

close STDOUT;
close STDERR;

open STDOUT, ">&SAVEOUT";
open STDERR, ">&SAVEERR";

print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";

$fd = $ENV{"MHCONTEXTFD"};
open(MHCONTEXT, "<&=$fdnum")
        or die "couldn't fdopen descriptor $fdnum: $!";

use Fcntl qw(F_GETFD F_SETFD);
$flags = fcntl(FH, F_SETFD, 0)
        or die "Can't clear close-on-exec flag on FH: $!\n";

With the one- or two-argument form of open, you have to be careful when you use a string variable as a filename, since the variable may contain arbitrarily weird characters (particularly when the filename has been supplied by arbitrarily weird characters on the Internet). If you're not careful, parts of the filename might get interpreted as a MODE string, ignorable whitespace, a dup specification, or a minus. Here's one historically interesting way to insulate yourself:

$path =~ s#^(\s)#./$1#;
open (FH, "< $path\0") or die "can't open $path: $!";

open(FH, "<", $path) or die "can't open $path: $!";

use Fcntl;
sysopen(FH, $path, O_RDONLY) or die "can't open $path: $!";

Systems that need the binmode function are distinguished from those that don't by the format used for text files. Those that don't need it terminate each line with a single character that corresponds to what C thinks is a newline, \n. Unix and Mac OS fall into this category. VMS, MVS, MS-whatever, and S&M operating systems of other varieties treat I/O on text files and binary files differently, so they need binmode.

Or its equivalent. As of the 5.6 release of Perl, you can specify binary mode in the open function without a separate call to binmode. As part of the MODE argument (but only in the three-argument form), you may specify various input and output disciplines. To do the equivalent of a binmode, use the three argument form of open and stuff a discipline of :raw in after the other MODE characters:

open(FH, "<:raw", $path) or die "can't open $path: $!";

Table 29.2. I/O Disciplines

You'll be able to stack disciplines that make sense to stack, so, for instance, you could say:

open(FH, "<:para:crlf:uni", $path) or die "can't open $path: $!";
while ($para = <FH>) { ... }

• read in some form of Unicode and translate to Perl's internal UTF-8 format if the file isn't already in UTF-8,

• look for variants of line-ending sequences, translating them all to \n, and

• process the file into paragraph-sized chunks, much as $/ = "" does.

If you want to set the default open mode (:def) to something other than :text, you can declare that at the top of your file with the open pragma:

use open IN => ":any", OUT => ":utf8";

29.2.105. opendir    

opendir DIRHANDLE, EXPR

29.2.106. ord    

ord EXPR
ord

29.2.107. our    

our TYPE EXPR : ATTRIBUTES
our EXPR : ATTRIBUTES
our TYPE EXPR
our EXPR

The primary use of an our declaration is to hide the variable from the effects of a use strict "vars" declaration; since the variable is masquerading as a my variable, you are permitted to use the declared global variable without qualifying it with its package. However, just like the my variable, this only works within the lexical scope of the our declaration. In this respect, it differs from use vars, which affects the entire package and is not lexically scoped.

our is also like my in that you are allowed to declare variables with a TYPE and with ATTRIBUTES. Here is the syntax:

our Dog $spot :ears(short) :tail(long);

Another way in which our is like my is in its visibility. An our declaration declares a global variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is located is determined at the point of the declaration, not at the point of use. This means the following behavior holds and is deemed to be a feature:

package Foo;
our $bar;      # $bar is $Foo::bar for rest of lexical scope
$bar = 582;

package Bar;
print $bar;    # prints 582, just as if "our" had been "my"

($x, $y) = ("one", "two");
print "before block, x is $x, y is $y\n";
{
    our $x = 10;
    local our $y = 20;
    print "in block, x is $x, y is $y\n";
}
print "past block, x is $x, y is $y\n";

before block, x is one, y is two
in block, x is 10, y is 20
past block, x is 10, y is two

use warnings;
package Foo;
our $bar;          # declares $Foo::bar for rest of lexical scope
$bar = 20;

package Bar;
our $bar = 30;     # declares $Bar::bar for rest of lexical scope
print $bar;        # prints 30

our $bar;          # emits warning

29.2.108. pack    

pack TEMPLATE, LIST

The template describes the structure of the string as a sequence of fields. Each field is represented by a single character that describes the type of the value and its encoding. For instance, a format character of N specifies an unsigned four-byte integer in big-endian byte order.

Fields are packed in the order given in the template. For example, to pack an unsigned one-byte integer and a single-precision floating-point value into a string, you'd say:

$string = pack("Cf", 244, 3.14);

Some important things to consider when packing are:

• the type of data (such as integer or float or string),

• the range of values (such as whether your integers will fit into one, two, four, or maybe even eight bytes; or whether you're packing 8-bit or Unicode characters),

• whether your integers are signed or unsigned, and

• the encoding to use (such as native, little-endian, or big-endian packing of bits and bytes).

Table 29-3 lists the format characters and their meanings. (Other characters can occur in formats as well; these are described later.)

Table 29.3. Template Characters for pack/unpack

You may freely place whitespace and comments in your TEMPLATEs. Comments start with the customary # symbol and extend up through the first newline (if any) in the TEMPLATE.

Each letter may be followed by a number indicating the count, interpreted as a repeat count or length of some sort, depending on the format. With all formats except a, A, b, B, h, H, P, and Z, count is a repeat count, so pack gobbles up that many values from the LIST. A * for the count means however many items are left.

The a, A, and Z formats gobble just one value, but pack it as a byte string of length count, padding with nulls or spaces as necessary. When unpacking, A strips trailing spaces and nulls, Z strips everything after the first null, and a returns the literal data unmolested. When packing, a and Z are equivalent.

Similarly, the b and B formats pack a string count bits long. Each byte of the input field generates 1 bit of the result based on the least-significant bit of each input byte (that is, on ord($byte) % 2). Conveniently, that means bytes 0 and 1 generate bits 0 and 1. Starting from the beginning of the input string, each 8-tuple of bytes is converted to a single byte of output. If the length of the input string is not divisible by 8, the remainder is packed as if padded by 0's. Similarly, during unpacking any extra bits are ignored. If the input string is longer than needed, extra bytes are ignored. A * for the count means to use all bytes from the input field. On unpacking, the bits are converted to a string of 0s and 1s.

The h and H formats pack a string of count nybbles (4-bit groups often represented as hexadecimal digits).

The p format packs a pointer to a null-terminated string. You are responsible for ensuring the string is not a temporary value (which can potentially get deallocated before you get around to using the packed result). The P format packs a pointer to a structure of the size indicated by count. A null pointer is created if the corresponding value for p or P is undef.

The / character allows packing and unpacking of strings where the packed structure contains a byte count followed by the string itself. You write length-item/string-item. The length-item can be any pack template letter, and describes how the length value is packed. The ones likely to be of most use are integer-packing ones like n (for Java strings), w (for ASN.1 or SNMP) and N (for Sun XDR). The string-item must, at present, be A*, a*, or Z*. For unpack, the length of the string is obtained from the length-item, but if you put in the *, it will be ignored.

unpack 'C/a', "\04Gurusamy";        # gives 'Guru'
unpack 'a3/A* A*', '007 Bond  J ';  # gives (' Bond','J')
pack 'n/a* w/a*','hello,','world';  # gives "\000\006hello,\005world"

The integer formats s, S, l, and L may be immediately followed by a ! to signify native shorts or longs instead of exactly 16 or 32 bits respectively. Today, this is an issue mainly in 64-bit platforms, where the native shorts and longs as seen by the local C compiler can be different than these values. (i! and I! also work but only because of completeness; they are identical to i and I.)

The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available via the Config module:

use Config;
print $Config{shortsize},    "\n";
print $Config{intsize},      "\n";
print $Config{longsize},     "\n";
print $Config{longlongsize}, "\n";

Integer formats of greater than one byte in length (s, S, i, I, l, and L) are inherently nonportable between processors because they obey the native byte order and endianness. If you want portable packed integers, use the formats n, N, v, and V; their byte endianness and size are known.

Floating-point numbers are in the native machine format only. Because of the variety of floating formats and lack of a standard "network" representation, no facility for interchange has been made. This means that packed floating-point data written on one machine may not be readable on another. This is a problem even when both machines use IEEE floating-point arithmetic, because the endian-ness of the memory representation is not part of the IEEE spec.

Perl uses doubles internally for all floating-point calculation, so converting from double into float, then back again to double will lose precision. This means that unpack("f", pack("f", $foo)) will not generally equal $foo.

You are responsible for any alignment or padding considerations expected by other programs, particularly those programs that were created by a C compiler with its own idiosyncratic notions of how to lay out a C struct on the particular architecture in question. You'll have to add enough x's while packing to make up for this. For example, a C declaration of:

struct foo {
        unsigned char c;
        float f;
};

Let's look at some examples. This first pair packs numeric values into bytes:

$out = pack "CCCC", 65, 66, 67, 68;      # $out eq "ABCD"
$out = pack "C4", 65, 66, 67, 68;        # same thing

$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);

$out = pack "CCxxCC", 65, 66, 67, 68;    # $out eq "AB\0\0CD"

$out = pack "s2", 1, 2;    # "\1\0\2\0" on little-endian
                           # "\0\1\0\2" on big-endian

$out = pack "B32", "01010000011001010111001001101100";
$out = pack "H8", "5065726c";    # both produce "Perl"

$out = pack "a4", "abcd", "x", "y", "z";      # "abcd"

$out = pack "aaaa",  "abcd", "x", "y", "z";   # "axyz"
$out = pack "a" x 4, "abcd", "x", "y", "z";   # "axyz"

$out = pack "a14", "abcdefg";   # "abcdefg\0\0\0\0\0\0\0"

$out = pack "i9pl", gmtime(), $tz, $toff;

If you want to join fixed-width text fields together, use pack with a TEMPLATE of several A or a formats:

$string = pack("A10" x 10, @data);

$string = join(" and ", @data);
$string = join("", @data);             # null separator

29.2.109. package    

package NAMESPACE
package

A package declaration affects only global variables--including those on which you've used local--not lexical variables created with my. It only affects unqualified global variables; global variables that are qualified with a package name of their own ignore the current declared package. Global variables declared with our are unqualified and therefore respect the current package, but only at the point of declaration, after which they behave like my variables. That is, for the rest of their lexical scope, our variables are "nailed" to the package in use at the point of declaration, even if a subsequent package declaration intervenes.

Typically, you would put a package declaration as the first thing in a file that is to be included by the require or use operator, but you can put one anywhere a statement would be legal. When creating a traditional or objected-oriented module file, it is customary to name the package the same name as the file to avoid confusion. (It's also customary to name such packages beginning with a capital letter because lowercase modules are by convention interpreted as pragmatic modules.)

You can switch into a given package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. (If the compiler sees another package declaration at the same level, the new declaration overrides the previous one.) Your main program is assumed to start with an invisible package main declaration.

You can refer to variables, subroutines, handles, and formats in other packages by qualifying the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package is assumed. That is, $::sail is equivalent to $main::sail, as well as to $main'sail, which is still occasionally seen in older code.

Here's an example:

package main;       $sail = "hale and hearty";
package Mizzen;     $sail = "tattered";
package Whatever;
print "My main sail is $main::sail.\n";
print "My mizzen sail is $Mizzen::sail.\n";

My main sail is hale and hearty.
My mizzen sail is tattered.

If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully qualified or declared as lexicals. This is stricter than use strict since it also extends to function names.

See Chapter 10, "Packages", for more information about packages. See my earlier in this chapter for other scoping issues.

29.2.110. pipe    

pipe READHANDLE, WRITEHANDLE

(As with open, if either filehandle is undefined, it will be autovivfied.)

Here's a small example:

pipe(README, WRITEME);
unless ($pid = fork) {  #               child
    defined $pid or die "can't fork: $!";
    close(README);
    for $i (1..5) { print WRITEME "line $i\n" }
    exit;
}
$SIG{CHLD} = sub { waitpid($pid, 0) };
close(WRITEME);
@strings = <README>;
close(README);
print "Got:\n", @strings;

29.2.111. pop    

pop ARRAY
pop

$tmp = $ARRAY[$#ARRAY--];

$tmp = splice @ARRAY, -1;

The pop requires its first argument to be an array, not a list. If you just want the last element of a list, use this:

( LIST )[-1]

29.2.112. pos    

pos SCALAR
pos

$graffito = "fee fie foe foo";
while ($graffito =~ m/e/g) {
    print pos $graffito, "\n";
}

$graffito = "fee fie foe foo";
pos $graffito = 4;  # Skip the fee, start at fie
while ($graffito =~ m/e/g) {
        print pos $graffito, "\n";
}

29.2.113. print    

print FILEHANDLE LIST
print LIST
print

print { $OK ? "STDOUT" : "STDERR" } "stuff\n";
print { $iohandle[$i] } "stuff\n";

print $a - 2;   # prints $a - 2 to default filehandle (usually STDOUT)
print $a (- 2); # prints -2 to filehandle specified in $a
print $a -2;    # also prints -2 (weird parsing rules :-)

print OUT <STDIN>;

print OUT scalar <STDIN>;

print (1+2)*3, "\n";            # WRONG
print +(1+2)*3, "\n";           # ok
print ((1+2)*3, "\n");          # ok

[9] Thus, STDOUT isn't really the default filehandle for print. It's merely the default default filehandle.

29.2.114. printf    

printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST

print FILEHANDLE sprintf FORMAT, LIST

An exception is raised only if an invalid reference type is used as the FILEHANDLE argument. Unrecognized formats are passed through intact. Both situations trigger warnings if they're enabled.

See the print and sprintf functions elsewhere in this chapter. The description of sprintf includes the list of format specifications. We'd duplicate them here, but this book is already an ecological disaster.

If you omit both the FORMAT and the LIST, $_ is used--but in that case, you should have been using print. Don't fall into the trap of using a printf when a simple print would do. The print function is more efficient and less error prone.

29.2.115. prototype    

prototype FUNCTION

If FUNCTION is a string starting with CORE::, the rest is taken as a name for Perl built-in, and an exception is raised if there is no such built-in. If the built-in is not overridable (such as qw//) or its arguments cannot be expressed by a prototype (such as system), the function returns undef because the built-in does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned.

29.2.116. push    

push ARRAY, LIST

foreach $value (listfunc()) {
    $array[++$#array] = $value;
}

splice @array, @array, 0, listfunc();

for (;;) {
    push @array, shift @array;
    ...
}

29.2.117. q/STRING/    

q/STRING/
qq/STRING/
qr/STRING/
qw/STRING/
qx/STRING/

29.2.118. quotemeta    

quotemeta EXPR
quotemeta

29.2.119. rand    

rand EXPR
rand

To get an integral value, such as for a die roll, combine this with int, as in:

$roll = int(rand 6) + 1;       # $roll now a number between 1 and 6

[10] Knuth, D.E. The Art of Computer Programming, Seminumerical Algorithms, vol. 2, 3d ed. (Reading, Mass.: Addison-Wesley, 1997). ISBN 0-201-89684-2.

29.2.120. read     [taintgray]

read FILEHANDLE, SCALAR, LENGTH, OFFSET
read FILEHANDLE, SCALAR, LENGTH

To copy data from filehandle FROM into filehandle TO, you could say:

while (read(FROM, $buf, 16384)) {
    print TO $buf;
}

Perl's read function is implemented in terms of standard I/O's fread(3) function, so the actual read(2) syscall may read more than LENGTH bytes to fill the input buffer, and fread(3) may do more than one read(2) syscall in order to fill the buffer. To gain greater control, specify the real syscall using sysread. Calls to read and sysread should not be intermixed unless you are into heavy wizardry (or pain). Whichever one you use, be aware that when reading from a file containing Unicode or any other multibyte encoding, the buffer boundary may fall in the middle of a character.

29.2.121. readdir    

readdir DIRHANDLE

opendir(THISDIR, ".") or die "serious dainbramage: $!";
@allfiles = readdir THISDIR;
closedir THISDIR;
print "@allfiles\n";

@allfiles = grep { $_ ne '.' and $_ ne '..' } readdir THISDIR;
@allfiles = grep { not /^[.][.]?\z/ } readdir THISDIR;
@allfiles = grep { not /^\.{1,2}\z/ } readdir THISDIR;
@allfiles = grep !/^\.\.?\z/, readdir THISDIR;

@allfiles = grep !/^\./, readdir THISDIR;

@textfiles = grep -T, readdir THISDIR;

opendir(THATDIR, $path) or die "can't opendir $path: $!";
@dotfiles = grep { /^\./ && -f } map { "$path/$_" } readdir(THATDIR);
closedir THATDIR;

29.2.122. readline     [taintgray]

readline FILEHANDLE

When slurping files in scalar context, if you happen to slurp an empty file, readline returns "" the first time, and undef each subsequent time. When slurping from magical ARGV filehandle, each file returns one chunk (again, null files return as ""), followed by a single undef when the files are exhausted.

The <FILEHANDLE> operator is discussed in more detail in the section "Input Operators" in Chapter 2, "Bits and Pieces".

$line = <STDIN>;
$line = readline(STDIN);          # same thing
$line = readline(*STDIN);         # same thing
$line = readline(\*STDIN);        # same thing

open my $fh, "<&=STDIN" or die;
bless $fh => 'AnyOldClass';
$line = readline($fh);            # same thing

29.2.123. readlink    

readlink EXPR
readlink

Be aware that the returned symlink may be relative to the location you specified. For instance, you may say:

readlink "/usr/local/src/express/yourself.h"

../express.1.23/includes/yourself.h

29.2.124. readpipe    

readpipe scalar EXPR
readpipe LIST (proposed)

29.2.125. recv     [taintgray]

recv SOCKET, SCALAR, LEN, FLAGS

29.2.126. redo    

redo LABEL
redo

# A loop that joins lines continued with a backslash.
while (<STDIN>) {
    if (s/\\\n$// && defined($nextline = <STDIN>)) {
        $_ .= $nextline;
        redo;
    }
    print;  # or whatever...
}

A block by itself is semantically identical to a loop that executes once. Thus redo inside such a block will effectively turn it into a looping construct. See the section "Loop Control" in Chapter 4, "Statements and Declarations".

29.2.127. ref    

ref EXPR
ref

SCALAR
ARRAY
HASH
CODE
GLOB
REF
LVALUE
IO::Handle

if (ref($r) eq "HASH") {
    print "r is a reference to a hash.\n";
}
elsif (ref($r) eq "Hump") {    # Naughty--see below.
    print "r is a reference to a Hump object.\n";
}
elsif (not ref $r) {
    print "r is not a reference at all.\n";
}

if ($r->isa("Hump") }
    print "r is a reference to a Hump object, or subclass.\n";
}

29.2.128. rename    

rename OLDNAME, NEWNAME

See the standard File::Copy module for cross-filesystem renames.

29.2.129. require    

require VERSION
require EXPR
require

If the argument is a string, require loads and executes the Perl code found in the separate file whose name is given by the string. This is similar to performing a do on a file, except that require checks to see whether the library file has been loaded already and raises an exception if any difficulties are encountered. (It can thus be used to express file dependencies without worrying about duplicate compilation.) Like its cousins do and use, require knows how to search the include path stored in the @INC array and to update %INC upon success. See Chapter 28, "Special Names".

The file must return true as the last value to indicate successful execution of any initialization code, so it's customary to end such a file with 1; unless you're sure it'll return true otherwise.

If require's argument is a version number of the form 5.6.2, require demands that the currently executing version of Perl be at least that version. (Perl also accepts a floating point number such as 5.005_03 for compatibility with older versions of Perl, but that form is now discouraged because folks from other cultures don't understand it.) Thus, a script that requires Perl version 5.6 can put as its first line:

require 5.6.0;             # or require v5.6.0

If require's argument is a bare package name (see package), require assumes an automatic .pm suffix, making it easy to load standard modules. This behavior is like use, except that it happens at run time rather than compile time, and the import method is not called. For example, to pull in Socket.pm without introducing any symbols into the current package, say this:

require Socket;            # instead of "use Socket;"

use Socket ();

require Foo::Bar;          # a splendid bare name

$class = 'Foo::Bar';
require $class;            # $class is not a bare name

require "Foo::Bar";        # quoted literal not a bare name

eval "require $class";

29.2.130. reset    

reset EXPR
reset

To reset all "X" variables, say this:

reset 'X';

reset 'a-z';

reset;

Lexical variables (created by my) are not affected. Use of reset is vaguely deprecated because it easily clears out entire namespaces and because the ?? operator is itself vaguely deprecated.

See also the delete_package() function from the standard Symbol module, and the whole issue of Safe compartments documented in the section "Safe Compartments" in Chapter 23, "Security".

29.2.131. return    

return EXPR
return

EXPR may be evaluated in list, scalar, or void context, depending on how the return value will be used, which may vary from one execution to the next. That is, the supplied expression will be evaluated in the context of the subroutine invocation. If the subroutine was called in a scalar context, EXPR is also evaluated in scalar context. If the subroutine was invoked in list context, then EXPR is also evaluated in list context and can return a list value. A return with no argument returns the scalar value undef in scalar context, an empty list () in list context, and (naturally) nothing at all in void context. The context of the subroutine call can be determined from within the subroutine by using the (misnamed) wantarray function.

29.2.132. reverse    

reverse LIST

for (reverse 1 .. 10) { ... }

%barfoo = reverse %foobar;

A small hint: reversing a list sorted earlier by a user-defined function can often be achieved more easily by sorting the list in the opposite direction in the first place.

29.2.133. rewinddir    

rewinddir DIRHANDLE

29.2.134. rindex    

rindex STR, SUBSTR, POSITION
rindex STR, SUBSTR

$pos = length $string;
while (($pos = rindex $string, $lookfor, $pos) >= 0) {
    print "Found at $pos\n";
    $pos--;
}

29.2.135. rmdir    

rmdir FILENAME
rmdir

29.2.136. s///     [taintgray]

s///

29.2.137. scalar    

scalar EXPR

my ($nextvar) = scalar <STDIN>;

Of course, a simpler, less-cluttered way would be to just leave the parentheses off, thereby changing the list context to a scalar one:

my $nextvar = <STDIN>;

print "Length is ", scalar(@ARRAY), "\n";

There's no "list" function corresponding to scalar since, in practice, one never needs to force evaluation in a list context. That's because any operation that wants LIST already provides a list context to its list arguments for free.

Because scalar is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. The following single statement:

print uc(scalar(&foo,$bar)),$baz;

&foo;
print(uc($bar),$baz);

29.2.138. seek    

seek FILEHANDLE, OFFSET, WHENCE

FILEHANDLE can be an expression whose value gives either the name of the actual filehandle or a reference to anything resembling a filehandle object. The function returns true upon success, false otherwise. For handiness, the function can calculate offsets from various file positions for you. The value of WHENCE specifies which file position your OFFSET uses for its starting point: 0, the beginning of the file; 1, the current position in the file; or 2, the end of the file. The OFFSET can be negative for a WHENCE of 1 or 2. If you'd like to use symbolic values for WHENCE, you may use SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or the POSIX module, or as of the 5.6 release of Perl, the Fcntl module.

If you want to position the file for sysread or syswrite, don't use seek; standard I/O buffering makes its effect on the file's system position unpredictable and nonportable. Use sysseek instead.

Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling the standard I/O library's clearerr(3) function. A WHENCE of 1 (SEEK_CUR) with an OFFSET 0 is useful for not moving the file position:

seek(TEST,0,1);

for (;;) {
    while (<LOG>) {
        grok($_);          # Process current line.
    }
    sleep 15;
    seek LOG,0,1;          # Reset end-of-file error.
}

for (;;) {
    for ($curpos = tell FILE; <FILE>; $curpos = tell FILE) {
        grok($_);          # Process current line.
    }
    sleep $for_a_while;
    seek FILE, $curpos, 0; # Reset end-of-file error.
}

29.2.139. seekdir    

seekdir DIRHANDLE, POS

29.2.140. select (output filehandle)    

select FILEHANDLE
select

select REPORT1;
$^ = 'MyTop';
select REPORT2;
$^ = 'MyTop';

my $oldfh = select STDERR; $| = 1; select $oldfh;

select((select(STDERR), $| = 1)[0])

You can also use the standard SelectSaver module to automatically restore the previous select upon scope exit.

However, now that we've explained all that, we should point out that you rarely need to use this form of select nowadays, because most of the special variables you would want to set have object-oriented wrapper methods to do it for you. So instead of setting $| directly, you might say:

use IO::Handle;             # Unfortunately, this is *not* a small module.
STDOUT->autoflush(1);

use IO::Handle;
REPORT1->format_top_name("MyTop");
REPORT2->format_top_name("MyTop");

29.2.141. select (ready file descriptors)    

select RBITS, WBITS, EBITS, TIMEOUT

$rin = $win = $ein = "";
vec($rin, fileno(STDIN), 1)  = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;

sub fhbits {
    my @fhlist = @_;
    my $bits;
    for (@fhlist) {
        vec($bits, fileno($_), 1) = 1;
    }
    return $bits;
}
$rin = fhbits(qw(STDIN TTY MYSOCK));

($nfound, $timeleft) =
    select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

The $wout=$win trick works because the value of an assignment is its left side, so $wout gets clobbered first by the assignment and then by the select, while $win remains unchanged.

Any of the arguments can also be undef, in which case they're ignored. The TIMEOUT, if not undef, is in seconds, which may be fractional. (A timeout of 0 effects a poll.) Not many implementations are capable of returning $timeleft. If not, they always return $timeleft equal to the supplied $timeout.

The standard IO::Select module provides a more user-friendly interface to select, mostly because it does all the bit mask work for you.

One use for select is to sleep with a finer resolution than sleep allows. To do this, specify undef for all the bitmasks. So, to sleep for (at least) 4.75 seconds, use:

select undef, undef, undef, 4.75;

One should probably not attempt to mix buffered I/O (like read or <HANDLE>) with select, except as permitted by POSIX, and even then only on truly POSIX systems. Use sysread instead.

29.2.142. semctl    

semctl ID, SEMNUM, CMD, ARG

See also the IPC::Semaphore module. This function is available only on machines supporting System V IPC.

29.2.143. semget    

semget KEY, NSEMS, SIZE, FLAGS

See also the IPC::Semaphore module. This function is available only on machines supporting System V IPC.

29.2.144. semop    

semop KEY, OPSTRING

OPSTRING must be a packed array of semop structures. You can make each semop structure by saying pack("s*", $semnum, $semop, $semflag). The number of semaphore operations is implied by the length of OPSTRING. The function returns true if successful, or false if there is an error.

The following code waits on semaphore $semnum of semaphore id $semid:

$semop = pack "s*", $semnum, -1, 0;
semop $semid, $semop or die "Semaphore trouble: $!\n";

See the section "System V IPC" in Chapter 16, "Interprocess Communication". See also the IPC::Semaphore module. This function is available only on machines supporting System V IPC.

29.2.145. send    

send SOCKET, MSG, FLAGS, TO
send SOCKET, MSG, FLAGS

(Some non-Unix systems improperly treat sockets as different from ordinary file descriptors, with the result that you must always use send and recv on sockets rather than the handier standard I/O operators.)

One error that at least one of us makes frequently is to confuse Perl's send with C's send and write:

send SOCK, $buffer, length $buffer      # WRONG

29.2.146. setpgrp    

setpgrp PID, PGRP

29.2.147. setpriority    

setpriority WHICH, WHO, PRIORITY

setpriority 0, 0, getpriority(0, 0) + 4;

See also the BSD::Resource module from CPAN.

29.2.148. setsockopt    

setsockopt SOCKET, LEVEL, OPTNAME, OPTVAL

use Socket;
socket(SOCK, ...) or die "Can't make socket: $!\n";
setsockopt(SOCK, SOL_SOCKET, SO_REUSEADDR, 1)
        or warn "Can't do setsockopt: $!\n";

See setsockopt(2) for other possible values.

29.2.149. shift    

shift ARRAY
shift

If ARRAY is omitted, the function shifts @_ within the lexical scope of subroutines and formats; it shifts @ARGV at file scopes (typically the main program) or within the lexical scopes established by the evalSTRING, BEGIN {}, CHECK {}, INIT {}, and END {} constructs.

Subroutines often start by copying their arguments into lexical variables, and shift can be used for this:

sub marine {
        my $fathoms = shift;  # depth
        my $fishies = shift;  # number of fish
        my $o2      = shift;  # oxygen concentration
        # ...
}

while (defined($_ = shift)) {
        /^[^-]/     && do { unshift @ARGV, $_; last };
        /^-w/       && do { $WARN = 1;         next };
        /^-r/       && do { $RECURSE = 1;      next };
        die "Unknown argument $_\n";
}

See also unshift, push, pop, and splice. The shift and unshift functions do the same thing to the left end of an array that pop and push do to the right end.

29.2.150. shmctl    

shmctl ID, CMD, ARG

If CMD is IPC_STAT, then ARG must be a variable that will hold the returned shmid_ds structure. Like ioctl and fcntl, the function returns undef for error, "0 but true" for zero, and the actual return value otherwise.

This function is available only on machines supporting System V IPC.

29.2.151. shmget    

shmget KEY, SIZE, FLAGS

This function is available only on machines supporting System V IPC.

29.2.152. shmread    

shmread ID, VAR, POS, SIZE

This function is available only on machines supporting System V IPC.

29.2.153. shmwrite    

shmwrite ID, STRING, POS, SIZE

This function is available only on machines supporting System V IPC. (You're probably tired of reading that--we're getting tired of saying it.)

29.2.154. shutdown    

shutdown SOCKET, HOW

shutdown(SOCK, 0);    # no more reading
shutdown(SOCK, 1);    # no more writing
shutdown(SOCK, 2);    # no more I/O at all

Imagine a server that wants to read its client's request until end of file, then send an answer. If the client calls close, that socket is now invalid for I/O, so no answer would ever come back. Instead, the client should use shutdown to half-close the connection:

print SERVER "my request\n";        # send some data
shutdown(SERVER, 1);                # send eof; no more writing
$answer = <SERVER>;                 # but you can still read

29.2.155. sin    

sin EXPR
sin

For the inverse sine operation, you may use Math::Trig or the POSIX module's asin function, or use this relation:

sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

29.2.156. sleep    

sleep EXPR
sleep

See also the POSIX module's sigpause function.

29.2.157. socket    

socket SOCKET, DOMAIN, TYPE, PROTOCOL

use Socket;

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See the $^F ($SYSTEM_FD_MAX) variable in Chapter 28, "Special Names".

29.2.158. socketpair    

socketpair SOCKET1, SOCKET2, DOMAIN, TYPE, PROTOCOL

This function is typically used just before a fork. One of the resulting processes should close SOCKET1, and the other should close SOCKET2. You can use these sockets bidirectionally, unlike the filehandles created by the pipe function. Some systems define pipe in terms of socketpair, in which a call to pipe(Rdr, Wtr) is essentially:

use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1);        # no more writing for reader
shutdown(Wtr, 0);        # no more reading for writer

29.2.159. sort    

sort USERSUB LIST
sort BLOCK LIST
sort LIST

USERSUB, if given, is the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the list are to be ordered. (The handy <=> and cmp operators can be used to perform three-way numeric and string comparisons.) If a USERSUB is given but that function is undefined, sort raises an exception.

In the interests of efficiency, the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine (nor may you exit the block or routine with a loop control operator), and the two elements to be compared are not passed into the subroutine via @_, but rather by temporarily setting the global variables $a and $b in the package in which the sort was compiled (see the examples that follow). The variables $a and $b are aliases to the real values, so don't modify them in the subroutine.

The comparison subroutine is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example), the results are not well defined. (That's another reason you shouldn't modify $a and $b.)

USERSUB may be a scalar variable name (unsubscripted), in which case the value provides either a symbolic or a hard reference to the actual subroutine to use. (A symbolic name rather than a hard reference is allowed even when the use strict 'refs' pragma is in effect.) In place of a USERSUB, you can provide a BLOCK as an anonymous, inline sort subroutine.

To do an ordinary numeric sort, say this:

sub numerically { $a <=> $b }
@sortedbynumber = sort numerically 53,29,11,32,7;

@descending = reverse sort numerically 53,29,11,32,7;

sub reverse_numerically { $b <=> $a }
@descending = sort reverse_numerically 53,29,11,32,7;

@unsorted = qw/sparrow Ostrich LARK catbird blueJAY/;
@sorted = sort { lc($a) cmp lc($b) } @unsorted;

Sorting hashes by value is a common use of the sort function. For example, if a %sales_amount hash records department sales, doing a hash lookup in the sort routine allows the hash keys to be sorted according to their corresponding values:

# sort from highest to lowest department sales
sub bysales { $sales_amount{$b} <=> $sales_amount{$a} }

for $dept (sort bysales keys %sale_amount) {
    print "$dept => $sales_amount{$dept}\n";
}

sub by_sales_then_dept {
    $sales_amount{$b} <=> $sales_amount{$a}
       ||
    $a cmp $b
}

for $dept (sort by_sales_then_dept keys %sale_amount) {
    print "$dept => $sales_amount{$dept}\n";
}

sub prospects {
    $b->{SALARY}    <=>  $a->{SALARY}
        ||
    $b->{HEIGHT}    <=>  $a->{HEIGHT}
        ||
    $a->{AGE}       <=>  $b->{AGE}
        ||
    $a->{LASTNAME}  cmp  $b->{LASTNAME}
        ||
    $a->{FIRSTNAME} cmp  $b->{FIRSTNAME}
}

@sorted = sort prospects @recs;

@sorted_lines = sort {
    @a_fields = split /:/, $a;     # colon-separated fields
    @b_fields = split /:/, $b;

    $a_fields[3] <=> $b_fields[3]  # numeric sort on 4th field, then
        ||
    $a_fields[0] cmp $b_fields[0]  # string sort on 1st field, then
        ||
    $b_fields[2] <=> $a_fields[2]  # reverse numeric sort on 3rd field
        ||
    ...                            # etc.

} @lines;

To avoid the expense of repeated derivations such as the splitting of lines in order to compare their fields, perform the derivation once per value prior to the sort and save the derived information. Here, anonymous arrays are created to encapsulate each line along with the results of splitting the line:

@temp = map { [$_, split /:/] } @lines;

@temp = sort {
    @a_fields = @$a[1..$#$a];
    @b_fields = @$b[1..$#$b];

    $a_fields[3] <=> $b_fields[3]  # numeric sort on 4th field, then
        ||
    $a_fields[0] cmp $b_fields[0]  # string sort on 1st field, then
        ||
    $b_fields[2] <=> $a_fields[2]  # reverse numeric sort on 3rd field
        ||
    ...                            # etc.

} @temp;

@sorted_lines = map { $_->[0] } @temp;

@sorted_lines = map { $_->[0] }
                sort {
                    @a_fields = @$a[1..$#$a];
                    @b_fields = @$b[1..$#$b];

                    $a_fields[3] <=> $b_fields[3]
                        ||
                    $a_fields[0] cmp $b_fields[0]
                        ||
                    $b_fields[2] <=> $a_fields[2]
                        ||
                    ...
                }
                map { [$_, split /:/] } @lines;

That being said, in version 5.6 you can write sort subroutines with the standard argument passing method (and, not coincidentally, use XS subroutines as sort subroutines), provided that you declare the sort subroutine with a prototype of ($$). And if you do that, then you can in fact declare $a and $b as lexicals:

sub numerically ($$) {
        my ($a, $b) = @_;
        $a <=> $b;
}

sub numerically ($a, $b) { $a <=> $b }

29.2.160. splice    

splice ARRAY, OFFSET, LENGTH, LIST
splice ARRAY, OFFSET, LENGTH
splice ARRAY, OFFSET
splice ARRAY

The splice function is also handy for carving up the argument list passed to a subroutine. For example, assuming list lengths are passed before lists:

sub list_eq {       # compare two list values
    my @a = splice(@_, 0, shift);
    my @b = splice(@_, 0, shift);
    return 0 unless @a == @b;       # same length?
    while (@a) {
        return 0 if pop(@a) ne pop(@b);
    }
    return 1;
}
if (list_eq($len, @foo[1..$len], scalar(@bar), @bar)) { ... }

29.2.161. split     [taintgray]

split /PATTERN/, EXPR, LIMIT
split /PATTERN/, EXPR
split /PATTERN/
split

[11]Scalar context also causes split to write its result to @_, but this usage is deprecated.

If LIMIT is specified and positive, the function splits into no more than that many fields (though it may split into fewer if it runs out of separators). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT has been specified. If LIMIT is omitted or zero, trailing null fields are stripped from the result (which potential users of pop would do well to remember). If EXPR is omitted, the function splits the $_ string. If PATTERN is also omitted or is the literal space, " ", the function splits on whitespace, /\s+/, after skipping any leading whitespace.

Strings of any length can be split:

@chars  = split //,   $word;
@fields = split /:/,  $line;
@words  = split " ",  $paragraph;
@lines  = split /^/,  $buffer;

print join ':', split / */, 'hi there';

The LIMIT parameter splits only part of a string:

($login, $passwd, $remainder) = split /:/, $_, 3;

We said earlier that the separators are not returned, but if the PATTERN contains parentheses, then the substring matched by each pair of parentheses is included in the resulting list, interspersed with the fields that are ordinarily returned. Here's a simple example:

split /([-,])/, "1-10,20";

(1, '-', 10, ',', 20)

split /(-)|(,)/, "1-10,20";

(1, '-', undef, 10, undef, ',', 20)

As a special case, if the expression is a single space (" "), the function splits on whitespace just as split with no arguments does. Thus, split(" ") can be used to emulate awk's default behavior. In contrast, split(/ /) will give you as many null initial fields as there are leading spaces. (Other than this special case, if you supply a string instead of a regular expression, it'll be interpreted as a regular expression anyway.) You can use this property to remove leading and trailing whitespace from a string and to collapse intervening stretches of whitespace into a single space:

$string = join(' ', split(' ', $string));

$header =~ s/\n\s+/ /g;      # Merge continuation lines.
%head = ('FRONTSTUFF', split /^(\S*?):\s*/m, $header);

open PASSWD, '/etc/passwd';
while (<PASSWD>) {
    chomp;        # remove trailing newline
    ($login, $passwd, $uid, $gid, $gcos, $home, $shell) =
            split /:/;
    ...
}

while (<>) {
    foreach $word (split) {
        $count{$word}++;
    }
}

29.2.162. sprintf    

sprintf FORMAT, LIST

Perl does its own sprintf formatting--it emulates the C function sprintf, but it doesn't use it.[12] As a result, any nonstandard extensions in your local sprintf(3) function are not available from Perl.

[12]Except for floating-point numbers, and even then only the standard modifiers are allowed.

Perl's sprintf permits the universally known conversions shown in Table 29-4.

Table 29.4. Formats for sprintf

In addition, Perl permits the following widely supported conversions:

Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary but widely supported conversions:

Perl permits the following universally known flags between the % and the conversion character:

There are also two Perl-specific flags:

If your Perl understands "quads" (64-bit integers) either because the platform natively supports them or because Perl has been specifically compiled with that ability, then the characters d u o x X b i D U O print quads, and they may optionally be preceded by ll, L, or q. For example, %lld %16LX %qo.

If Perl understands "long doubles" (this requires that the platform support long doubles), the flags e f g E F G may optionally be preceded by ll or L. For example, %llf %Lg.

Where a number would appear in the flags, an asterisk ("*") may be used instead, in which case Perl uses the next item in the argument list as the given number (that is, as the field width or precision). If a field width obtained through "*" is negative, it has the same effect as the "-" flag: left-justification.

The v flag is useful for displaying ordinal values of characters in arbitrary strings:

sprintf "version is v%vd\n", $^V;           # Perl's version
sprintf "address is %vd\n", $addr;          # IPv4 address
sprintf "address is %*vX\n", ":", $addr;    # IPv6 address
sprintf "bits are %*vb\n", " ", $bits;      # random bit strings

29.2.163. sqrt    

sqrt EXPR
sqrt

use Math::Complex;
print sqrt(-2);    # prints 1.4142135623731i

29.2.164. srand    

srand EXPR
srand

Frequently called programs (like CGI scripts) that simply use time ^ $$ for a seed can fall prey to the mathematical property that a^b == (a+1)^(b+1) one-third of the time. So don't do that. Use this instead:

srand( time() ^ ($$ + ($$ << 15)) );

srand (time ^ $$ ^ unpack "%32L*", `ps wwaxl | gzip`);

Do not call srand multiple times in your program unless you know exactly what you're doing and why you're doing it. The point of the function is to "seed" the rand function so that rand can produce a different sequence each time you run your program. Just do it once at the top of your program, or you won't get random numbers out of rand!

29.2.165. stat    

stat FILEHANDLE
stat EXPR
stat

($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
    $atime,$mtime,$ctime,$blksize,$blocks)
            = stat $filename;

Table 29.5. Fields Returned by stat

$dev and $ino, taken together, uniquely identify a file on the same system. The $blksize and $blocks are likely defined only on BSD-derived filesystems. The $blocks field (if defined) is reported in 512-byte blocks. The value of $blocks*512 can differ greatly from $size for files containing unallocated blocks, or "holes", which aren't counted in $blocks.

If stat is passed the special filehandle consisting of an underline, no actual stat(2) is done, but the current contents of the stat structure from the last stat, lstat, or stat-based file test operator (such as -r, -w, and -x) are returned.

Because the mode contains both the file type and its permissions, you should mask off the file type portion and printf or sprintf using a "%o" if you want to see the real permissions:

$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;

use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
   $filename, $sb->size, $sb->mode & 07777,
   scalar localtime $sb->mtime;

Hint: if you need only the size of the file, check out the -s file test operator, which returns the size in bytes directly. There are also file tests that return the ages of files in days.

29.2.166. study    

study SCALAR
study

The way study works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the "k" characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this rarest character are examined.

For example, here is a loop that inserts index-producing entries before any line containing a certain pattern:

while (<>) {
    study;
    print ".IX foo\n"      if /\bfoo\b/;
    print ".IX bar\n"      if /\bbar\b/;
    print ".IX blurfl\n"   if /\bblurfl\b/;
    ...
    print;
}

If you have to look for strings that you don't know until run time, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with setting $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a case-insensitive match:

$search = 'while (<>) { study;';
foreach $word (@words) {
    $search .= "++\$seen{\$ARGV} if /\\b$word\\b/i;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;               # slurp each entire file
eval $search;           # this screams
die $@ if $@;           # in case eval failed
$/ = "\n";              # restore normal input terminator
foreach $file (sort keys(%seen)) {
    print "$file\n";
}

@pats = ();
foreach $word (@words) {
    push @pats, qr/\b${word}\b/i;
}
@ARGV = @files;
undef $/;               # slurp each entire file
while (<>) {
    for $pat (@pats) {
        $seen{$ARGV}++ if /$pat/;
    }
}
$/ = "\n";              # restore normal input terminator
foreach $file (sort keys(%seen)) {
    print "$file\n";
}

29.2.167. sub    

Named declarations:

sub NAME PROTO ATTRS
sub NAME ATTRS
sub NAME PROTO
sub NAME

sub NAME PROTO ATTRS BLOCK
sub NAME ATTRS BLOCK
sub NAME PROTO BLOCK
sub NAME BLOCK

sub PROTO ATTRS BLOCK
sub ATTRS BLOCK
sub PROTO BLOCK
sub BLOCK

sub NAME PROTO ATTRS BLOCK

• If you just have a NAME and no BLOCK, it's a declaration of that name (and if you ever want to call the subroutine, you'll have to supply a definition with both a NAME and a BLOCK later). Named declarations are useful because the parser treats a name specially if it knows it's a user-defined subroutine. You can call such a subroutine either as a function or as an operator, just like built-in functions. These are sometimes called forward declarations.

• If you have both a NAME and a BLOCK, it's a standard named subroutine definition (and a declaration too, if you didn't declare the name previously). Named definitions are useful because the BLOCK associates an actual meaning (the body of the subroutine) with the declaration. That's all we mean when we say it defines the subroutine rather than just declaring it. The definition is like the declaration, however, in that the surrounding code doesn't see it, and it returns no inline value by which you could reference the subroutine.

• If you have just have a BLOCK without a NAME, it's a nameless definition, that is, an anonymous subroutine. Since it doesn't have a name, it's not a declaration at all, but a real operator that returns a reference to the anonymous subroutine body at run time. This is extremely useful for treating code as data. It allows you to pass odd chunks of code around to be used as callbacks, and maybe even as closures if the sub definition operator refers to any lexical variables outside of itself. That means that different calls to the same sub operator will do the bookkeeping necessary to keep the correct "version" of each such lexical variable in sight for the life of the closure, even if the original scope of the lexical variable has been destroyed.

In any of these three cases, either one or both of the PROTO and ATTRS may occur after the NAME and/or before the BLOCK. A prototype is a list of characters in parentheses that tell the parser how to treat arguments to the function. Attributes are introduced by a colon and supply additional information to the parser about the function. Here's a typical definition that includes all four fields:

sub numstrcmp ($$) : locked {
    my ($a, $b) = @_;
    return $a <=> $b || $a cmp $b;
}

29.2.168. substr    

substr EXPR, OFFSET, LENGTH, REPLACEMENT
substr EXPR, OFFSET, LENGTH
substr EXPR, OFFSET

You may use substr as an lvalue (something to assign to), in which case EXPR must also be a legal lvalue. If you assign something shorter than the length of your substring, the string will shrink, and if you assign something longer than the length, the string will grow to accommodate it. To keep the string the same length, you may need to pad or chop your value using sprintf or the x operator. If you attempt to assign to an unallocated area past the end of the string, substr raises an exception.

To prepend the string "Larry" to the current value of $_, use:

substr($var, 0, 0) = "Larry";

substr($var, 0, 1) = "Moe";

substr($var, -1) = "Curly";

$oldstr = substr($var, -1, 1, "Curly");

substr($var, -10) =~ s/ /./g;

29.2.169. symlink    

symlink OLDNAME, NEWNAME

$can_symlink = eval { symlink("",""); 1 };

See also link and readlink earlier in this chapter.

29.2.170. syscall    

syscall LIST

The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as a C integer. If not, a pointer to the string value is passed. You are responsible for making sure the string is long enough to receive any result that might be written into it; otherwise, you're looking at a core dump. You can't use a string literal (or other read-only string) as an argument to syscall because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers.

syscall returns whatever value was returned by the system call invoked. By C coding conventions, if that system call fails, syscall returns -1 and sets $! (errno). Some system calls legitimately return -1 if successful. The proper way to handle such calls is to assign $!=0; before the call and check the value of $! if syscall returns -1.

Not all system calls can be accessed this way. For example, Perl supports passing up to 14 arguments to your system call, which in practice should usually suffice. However, there's a problem with syscalls that return multiple values. Consider syscall(&SYS_pipe): it returns the file number of the read end of the pipe it creates. There is no way to retrieve the file number of the other end. You can avoid this instance of the problem by using pipe instead. To solve the generic problem, write XSUBs (external subroutine modules, a dialect of C) to access the system calls directly. Then put your new module onto CPAN, and become wildly popular.

The following subroutine returns the current time as a floating-point number rather than as integer seconds as time returns. (It will only work on machines that support the gettimeofday(2) syscall.)

sub finetime() {
    package main;   # for next require
    require 'syscall.ph';
    # presize buffer to two 32-bit longs...
    my $tv = pack("LL", ());
    syscall(&SYS_gettimeofday, $tv, undef) >= 0
        or die "gettimeofday: $!";
    my($seconds, $microseconds) = unpack("LL", $tv);
    return $seconds + ($microseconds / 1_000_000);
}

require 'syscall.ph';
syscall(&SYS_setgroups, scalar @newgids, pack("i*", @newgids))
        or die "setgroups: $!";

[13]Although through $(, it does.

See the Time::HiRes module from CPAN for a more rigorous approach to fine-grained timing issues.

29.2.171. sysopen    

sysopen FILEHANDLE, FILENAME, MODE, MASK
sysopen FILEHANDLE, FILENAME, MODE

This function is a direct interface to your operating system's open(2) syscall followed by an fdopen(3) library call. As such, you'll need to pretend you're a C programmer for a bit here. The possible values and flag bits of the MODE parameter are available through the Fcntl module. Because different systems support different flags, don't count on all of them being available on your system. Consult your open(2) manpage or its local equivalent for details. Nevertheless, the following flags should be present on any system with a reasonably standard C library:

Many other options are possible, however. Here are some less common flags:

The O_EXCL flag is not for locking: here, exclusiveness means that if the file already exists, sysopen fails.

If the file named by FILENAME does not exist and the MODE includes the O_CREAT flag, then sysopen creates the file with initial permissions determined by the MASK argument (or 0666 if omitted) as modified by your process's current umask. This default is reasonable: see the entry on umask for an explanation.

Filehandles opened with open and sysopen may be used interchangeably. You do not need to use sysread and friends just because you happened to open the file with sysopen, nor are you precluded from doing so if you opened it with open. Both can do things that the other can't. Regular open can open pipes, fork processes, set disciplines, duplicate file handles, and convert a file descriptor number into a filehandle. It also ignores leading and trailing whitespace in filenames and respects "-" as a special filename. But when it comes to opening actual files, sysopen can do anything that open can.

The following examples show equivalent calls to both functions. We omit the or die $! checks for clarity, but make sure to always check return values in your programs. We'll restrict ourselves to using only flags available on virtually all operating systems. It's just a matter of controlling the values that you OR together using the bitwise | operator to pass in MODE argument.

• Open a file for reading:

  open(FH, "<", $path);
  sysopen(FH, $path, O_RDONLY);

• Open a file for writing, creating a new file if needed, or truncating an old file:

  open(FH, ">", $path);
  sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);

• Open a file for appending, creating one if necessary:

  open(FH, ">>", $path);
  sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);

• Open a file for update, where the file must already exist:

  open(FH, "+<", $path);
  sysopen(FH, $path, O_RDWR);

And here are things you can do with sysopen but not with regular open:

• Open and create a file for writing, which must not previously exist:

  sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);

• Open a file for appending, which must already exist:

  sysopen(FH, $path, O_WRONLY | O_APPEND);

• Open a file for update, creating a new file if necessary:

  sysopen(FH, $path, O_RDWR | O_CREAT);

• Open a file for update, which must not already exist:

  sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);

• Open a write-only file without blocking, but not creating it if it doesn't exist:

  sysopen(FH, $path, O_WRONLY | O_NONBLOCK);

The FileHandle module described in Chapter 32, "Standard Modules" provides a set of object-oriented synonyms (plus a small bit of new functionality) for opening files. You are welcome to call the appropriate FileHandle methods[14] on any handle created with open, sysopen, pipe, socket, or accept, even if you didn't use the module to initialize those handles.

[14]Really IO::File or IO::Handle methods.

29.2.172. sysread     [taintgray]

sysread FILEHANDLE, SCALAR, LENGTH, OFFSET
sysread FILEHANDLE, SCALAR, LENGTH

[15]There is no syseof function, which is okay, since eof doesn't work well on device files (like terminals) anyway. Use sysread and check for a return value for 0 to decide whether you're done.

You should be prepared to handle the problems (like interrupted syscalls) that standard I/O normally handles for you. Because it bypasses standard I/O, do not mix sysread with other kinds of reads, print, printf, write, seek, tell, or eof on the same filehandle unless you are into heavy wizardry (and/or pain). Also, please be aware that, when reading from a file containing Unicode or any other multibyte encoding, the buffer boundary may fall in the middle of a character.

29.2.173. sysseek    

sysseek FILEHANDLE, POSITION, WHENCE

Returns the new position, or undef on failure. A position of zero is returned as the special string "0 but true", which can be used numerically without producing warnings.

29.2.174. system    

system PATHNAME LIST
system LIST

Argument processing varies depending on the number of arguments, as described under exec, including determining whether the shell will be called and whether you've lied to the program about its name by specifying a separate PATHNAME.

Because system and backticks block SIGINT and SIGQUIT, sending one of those signals (such as from a Control-C) to the program being run doesn't interrupt your main program. But the other program you're running does get the signal. Check the return value from system to see whether the program you were running exited properly or not.

@args = ("command", "arg1", "arg2");
system(@args) == 0
     or die "system @args failed: $?"

$exit_value  = $? >> 8;
$signal_num  = $? & 127;    # or 0x7f, or 0177, or 0b0111_1111
$dumped_core = $? & 128;    # or 0x80, or 0200, or 0b1000_0000

[16]That's /bin/sh by definition, or whatever makes sense on your platform, but not whatever shell the user just happens to be using at the time.

29.2.175. syswrite    

syswrite FILEHANDLE, SCALAR, LENGTH, OFFSET
syswrite FILEHANDLE, SCALAR, LENGTH
syswrite FILEHANDLE, SCALAR

To copy data from filehandle FROM into filehandle TO, you can use something like:

use Errno qw/EINTR/;
$blksize = (stat FROM)[11] || 16384;  # preferred block size?
while ($len = sysread FROM, $buf, $blksize) {
    if (!defined $len) {
        next if $! == EINTR;
        die "System read error: $!\n";
    }
    $offset = 0;
    while ($len) {          # Handle partial writes.
        $written = syswrite TO, $buf, $len, $offset;
        die "System write error: $!\n" unless defined $written;
        $offset += $written;
        $len    -= $written;
    }
}

[17]Or pain.

29.2.176. tell    

tell FILEHANDLE
tell

There is no systell function. Use sysseek(FH, 0, 1) for that. Seek seek for an example telling how to use tell.

29.2.177. telldir    

telldir DIRHANDLE

29.2.178. tie    

tie VARIABLE, CLASSNAME, LIST

Any additional arguments are passed to the appropriate constructor method of the class, meaning one of TIESCALAR, TIEARRAY, TIEHASH, or TIEHANDLE. (If the appropriate method is not found, an exception is raised.) Typically, these are arguments such as might be passed to the dbm_open(3) function of C, but their meaning is package dependent. The object returned by the constructor is in turn returned by the tie function, which can be useful if you want to access other methods in CLASSNAME. (The object can also be accessed through the tied function.) So, a class for tying a hash to an ISAM implementation might provide an extra method to traverse a set of keys sequentially (the "S" of ISAM), since your typical DBM implementation can't do that.

Functions such as keys and values may return huge list values when used on large objects like DBM files. You may prefer to use the each function to iterate over such. For example:

use NDBM_File;
tie %ALIASES, "NDBM_File", "/etc/aliases", 1, 0
    or die "Can't open aliases: $!\n";
while (($key,$val) = each %ALIASES) {
    print $key, ' = ', $val, "\n";
}
untie %ALIASES;

TIEHASH CLASS, LIST
FETCH SELF, KEY
STORE SELF, KEY, VALUE
DELETE SELF, KEY
CLEAR SELF
EXISTS SELF, KEY
FIRSTKEY SELF
NEXTKEY SELF, LASTKEY
DESTROY SELF

TIEARRAY CLASS, LIST
FETCH SELF, SUBSCRIPT
STORE SELF, SUBSCRIPT, VALUE
FETCHSIZE SELF
STORESIZE SELF, COUNT
CLEAR SELF
PUSH SELF, LIST
POP SELF
SHIFT SELF
UNSHIFT SELF, LIST
SPLICE SELF, OFFSET, LENGTH, LIST
EXTEND SELF, COUNT
DESTROY SELF

TIESCALAR CLASS, LIST
FETCH SELF,
STORE SELF, VALUE
DESTROY SELF

TIEHANDLE CLASS, LIST
READ SELF, SCALAR, LENGTH, OFFSET
READLINE SELF
GETC SELF
WRITE SELF, SCALAR, LENGTH, OFFSET
PRINT SELF, LIST
PRINTF SELF, FORMAT, LIST
CLOSE SELF
DESTROY SELF

29.2.179. tied    

tied VARIABLE

ref tied %hash

29.2.180. time    

time

$start = time();
system("some slow command");
$end   = time();
if ($end - $start > 1) {
    print "Program started: ", scalar localtime($start), "\n";
    print "Program ended:   ", scalar localtime($end), "\n";
}

[18]Not to be confused with the "epic", which is about the making of Unix. (Other operating systems may have a different epoch, not to mention a different epic.)

29.2.181. times    

times

($user, $system, $cuser, $csystem) = times();
printf "This pid and its kids have consumed %.3f seconds\n",
            $user + $system + $cuser + $csystem;

$start = times();
...
$end = times();
printf "that took %.2f CPU seconds of user time\n",
        $end - $start;

29.2.182. tr///    

tr///
y///

29.2.183. truncate    

truncate FILEHANDLE, LENGTH
truncate EXPR, LENGTH

29.2.184. uc     [taintgray]

uc EXPR
uc

29.2.185. ucfirst     [taintgray]

ucfirst EXPR
ucfirst

To force the initial character to titlecase and everything else to lowercase, use:

ucfirst lc $word

29.2.186. umask    

umask EXPR
umask

umask((umask() & 077) | 7);   # don't change the group bits

The Unix permission rwxr-x--- is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indicates octal and doesn't count as one of the digits). Since the umask's bits are flipped, it represents disabled permissions bits. The permission (or "mode") values you supply to mkdir or sysopen are modified by your umask, so even if you tell sysopen to create a file with permissions 0777, if your umask is 0022, the file is created with permissions 0755. If your umask were 0027 (group can't write; others can't read, write, or execute), then passing sysopen a MASK of 0666 would create a file with mode 0640 (since 0666 & ~0027 is 0640).

Here's some advice: supply a creation mode of 0666 for regular files (in sysopen) and one of 0777 both for directories (in mkdir) and for executable files. This gives users the freedom of choice: if they want protected files, they choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. The exception to this rule is programs that write files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.

If umask(2) is not implemented on your system and you are trying to restrict your own access (that is, if EXPR& 0700) > 0), you'll trigger a run-time exception. If umask(2) is not implemented and you are not trying to restrict your own access, the function simply returns undef.

29.2.187. undef    

undef EXPR
undef

Coincidentally, the undef function can also explicitly undefine an entity if you supply its name as an argument. The EXPR argument, if specified, must be an lvalue. Hence you may only use this on a scalar value, an entire array or hash, a subroutine name (using the & prefix), or a typeglob. Any storage associated with the object will be recovered for reuse (though not returned to the system, for most operating systems). The undef function will probably not do what you expect on most special variables. Using it on a read-only variable like $1 raises an exception.

The undef function is a unary operator, not a list operator, so you can only undefine one thing at a time. Here are some uses of undef as a unary operator:

undef $foo;
undef $bar{'blurfl'};   # Different from delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz;             # destroys $xyz, @xyz, %xyz, &xyz, etc.

select(undef, undef, undef, $naptime);

return (wantarray ? () : undef) if $they_blew_it;
return if $they_blew_it;  # same thing

($a, $b, undef, $c) = &foo;       # Ignore third value returned

29.2.188. unlink    

unlink LIST
unlink

$count = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob("*.orig");

[19] Actually, under a POSIX filesystem, it removes the directory entries (filenames) that refer to the real files. Since a file may be referenced (linked) from more than one directory, the file isn't removed until the last reference to it is removed.

Here's a simple rm command with very simple error checking:

#!/usr/bin/perl
@cannot = grep {not unlink} @ARGV;
die "$0: could not unlink @cannot\n" if @cannot;

29.2.189. unpack    

unpack TEMPLATE, EXPR

The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, the bytes of the string either are the result of a pack, or represent a C structure of some kind.

If the repeat count of a field is larger than the remainder of the input string allows, the repeat count is silently decreased. (Normally, you'd use a repeat count of * here, anyway.) If the input string is longer than what TEMPLATE describes, the rest of the string is ignored.

The unpack function is also useful for plain text data, too, not just binary data. Imagine that you had a data file that contained records that looked like this:

1986 Ender's Game           Orson Scott Card
1985 Neuromancer            William Gibson
1984 Startide Rising        David Brin
1983 Foundation's Edge      Isaac Asimov
1982 Downbelow Station      C. J. Cherryh
1981 The Snow Queen         Joan D. Vinge

while (<>) {
        ($year, $title, $author) = unpack("A4 x A23 A*", $_);
        print "$author won ${year}'s Hugo for $title.\n";
}

Here's a complete uudecode program:

#!/usr/bin/perl
$_ = <> until ($mode,$file) = /^begin\s*(\d*)\s*(\S*)/;
open(OUT,"> $file") if $file ne "";
while (<>) {
    last if /^end/;
    next if /[a-z]/;
    next unless int((((ord() - 32) & 077) + 2) / 3) ==
                int(length() / 4);
    print OUT unpack "u", $_;
}
chmod oct($mode), $file;

undef $/;
$checksum = unpack ("%32C*", <>) % 65535;

$setbits = unpack "%32b*", $selectmask;

while (<>) {
    tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
    tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
    $len = pack("c", 32 + 0.75*length);   # compute length byte
    print unpack("u", $len . $_);         # uudecode and print
}

29.2.190. unshift    

unshift ARRAY, LIST

unshift @ARGV, '-e', $cmd unless $ARGV[0] =~ /^-/;

29.2.191. untie    

untie VARIABLE

29.2.192. use    

use MODULE VERSION LIST
use MODULE VERSION ()
use MODULE VERSION
use MODULE LIST
use MODULE ()
use MODULE
use VERSION

use MODULE LIST;

BEGIN { require MODULE; import MODULE LIST; }

If you don't want your namespace altered, explicitly supply an empty list:

use MODULE ();

BEGIN { require MODULE; }

Speaking of not breaking things, Perl still accepts old version numbers of the form:

use 5.005_03;

use 5.6.0;   # That's version 5, subversion 6, patchlevel 0.

See Chapter 32, "Standard Modules" for a list of standard modules.

Because use provides a wide-open interface, pragmas (compiler directives) are also implemented via modules. Examples of currently implemented pragmas include:

use autouse 'Carp' => qw(carp croak);
use bytes;
use constant PI => 4 * atan2(1,1);
use diagnostics;
use integer;
use lib '/opt/projects/spectre/lib';
use locale;
use sigtrap qw(die INT QUIT);
use strict  qw(subs vars refs);
use warnings "deprecated";

There's a corresponding declaration, no, that "unimports" any meanings originally imported by use that have since become, er, unimportant:

no integer;
no strict 'refs';
no utf8;
no warnings "unsafe";

29.2.193. utime    

utime LIST

#!/usr/bin/perl
# montouch - post-date files now + 1 month
$day = 24 * 60 * 60;            # 24 hours of seconds
$later = time() + 30 * $day;    # 30 days is about a month
utime $later, $later, @ARGV;

#!/usr/bin/perl
# montouch - post-date files now + 1 month
$later = time() + 30 * 24 * 60 * 60;
@cannot = grep {not utime $later, $later, $_} @ARGV;
die "$0: Could not touch @cannot.\n" if @cannot;

29.2.194. values    

values HASH

You can modify the values of a hash using this function because the returned list contains aliases of the values, not just copies. (In earlier versions, you needed to use a hash slice for that.)

for (@hash{keys %hash}) { s/foo/bar/g }   # old way
for (values %hash)      { s/foo/bar/g }   # now changes values

29.2.195. vec    

vec EXPR, OFFSET, BITS

OFFSET specifies the index of the particular element you're interested in. The syntaxes for reading and writing the element are the same, since vec stores or returns the value of the element depending on whether you use it in an lvalue or an rvalue context.

BITS specifies how wide each element is in bits, which must be a power of two: 1, 2, 4, 8, 16, or 32 (and also 64 on some platforms). (An exception is raised if any other value is used.) Each element can therefore contain an integer in the range 0..(2**BITS)-1. For the smaller sizes, as many elements as possible are packed into each byte. When BITS is 1, there are eight elements per byte. When BITS is 2, there are four elements per byte. When BITS is 4, there are two elements (traditionally called nybbles) per byte. And so on. Integers larger than a byte are stored in big-endian order.

A list of unsigned integers can be stored in a single scalar variable by assigning them individually to the vec function. (If EXPR is not a valid lvalue, an exception is raised.) In the following example, the elements are each 4 bits wide:

$bitstring = "";
$offset = 0;

foreach $num (0, 5, 5, 6, 2, 7, 12, 6) {
    vec($bitstring, $offset++, 4) = $num;
}

The vectors stored in the scalar variable can be subsequently retrieved by specifying the correct OFFSET.

$num_elements = length($bitstring)*2;  # 2 elements per byte

foreach $offset (0 .. $num_elements-1) {
    print vec($bitstring, $offset, 4), "\n";
}

Strings created with vec can also be manipulated with the logical operators |, &, ^, and ~. These operators will assume that a bit string operation is desired when both operands are strings. See the examples of this in Chapter 3, "Unary and Binary Operators", in the section "Bitwise Operators".

If BITS == 1, a bitstring can be created to store a series of bits all in one scalar. The ordering is such that vec($bitstring,0,1) is guaranteed to go into the lowest bit of the first byte of the string.

@bits = (0,0,1,0, 1,0,1,0, 1,1,0,0, 0,0,1,0);

$bitstring = "";
$offset = 0;

foreach $bit (@bits) {
    vec($bitstring, $offset++, 1) = $bit;
}

print "$bitstring\n";        # "TC", ie. '0x54', '0x43'

$bitstring = pack "b*", join('', @bits);
print "$bitstring\n";   # "TC", same as before

@bits = split(//, unpack("b*", $bitstring));
print "@bits\n";        # 0 0 1 0 1 0 1 0 1 1 0 0 0 0 1 0

See select for additional examples of using bitmaps generated with vec. See pack and unpack for higher-level manipulation of binary data.

29.2.196. wait    

wait

If you expected a child and didn't find it with wait, you probably had a call to system, a close on a pipe, or backticks between the fork and the wait. These constructs also do a wait(2) and may have harvested your child process. Use waitpid to avoid this problem.

29.2.197. waitpid    

waitpid PID, FLAGS

use POSIX ":sys_wait_h";
do {
    $kid = waitpid(-1,&WNOHANG);
} until $kid == -1;

On some systems, a return value of -1 could mean that child processes are being automatically reaped because you set $SIG{CHLD} = 'IGNORE'.

29.2.198. wantarray    

wantarray

Here's are examples of typical usage:

return unless defined wantarray;    # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : \@a;

29.2.199. warn    

warn LIST
warn

warn "Debug enabled" if $debug;

As with die, if the strings supplied don't end in a newline, file and line number information is automatically appended. The warn function is unrelated to Perl's -w command-line option, but can be used in conjunction with it, such as when you wish to emulate built-ins:

warn "Something wicked\n" if $^W;

local $SIG{__WARN__} = sub {
    my $msg = shift;
    die $msg if $msg =~ /isn't numeric/;
};

Using a __WARN__ handler provides a powerful way to silence all warnings, even the so-called mandatory ones. Sometimes you need to wrap this in a BEGIN{} block so that it can happen at compile time:

# wipe out *all* compile-time warnings
BEGIN { $SIG{__WARN__} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20;          # no warning about duplicate my $foo,
                       # but hey, you asked for it!

# no compile-time or run-time warnings before here
$DOWARN = 1;           # *not* a built-in variable

# run-time warnings enabled after here
warn "\$foo is alive and $foo!";     # does show up

29.2.200. write    

write FILEHANDLE
write

$old_fh = select(HANDLE);
$~ = "NEWNAME";
select($old_fh);

use IO::Handle;
HANDLE->format_name("NEWNAME");

$~ = "OtherPack::NEWNAME";

use IO::Handle;
HANDLE->format_top_name("NEWNAME_TOP");

If a specified format or the current top-of-page format does not exist, an exception is raised.

The write function is not the opposite of read. Unfortunately. Use print for simple string output. If you looked up this entry because you wanted to bypass standard I/O, see syswrite.

29.2.201. y//    

y///

29.1. Perl Functions by Category		30. The Standard Perl Library

Copyright © 2001 O'Reilly & Associates. All rights reserved.