PERLFUNC
NAME DESCRIPTION
perlfunc - Perl builtin functions
The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in perlop.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, the scalar arguments will be first, and the list argument will follow. (Note that there can ever be only one such list argument.) For instance, splice() has three scalar arguments followed by a list, whereas gethostbyname() has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count--so you need to be
print(1+2) + 4; # Prints 3. print (1+2)+4; # Also prints 3! print +(1+2)+4; # Prints 7. print ((1+2)+4); # Prints 7.
Useless use of integer addition in void context at - line 1. A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as time and endpwent. For example, time+86_400 always means time() + 86_400.
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
An named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list construction version of the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2), closedir(2)?, etc.) all return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1 on failure. Exceptions to this rule are wait, waitpid, and syscall. System calls also set the special $! variable on failure. Other functions do not, except accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.
Functions for SCALARs or strings
chomp, chop, chr, crypt, hex, index, lc, lcfirst, length, oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex, sprintf, substr, tr///, uc, ucfirst, y///
Regular expressions and pattern matching
m//, pos, quotemeta, s///, split, study, qr//
Numeric functions
abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand
Functions for real @ARRAYs
pop, push, shift, splice, unshift
Functions for list data
grep, join, map, qw/STRING/, reverse, sort, unpack
Functions for real %HASHes
delete, each, exists, keys, values
Input and output functions
binmode, close, closedir, dbmclose, dbmopen, die, eof, fileno, flock, format, getc, print, printf, read, readdir, rewinddir, seek, seekdir, select, syscall, sysread, sysseek, syswrite, tell, telldir, truncate, warn, write
Functions for fixed length data or records
pack, read, syscall, sysread, syswrite, unpack, vec
Functions for filehandles, files, or directories
chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, stat, symlink, umask, unlink, utime
Keywords related to the control flow of your perl program
caller, continue, die, do, dump, eval, exit, goto, last, next, redo, return, sub, wantarray
Keywords related to scoping
caller, import, local, my, our, package, use
Miscellaneous functions
defined, dump, eval, formline, local, my, our, reset, scalar, undef, wantarray
Functions for processes and process groups
alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx/STRING/, setpgrp, setpriority, sleep, system, times, wait, waitpid
Keywords related to perl modules
do, import, no, package, require, use
Keywords related to classes and object-orientedness
bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
Low-level socket functions
accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair
System V interprocess communication functions
msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite
Fetching user and group info
endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent
Fetching network info
endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent
Time-related functions
gmtime, localtime, time, times
Functions new in perl5
abs, bless, chomp, chr, exists, formline, glob, import, lc, lcfirst, map, my, no, our, prototype, qx, qw, readline, readpipe, ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
an operator, which can be used in expressions.
Functions obsoleted in perl5
dbmclose, dbmopen
Portability
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. The Perl functions affected by this are:
chown, chroot, crypt, dbmclose, dbmopen, dump, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, exec, fcntl, flock, fork, getgrent, getgrgid, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getppid, getprgp, getpriority, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyport, getservent, getsockopt, glob, ioctl, kill, link, lstat, msgctl, msgget, msgrcv, msgsnd, open, pipe, readlink, rename, select, semctl, semget, semop, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shmctl, shmget, shmread, shmwrite, socket, socketpair, stat, symlink, syscall, sysopen, system, times, truncate, umask, unlink, utime, wait, waitpid
For more information about the portability of these functions, see perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X
A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for -t, which tests STDIN . Unless otherwise documented, it returns 1 for true and '' for false, or the undefined value if the file doesn't exist. Despite the funny names, precedence is the same as any other named unary operator, and the argument may be parenthesized like any other unary operator. The operator may be any
o File is owned by effective uid.
O File is owned by real uid.
s File has nonzero size (returns size in bytes).
t Filehandle is opened to a tty.
k File has sticky bit set.
B File is a
The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the -r, -R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called filetest that may produce more accurate results than the bare stat() mode bits. When under the use filetest 'access' the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the -x and -X may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Read the documentation for the filetest pragma for more information.
Note that -s/a/b/ does not do a negated substitution. Saying -exp($foo) still works as expected, however--only single letters following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or characters with the high bit set. If too many strange characters (
first block is considered a binary file. If -T or
buffer is examined rather than the first block. Both
file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file .
If any of the file tests (or either the stat or lstat operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat() and -l will leave values in the stat structure for the symbolic link,
abs VALUE
abs
Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET ,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call does. Returns the packed address if it succeeded, false otherwise. See the example in ``Sockets: Client/Server Communication'' in perlipc.
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 $^F. See ``$^F'' in perlvar.
alarm SECONDS
alarm
Arranges to have a SIGALRM delivered to this process after the specified number of seconds have elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less than you specified because of how seconds are counted.) Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's four-argument version of select() leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. The Time::!HiRes? module from CPAN may also prove useful.
It is usually a mistake to intermix alarm and sleep calls. (sleep may be internally implemented in your system with alarm)
If you want to use alarm to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works, modulo the caveats given in ``Signals'' in perlipc.
eval {
local $SIG{ALRM} = sub { die
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI .
For the tangent operation, you may use the Math::Trig::tan function, or use the familiar
bind SOCKET ,NAME
Binds a network address to a socket, just as the bind system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in ``Sockets: Client/Server Communication'' in perlipc.
binmode FILEHANDLE , DISCIPLINE
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in ``binary or ``text mode on systems where the run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. DISCIPLINE can be either of
for binary mode or for ``text'' mode. If the
DISCIPLINE is omitted, it defaults to .
binmode() should be called after open() but before any I/O is done on the filehandle.
On many systems binmode() currently has no effect, but in future, it will be extended to support user-defined input and output disciplines. On some systems binmode() is necessary when you're not working with a text file. For the sake of portability it is a good idea to always use it when appropriate, and to never use it when it isn't appropriate.
In other words: Regardless of platform, use binmode() on binary files, and do not use binmode() on text files.
The open pragma can be used to establish default disciplines. See open.
The operating system, device drivers, C libraries, and Perl run-time system all work together to let the programmer treat a single character (n) as the line terminator, irrespective of the external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of n is made up of more than one character.
Mac OS and all variants of Unix use a single character to end each line in the external representation of text (even though that single character is not necessarily the same across these platforms). Consequently binmode() has no effect on these operating systems. In other systems like VMS , MS-DOS and the various flavors of MS-Windows your program sees a n as a simple cJ, but what's stored in text files are the two characters cMcJ. That means that, if you don't use binmode() on these systems, cMcJ sequences on disk will be converted to n on input, and any n in your program will be converted back to cMcJ on output. This is what you want for text files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that if your binary data contains cZ, the I/O subsystem will regard it as the end of the file, unless you use binmode().
binmode() is not only important for readline() and print() operations, but also when using read(), seek(), sysread(), syswrite() and tell() (see perlport for more details). See the $/ and $\ variables in perlvar for how to manually set your input and output line-termination sequences.
bless REF ,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the current package is used. Because a bless is often the last thing in a constructor, it returns the reference for convenience. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See perltoot and perlobj for more about the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
See ``Perl Modules'' in perlmod.
caller EXPR
caller
Returns the context of the current subroutine call. In scalar context, returns the caller's package name if there is a caller, that is, if we're in a subroutine or eval or require, and the undefined value otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR , it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); Here $subroutine may be (eval) if the frame is not a subroutine call, but an eval. In such a case additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a require or use statement, $evaltext contains the text of the eval EXPR statement. In particular, for an eval BLOCK statement, $filename is (eval), but $evaltext is undefined. (Note also that each use statement creates a require frame inside an eval EXPR) frame. $hasargs is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.
Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before caller had a chance to get the information. That means that caller(N) might not return information about the call frame you expect it do, for N . In particular, @DB::args might have information from the previous time caller was called.
chdir EXPR
Changes the working directory to EXPR , if possible. If EXPR is omitted, changes to the directory specified by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}. If neither is set, chdir does nothing. It returns true upon success, false otherwise. See the example under die.
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numerical mode, which should probably be an octal number, and which definitely should not a string of octal digits: 0644 is okay, '0644' is not. Returns the number of files successfully changed. See also ``oct'', if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables; $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
$mode = '0644'; chmod oct($mode), 'foo'; # this is better $mode = 0644; chmod $mode, 'foo'; # this is best
chomp VARIABLE
chomp LIST
chomp
This safer version of ``chop removes any trailing string that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode ($/ = ), it removes all trailing newlines from the string. When in slurp mode ($/ = undef) or fixed-length record mode ($/ is a reference to an integer or the like, see perlvar) chomp()'' won't remove anything. If VARIABLE is omitted, it
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including
chomp($answer = If you chomp a list, each element is chomped, and the total number of characters removed is returned.
chop VARIABLE
chop LIST
chop
Chops off the last character of a string and returns the character chopped. It is much more efficient than s/.$//s because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. If VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last chop is returned.
Note that chop returns the last character. To return all but the last character, use substr($string, 0, -1).
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
or die
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr
Returns the character represented by that NUMBER in the character set. For example, chr(65) is in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face (but only within the scope of a use utf8). For the reverse, use ``ord''. See utf8 for more about Unicode.
If NUMBER is omitted, uses $_.
chroot FILENAME
chroot
This function works like the system call by the same name: it makes the named directory the new root directory for all further pathnames that begin with a / by your process and all its children. (It doesn't change your current working directory, which is unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does a chroot to $_.
close FILEHANDLE
close
Closes the file or pipe associated with the file handle, returning true only if stdio successfully flushes buffers and closes the system file descriptor. Closes the currently selected filehandle if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do another open on it, because open will close it for you. (See open.) However, an explicit close on an input file resets the line counter ($.), while the implicit close done by open does not.
If the file handle came from a piped open close will additionally return false if one of the other system calls involved fails or if the program exits with non-zero status. (If the only problem was that the program exited non-zero $! will be set to 0.) Closing a pipe also waits for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards, and implicitly puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe.
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name.
closedir DIRHANDLE
Closes a directory opened by opendir and returns the success of that system call.
DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name.
connect SOCKET ,NAME
Attempts to connect to a remote socket, just as the connect system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in ``Sockets: Client/Server Communication'' in perlipc.
continue BLOCK
Actually a flow control statement rather than a function. If there is a continue BLOCK attached to a BLOCK (typically in a while or foreach), it is always executed just before the conditional is about to be evaluated again, just like the third part of a for loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (which is similar to the C continue statement).
last, next, or redo may appear within a continue block. last and redo will behave as if they had been executed within the main block. So will next, but since it will execute a continue block, it may be more entertaining.
while (EXPR) {
do_something; } continue {
do_something_else;
}
Omitting the continue section is semantically equivalent to using an empty one, logically enough. In that case, next goes directly back to check the condition at the top of the loop.
cos EXPR
cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the Math::Trig::acos() function, or use this
crypt PLAINTEXT ,SALT
Encrypts a string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition). This can prove useful for checking the password file for lousy passwords, amongst other things. Only the guys wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function. As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted text as the salt (like crypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt and with more exotic implementations. When choosing a new salt create a random two character string whose characters come from the set [./0-9A-Za-z? (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64?).
Here's an example that makes sure that whoever runs this
die Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.
dbmclose HASH
Breaks the binding between a DBM file and a hash.
dbmopen HASH ,DBNAME,MASK
This binds a dbm(3)?, ndbm(3)?, sdbm(3)?, gdbm(3)?, or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal open, the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the umask). If your system supports only the older DBM functions, you may perform only one dbmopen in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen produced a fatal error; it now falls back to sdbm(3)?.
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an eval, which will trap the error.
Note that functions such as keys and values may return huge lists when used on large DBM files. You may prefer to use the each function to iterate over large
dbmopen(%HIST,'/usr/lib/news/history',0666); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call
dbmopen(%NS_Hist,
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ will be checked.
Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and , which are all equally false.) Note that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop returns undef when its argument is an empty array, or when the element to return happens to be undef.
You may also use defined( to check whether subroutine has ever been defined. The return value is unaffected by any forward declarations of . Note that a subroutine which is not defined may still be callable: its package may have an AUTOLOAD method that makes it spring into existence the first time that it is called -- see perlsub.
Use of defined on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. You should instead use
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use ``exists'' for the latter purpose.
print Note: Many folks tend to overuse defined, and then are surprised to discover that the number 0 and (the zero-length string) are, in fact, defined values. For example, if you say
The pattern match succeeds, and $1 is defined, despite the fact that it matched ``nothing''. But it didn't really match nothing--rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use defined only when you're questioning the integrity of what you're trying to do. At other times, a simple comparison to 0 or is what you want.
See also ``undef, ``exists, ``ref''.
delete EXPR
Given an expression that specifies a hash element, array element, hash slice, or array slice, deletes the specified element(s) from the hash or array. In the case of an array, if the array elements happen to be at the end, the size of the array will shrink to the highest element that tests true for exists() (or 0 if no such element exists).
Returns each element so deleted or the undefined value if there was no such element. Deleting from $ENV{} modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a tied hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array to its initial, uninitialized state. Subsequently testing for the same element with exists() will return false. Note that deleting array elements in the middle of an array will not shift the index of the ones after them down--use splice() for that. See ``exists''.
The following (inefficiently) deletes all the values of
delete $HASH{$key}; }
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index?; }
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
die LIST
Outside an eval, prints the value of LIST to STDERR and exits with the current value of $! (errno). If $! is 0, exits with the value of ($? (backtick `command` status). If ($? is 0, exits with 255. Inside an eval(), the error message is stuffed into $@ and the eval is terminated with the undefined value. This makes die the way to raise an exception.
If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Note that the ``input line number (also known as ``chunk) is subject to whatever notion of ``line happens to be currently in effect, and is also available as the special variable $.. See ``$/ in perlvar and ``$.'' in perlvar.
Hint: sometimes appending to your message will cause it to make better sense when the string is appended. Suppose you are running script ``canasta''.
die
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123. See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is reused after appending . This is useful for
die unless $@ = /Expected exception/; If $@ is empty then the string is used.
die() can also be called with a reference argument. If this happens to be trapped within an eval(), $@ contains the reference. This behavior permits a more elaborate exception handling implementation using objects that maintain arbitrary state about the nature of the exception. Such a scheme is sometimes preferable to matching particular string values of $@ using regular expressions.
Because perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See overload for details about that.
You can arrange for a callback to be run just before the die does its deed, by setting the $SIG{DIE} hook. The associated handler will be called with the error text and can change the error message, if it sees fit, by calling die again. See ``$SIG{expr} in perlvar for details on setting %SIG entries, and ``eval BLOCK for some examples. Although this feature was meant to be run only right before your program was to exit, this is not currently the case--the $SIG{DIE} hook is currently called even inside eval()ed blocks/strings! If one wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see ``$^S'' in perlvar). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK . When modified by a loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.)
do BLOCK does not count as a loop, so the loop control statements next, last, or redo cannot be used to leave or restart the block. See perlsyn for alternative strategies.
do SUBROUTINE ( LIST )
A deprecated form of subroutine call. See perlsub.
do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a Perl script. Its primary use is to include subroutines from a Perl subroutine library.
do 'stat.pl';
is just like
scalar eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error messages, searches the @INC libraries, and updates %INC if the file is found. See ``Predefined Names'' in perlvar for these variables. It also differs in that code evaluated with do FILENAME cannot see lexicals in the enclosing scope; eval STRING does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.
If do cannot read the file, it returns undef and sets $! to the error. If do can read the file but cannot 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.
Note that inclusion of library modules is better done with the use and require operators, which also do automatic error checking and raise an exception if there's a problem.
You might like to use do to read in a program configuration file. Manual error checking can be done this
for $file (
dump LABEL
dump
This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does the same thing. Primarily this is so that you can use the undump program (not supplied) to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a goto LABEL (with all the restrictions that goto suffers). Think of it as a goto with an intervening core dump and reincarnation. If LABEL is omitted, restarts the program from the top.
WARNING : Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very hard to convert a core file into an executable, and because the real 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, consider generating bytecode or native C code as described in perlcc. If you're just trying to accelerate a CGI script, consider using the mod_perl extension to Apache, or the CPAN module, Fast::CGI. You might also consider autoloading or selfloading, which at least make your program appear to run faster.
each HASH
When called in list context, returns a 2-element list consisting of the key and value for the next element of a hash, so that you can iterate over it. When called in scalar context, returns only the key for the next element in the hash.
Entries are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be in the same order as either the keys or values function would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context (which when assigned produces a false (0) value), and undef in scalar context. The next call to each after that will start iterating again. 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, you may get entries skipped or duplicated, so don't. Exception: It is always safe to delete the item most recently returned by each(), which means that the
while (($key, $value) = each %hash) {
print $key,
print See also keys, values and sort.
eof FILEHANDLE
eof ()
eof
Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function actually reads a character and then ungetcs it, so isn't very useful in an interactive context.) Do not read from a terminal file (or call eof(FILEHANDLE) on it) after end-of-file is reached. File types such as terminals may lose the end-of-file condition if you do.
An eof without an argument uses the last file read. Using eof() with empty parentheses is very different. It refers to the pseudo file formed from the files listed on the command line and accessed via the
operator. Since isn't
explicitly opened, as a normal filehandle is, an eof() before has been used will cause @ARGV to be examined to determine if input is available.
In a while ( loop, eof or eof(ARGV) can be used to detect the end of each file, eof() will only detect the end of the last
while (
while ( Practical hint: you almost never need to use eof in Perl, because the input operators typically return undef when they run out of data, or if there was an error.
eval EXPR
eval BLOCK
In the first form, the return value of EXPR is parsed and executed as if it were a little Perl program. The value of the expression (which is itself determined within scalar context) is first parsed, and if there weren't any errors, executed in the lexical context of the current Perl program, so that any variable settings or subroutine and format definitions remain afterwards. Note that the value is parsed every time the eval executes. If EXPR is omitted, evaluates $_. This form is typically used to delay parsing and subsequent execution of the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only once--at the same time the code surrounding the eval itself was parsed--and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK .
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also used, just as with subroutines. 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 syntax error or runtime error, or a die statement is executed, an undefined value is returned by eval, and $@ is set to the error message. If there was no error, $@ is guaranteed to be a null string. Beware that using eval neither silences perl from printing warnings to STDERR , nor does it stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{WARN} facility. See ``warn'' and perlvar.
Note that, because eval traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as socket or symlink) is implemented. It is also Perl's exception trapping mechanism, where the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is
eval { $answer = $a / $b; }; warn $@ if $@;
eval '$answer = $a / $b'; warn $@ if $@;
eval { $answer = }; # WRONG
eval '$answer ='; # sets $@
eval { local $SIG{'DIE'}; $answer = $a / $b; }; warn $@ if $@;
{ local $SIG{'DIE'} = sub { (my $x = $_[0?) = s/foo/bar/g; die $x }; eval { die Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
With an eval, you should be especially careful to
eval
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval
Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6.
eval BLOCK does not count as a loop, so the loop control statements next, last, or redo cannot be used to leave or restart the block.
exec LIST
exec PROGRAM LIST
The exec function executes a system command and never returns-- use system instead of exec if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).
Since it's a common mistake to use exec instead of system, Perl warns you if there is a following statement which isn't die, warn, or exit (if -w is set - but you always do that). If you really want to follow an exec with some other statement, you can use one of these styles
exec
exec $shell '-sh'; # pretend it's a login shell or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will be subject to its quirks and capabilities. See ```STRING`'' in perlop for details.
Using an indirect object with exec or system is also more secure. This usage (which also works fine with system()) forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.
@args = ( exec @args; # subject to shell escapes
exec { $args[0? } @args; # safe even with one-arg list The first version, the one without the indirect object, ran the echo program, passing it an argument. The second version didn't--it tried to run a program literally called ``echo surprise'', didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $ ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles in order to avoid lost output.
Note that exec will not call your END blocks, nor will it call any DESTROY methods in your objects.
exists EXPR
Given an expression that specifies a hash element or array element, returns true if the specified element in the hash or array has ever been initialized, even if the corresponding value is undefined. The element is not autovivified if it doesn't exist.
print print
A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for exists or defined does not count as declaring it. Note that a subroutine which does not exist may still be callable: its package may have an AUTOLOAD method that makes it spring into existence the first time that it is called -- see perlsub.
if (exists $ref- This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release.
See ``Pseudo-hashes: Using an array as a hash in perlref for specifics on how exists()'' acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists
exit EXPR
Evaluates EXPR and exits immediately with
See also die. If EXPR is omitted, exits with 0 status. The only universally recognized values for EXPR are 0 for success and 1 for error; other values are subject to interpretation depending on the environment in which the Perl program is running. For example, exiting 69 ( EX_UNAVAILABLE ) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered, but that's not true everywhere.
Don't use exit to abort a subroutine if there's any chance that someone might want to trap whatever error happened. Use die instead, which can be trapped by an eval.
The exit() function does not always exit immediately. It calls any defined END routines first, but these END routines may not themselves abort the exit. Likewise any object destructors that need to be called are called before the real exit. If this is a problem, you can call POSIX:_exit($status) to avoid END and destructor processing. See perlmod for details.
exp EXPR
exp
Returns e (the natural logarithm base) to the power of EXPR . If EXPR is omitted, gives exp($_).
fcntl FILEHANDLE ,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer) or die You don't have to check for defined on the return from fnctl. Like ioctl, it maps a 0 return from the system call into in Perl. This string is true in boolean context and 0 in numeric context. It is also exempt from the normal -w warnings on improper numeric conversions.
Note that fcntl will produce a fatal error if used on a machine that doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. This is mainly useful for constructing bitmaps for select and low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to
flock FILEHANDLE ,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE . Returns true for success, false on failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock is Perl's portable file locking interface, although it locks only entire files, not records.
Two potentially non-obvious but traditional flock semantics are that it waits indefinitely until the lock is granted, and that its locks merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with flock may be modified by programs that do not also use flock. See perlport, your port's specific documentation, or your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called ``features''). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH , LOCK_EX , or LOCK_UN , possibly combined 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, LOCK_EX requests an exclusive lock, and LOCK_UN releases a previously requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX then flock will return immediately rather than blocking waiting for the lock (check the return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most if not all systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people.
Note also that some versions of flock cannot lock things over the network; you would need to use the more system-specific fcntl for that. If you like you can force Perl to ignore your system's flock(2) function, and so provide its own fcntl(2)-based emulation, by passing the switch -Ud_flock to the Configure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants sub lock {
flock(MBOX,LOCK_EX);
seek(MBOX, 0, 2); }
sub unlock {
flock(MBOX,LOCK_UN); }
open(MBOX, lock();
print MBOX $msg, On systems that support a real flock(), locks are inherited across fork() calls, whereas those that must resort to the more capricious fcntl() function lose the locks, making it harder to write servers.
See also DB_File for other flock() examples.
fork
Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or undef if the fork is unsuccessful. File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied. On most systems supporting fork(), 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.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $ ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles in order to avoid duplicate output.
If you fork without ever waiting on your children, you will accumulate zombies. On some systems, you can avoid this by setting $SIG{CHLD} to . See also perlipc for more examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue.
format
Declare a picture format for use by the write
Test: @
$str =
See perlform for many details and examples.
formline PICTURE ,LIST
This is an internal function used by formats, though you may call it, too. It formats (see perlform) a list of values according to the contents of PICTURE , placing the output into the format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a write is done, the contents of $^A are written to some filehandle, but you could also read $^A yourself and then set $^A back to . Note that a format typically does one formline per line of form, but the formline function itself doesn't care how many newlines are embedded in the PICTURE . This means that the and ~ tokens will treat the entire PICTURE as a single line. You may therefore need to use multiple formlines to implement a single record format, just like the format compiler.
Be careful if you put double quotes around the picture, because an @ character may be taken to mean the beginning of an array name. formline always returns true. See perlform for other examples.
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE , or the undefined value at end of file, or if there was an error. If FILEHANDLE is omitted, reads from STDIN . This is not particularly efficient. However, it cannot be used by itself to fetch single characters without waiting for the user to
system
$key = getc(STDIN); if ($BSD_STYLE) {
system Determination of whether $BSD_STYLE should be set is left as an exercise to the reader.
The POSIX::getattr function can do this more portably on systems purporting POSIX compliance. See also the Term::!ReadKey? module from your nearest CPAN site; details on CPAN can be found on `` CPAN '' in perlmodlib.
getlogin
Implements the C library function of the same name, which on most systems returns the current login from /etc/utmp, if any. If null, use getpwuid.
$login = getlogin getpwuid($
Do not consider getlogin for authentication: it is not as secure as getpwuid.
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK); ($port, $iaddr) = sockaddr_in($hersockaddr); $herhostname = gethostbyaddr($iaddr, AF_INET); $herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID . Use a PID of 0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns process group of current process. Note that the POSIX version of getpgrp does not accept a PID argument, so only PID==0 is truly portable.
getppid
Returns the process id of the parent process.
getpriority WHICH ,WHO
Returns the current priority for a process, a process group, or a user. (See getpriority(2).) Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME ,PROTO
gethostbyaddr ADDR ,ADDRTYPE
getnetbyaddr ADDR ,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT ,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts in the system library. In list context, the return values from the various get routines are as
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw* ($name,$passwd,$gid,$members) = getgr* ($name,$aliases,$addrtype,$length,@addrs) = gethost* ($name,$aliases,$addrtype,$net) = getnet* ($name,$aliases,$proto) = getproto* ($name,$aliases,$port,$proto) = getserv* (If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the
$name = getpwuid($num); $name = getpwent(); $gid = getgrnam($name); $name = getgrgid($num; $name = getgrent();
In getpw*() the fields $quota, $comment, and $expire are special cases in the sense that in many systems they are unsupported. If the $quota is unsupported, it is an empty scalar. If it is supported, it usually encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If it is supported it usually encodes some administrative comment about the user. In some systems the $quota field may be $change or $age, fields that have to do with password aging. In some systems the $comment field may be $class. The $expire field, if present, encodes the expiration period of the account or the password. For the availability and the exact meaning of these fields in your system, please consult your getpwnam(3) documentation and your pwd.h file. You can also find out from within Perl what your $quota and $comment fields mean and whether you have the $expire field by using the Config module and the values d_pwquota, d_pwage, d_pwchange, d_pwcomment, and d_pwexpire. Shadow password files are only supported if your vendor has implemented them in the intuitive fashion that calling the regular C library routines gets the shadow versions if you're running under privilege or if there exists the shadow(3)? functions as found in System V ( this includes Solaris and Linux.) Those systems which implement a proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the members of the group.
For the gethost*() functions, if the h_errno variable is supported in C, it will be returned to you via $? if the function call fails. The @addrs value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and
$iaddr = inet_aton(
$straddr = inet_ntoa($iaddr);
User::grent. These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For exampleuse User::pwent;
$is_his = (stat($filename)-
Even though it looks like they're the same method calls (uid), they aren't, because a File::stat object is different from a User::pwent object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you don't know the address because you have several different IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK); ($port, $myaddr) = sockaddr_in($mysockaddr); printf
getsockopt SOCKET ,LEVEL,OPTNAME
Returns the socket option requested, or undef if there is an error.
glob EXPR
glob
Returns the value of EXPR with filename expansions such as the standard Unix shell /bin/csh would do. This is the internal function implementing the
operator, but you can use it directly.
If EXPR is omitted, $_ is used. The
operator is discussed in more detail in
``I/O Operators'' in perlop.
Beginning with v5.6.0, this operator is implemented using the standard File::Glob extension. See File::Glob for details.
gmtime EXPR
Converts a time as returned by the time function to a 8-element list with the time localized for the standard
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = gmtime(time); All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time. $mday is the day of the month, and $mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December. $year is the number of years since 1900. That is, $year is 123 in year 2023. $wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)
Note that the $year element is not simply the last two digits of the year. If you assume it is, then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is
$year += 1900;
$year = sprintf(
If EXPR is omitted, gmtime() uses the current time (gmtime(time)).
In scalar context, gmtime() returns the
Also see the timegm function provided by the Time::Local module, and the strftime(3) function available via the POSIX module.
This scalar value is not locale dependent (see perllocale), but is instead a Perl builtin. 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 perllocale)
$now_string = strftime Note that the %a and %b escapes, which represent the short forms of the day of the week and the month of the year, may not necessarily be three characters wide in all locales.
goto LABEL
goto EXPR
goto
The goto-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used to go into any construct that requires initialization, such as a subroutine or a foreach loop. It also can't be used to go into a construct that is optimized away, or to get out of a block or subroutine given to sort. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such as last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN , but isn't necessarily recommended if you're optimizing for
The goto- form is quite different from the other forms of goto. In fact, it isn't a goto in the normal sense at all, and doesn't have the stigma associated with other gotos. Instead, it substitutes a call to the named subroutine for the currently running subroutine. This is used by AUTOLOAD subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to @_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller will be able to tell that this routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference, or a block which evaluates to a code reference.
grep BLOCK LIST
grep EXPR ,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value consisting of those elements for which the expression evaluated to true. In scalar context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST . While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Similarly, grep returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by grep (for example, in a foreach, map or another grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code.
See also ``map'' for a list composed of the results of the BLOCK or EXPR .
hex EXPR
hex
Interprets EXPR as a hex string and returns the corresponding value. (To convert strings that might start with either 0, 0x, or 0b, see ``oct''.) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning.
import
There is no builtin import function. It is just an ordinary method (subroutine) defined (or inherited) by modules that wish to export names to another module. The use function calls the import method for the package used. See also ``use'', perlmod, and Exporter.
index STR ,SUBSTR,POSITION
index STR ,SUBSTR
The index function searches for one string within another, but without the wildcard-like behavior of a full regular-expression pattern match. It returns the position of the first occurrence of SUBSTR in STR at or after POSITION . If POSITION is omitted, starts searching from the beginning of the string. The return value is based at 0 (or whatever you've set the $[[ variable to--but don't do that). If the substring is not found, returns one less than the base, ordinarily
int EXPR
int
Returns the integer portion of EXPR . If EXPR is omitted, uses $_. You should not use this function for rounding: one because it truncates towards 0, and two because machine representations of floating point numbers can sometimes produce counterintuitive results. For example, int(-6.725/0.025) produces -268 rather than the correct -269; that's because it's really more like
sprintf, printf, or the POSIX::floor and POSIX::ceil functions will serve you better than will int().
ioctl FILEHANDLE ,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require
to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as . (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION--a pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The pack and unpack functions may be needed to manipulate the values of structures used by ioctl.
The return value of ioctl (and fcntl) is
if OS returns: then Perl returns:
0 string
printf The special string 0 but true-w__ complaints about improper numeric conversions.
Here's an example of setting a filehandle named REMOTE to be non-blocking at the system level. You'll have to negotiate $ on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); $flags = fcntl(REMOTE, F_GETFL, 0)
or die
$flags = fcntl(REMOTE, F_SETFL, $flags O_NONBLOCK)
or die
join EXPR ,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of EXPR , and returns that new string.
Beware that unlike split, join doesn't take a pattern as its first argument. Compare ``split''.
keys HASH
Returns a list consisting of all the keys of the named hash. (In scalar context, returns the number of keys.) The keys are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be the same order as either the values or each function produces (given that the hash has not been modified). As a side effect, it resets HASH 's iterator.
Here is yet another way to print your
@values = values %ENV; while (@keys) { print pop(@keys), '=', pop(@values),
print $key, '=', $ENV{$key}, The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare ``values''.
To sort a hash by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its
As an lvalue keys allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do %hash = (), use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys in this way (but you needn't worry about doing this by accident, as trying has no effect).
See also each, values and sort.
kill SIGNAL , LIST
Sends a signal to a list of processes. Returns the number of processes successfully signaled (which is not necessarily the same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners; If SIGNAL is zero, no signal is sent to the process. This is a useful way to check that the process is alive and hasn't changed its UID . See perlport for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes. See ``Signals'' in perlipc for details.
last LABEL
last
The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The continue block, if any, is not
last cannot be used to exit a block which returns a value such as eval {}, sub {} or do {}, and should not be used to exit a grep() or map() operation.
Note that 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 ``continue'' for an illustration of how last, next, and redo work.
lc EXPR
lc
Returns an lowercased version of EXPR . This is the internal function implementing the L escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale and utf8.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst
Returns the value of EXPR with the first character lowercased. This is the internal function implementing the l escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.
If EXPR is omitted, uses $_.
length EXPR
length
Returns the length in characters of the value of EXPR . If EXPR is omitted, returns length of $_. Note that this cannot be used on an entire array or hash to find out how many elements these have. For that, use scalar @array and scalar keys %hash respectively.
link OLDFILE ,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise.
listen SOCKET ,QUEUESIZE
Does the same thing that the listen system call does. Returns true if it succeeded, false otherwise. See the example in ``Sockets: Client/Server Communication'' in perlipc.
local EXPR
You really probably want to be using my instead, because local isn't what most people think of as ``local. See ``Private Variables via my() in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See ``Temporary Values via local()'' in perlsub for details, including issues with tied arrays and hashes.
localtime EXPR
Converts a time as returned by the time function to a 9-element list with the time analyzed for the local time
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time); All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time. $mday is the day of the month, and $mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December. $year is the number of years since 1900. That is, $year is 123 in year 2023. $wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.) $isdst is true if the specified time occurs during daylight savings time, false otherwise.
Note that the $year element is not simply the last two digits of the year. If you assume it is, then you create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is
$year += 1900;
$year = sprintf(
If EXPR is omitted, localtime() uses the current time (localtime(time)).
In scalar context, localtime() returns the
$now_string = strftime Note that the %a and %b, the short forms of the day of the week and the month of the year, may not necessarily be three characters wide.
lock
lock I
This function places an advisory lock on a variable, subroutine, or referenced object contained in THING until the lock goes out of scope. This is a built-in function only if your version of Perl was built with threading enabled, and if you've said use Threads. Otherwise a user-defined function by this name will be called. See Thread.
log EXPR
log
Returns the natural logarithm (base e) of EXPR . If EXPR is omitted, returns log of $_. To get the log of another base, use basic algebra: The base-N log of a number is equal to the natural log of that number divided by the natural log of
my $n = shift; return log($n)/log(10); } See also ``exp'' for the inverse operation.
lstat FILEHANDLE
lstat EXPR
lstat
Does the same thing as the stat function (including setting the special _ filehandle) but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal stat is done.
If EXPR is omitted, stats $_.
m//
The match operator. See perlop.
map BLOCK LIST
map EXPR ,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value composed of the results of each such evaluation. In scalar context, returns the total number of elements so generated. Evaluates BLOCK or EXPR in list context, so each element of LIST may produce zero, one, or more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map { getkey($_) =
is just a funny way to write
%hash = ();
foreach $_ (@array) { $hash{getkey($_)} = $_; } Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST . While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regular foreach loop for this purpose would be clearer in most cases. See also ``grep'' for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true.
{ starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR , LIST . Because perl doesn't look ahead for the closing } it has to take a guess at which its dealing with based what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the } and encounters the missing (or unexpected) comma. The syntax error will be reported close to the } but you'll need to change something near the { such as using a
or to force an anon hash constructor use +{
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
mkdir FILENAME ,MASK
mkdir FILENAME
Creates the directory specified by FILENAME , with permissions specified by MASK (as modified by umask). If it succeeds it returns true, otherwise it returns false and sets $! (errno). If omitted, MASK defaults to 0777.
In general, it is better to create directories with permissive MASK , and let the user modify that with their umask, than it is to supply a restrictive MASK and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The perlfunc(1) entry on umask discusses the choice of MASK in more detail.
msgctl ID ,CMD,ARG
Calls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable which will hold the returned msqid_ds structure. Returns like ioctl: the undefined value for error, for zero, or the actual return value otherwise. See also ``SysV IPC '' in perlipc, IPC::SysV, and IPC::Semaphore documentation.
msgget KEY ,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue id, or the undefined value if there is an error. See also ``SysV IPC '' in perlipc and IPC::SysV and IPC::Msg documentation.
msgrcv ID ,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE . Note that when a message is received, the message type as a native long integer will be the first thing in VAR , followed by the actual message.