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Virtual Solutions : Resources : Documentation : Perl POD : perlfunc - Perl builtin functions

NAME

perlfunc - Perl builtin functions

DESCRIPTION

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 the perlop manpage .) 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 careful sometimes:

print 1+2+4; # Prints 7. 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.

If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:

print (...) interpreted as function at - line 1. 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.

A 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

-I >, chdir , chmod , chown , chroot , fcntl , glob , ioctl , link , lstat , mkdir , open , opendir , readlink , rename , rmdir , stat , symlink , sysopen , umask , unlink , utime

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

* - sub was a keyword in perl4, but in perl5 it is 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:

-X , binmode , chmod , 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 the perlport manpage 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 of: X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>

-r File is readable by effective uid/gid. -w File is writable by effective uid/gid. -x File is executable by effective uid/gid. -o File is owned by effective uid. -R File is readable by real uid/gid. -W File is writable by real uid/gid. -X File is executable by real uid/gid. -O File is owned by real uid. -e File exists. -z File has zero size (is empty). -s File has nonzero size (returns size in bytes). -f File is a plain file. -d File is a directory. -l File is a symbolic link. -p File is a named pipe (FIFO), or Filehandle is a pipe. -S File is a socket. -b File is a block special file. -c File is a character special file. -t Filehandle is opened to a tty. -u File has setuid bit set. -g File has setgid bit set. -k File has sticky bit set. -T File is an ASCII text file (heuristic guess). -B File is a "binary" file (opposite of -T). -M Script start time minus file modification time, in days. -A Same for access time. -C Same for inode change time (Unix, may differ for other platforms)

Example:

while (<>) { chomp; next unless -f $_; # ignore specials #... }

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 (>30%) are found, it's a -B file, otherwise it's a -T file. Also, any file containing null in the first block is considered a binary file. If -T or -B is used on a filehandle, the current IO buffer is examined rather than the first block. Both -T and -B return true on a null file, or a 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 && -T $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, not the real file.) (Also, if the stat buffer was filled by a lstat call, -T and -B will reset it with the results of stat _ ). Example:

print "Can do.\n" if -r $a || -w _ || -x _; stat($filename); print "Readable\n" if -r _; print "Writable\n" if -w _; print "Executable\n" if -x _; print "Setuid\n" if -u _; print "Setgid\n" if -g _; print "Sticky\n" if -k _; print "Text\n" if -T _; print "Binary\n" if -B _;

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''.

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 $^/EM.

alarm SECONDS

alarm

Arranges to have a SIGALRM delivered to this process after the specified number of wallclock 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 or more than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further.)
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, and starting from Perl 5.8 part of the standard distribution) 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''.

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 relation:

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''.

binmode FILEHANDLE, LAYER

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. Returns true on success, otherwise it returns undef and sets $! (errno).
If LAYER is omitted or specified as :raw the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that as desipite what may be implied in ``Programming Perl'' (the Camel) or elsewhere :raw is not the simply inverse of :crlf -- other layers which would affect binary nature of the stream are also disabled. See PerlIO, the perlrun manpage and the discussion about the PERLIO environment variable.
Ibinmode() function is described as ``DISCIPLINE'' in ``Programming Perl, 3rd Edition''. However, since the publishing of this book, by many known as ``Camel III'', the consensus of the naming of this functionality has moved from ``discipline'' to ``layer''. All documentation of this version of Perl therefore refers to ``layers'' rather than to ``disciplines''. Now back to the regularly scheduled documentation...>
On some systems (in general, DOS and Windows-based 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 (like for example images).
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the file handle. When LAYER is present using binmode on text file makes sense.
To mark FILEHANDLE as UTF-8, use :utf8 .
The :bytes , :crlf , and :utf8 , and any other directives of the form :..., are called I/O layers. The open pragma can be used to establish default I/O layers. See the open manpage .
In general, binmode() should be called after open() but before any I/O is done on the filehandle. Calling binmode() will normally flush any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the :encoding layer that changes the default character encoding of the handle, see the open manpage . The :encoding layer sometimes needs to be called in mid-stream, and it doesn't flush the stream.
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, all variants of Unix, and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on Mac OS and LINE FEED on Unix and most VMS files). In other systems like OS/2, 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 \cM\cJ. That means that, if you don't use binmode() on these systems, \cM\cJ sequences on disk will be converted to \n on input, and any \n in your program will be converted back to \cM\cJ 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 the perlport manpage for more details). See the $/ and $\ variables in the perlvar manpage 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 the perltoot manpage and the perlobj manpage 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''.

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

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.

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.) $subroutine may also be (unknown) if this particular subroutine happens to have been deleted from the symbol table. $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 C<< N > 1 >>. 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}. (Under VMS, the variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) 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.

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

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 the perlvar manpage ) chomp() won't remove anything. If VARIABLE is omitted, it chomps $_ . Example:

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 an assignment:

If you chomp a list, each element is chomped, and the total number of characters removed is returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because chomp $cwd = `pwd`; is interpreted as (chomp $cwd) = `pwd`;, rather than as chomp( $cwd = `pwd` ) which you might expect. Similarly, chomp $a , $b is interpreted as chomp( $a ), $b rather than as chomp( $a , $b ) .

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) .
See also chomp .

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.

Here's an example that looks up nonnumeric uids in the passwd file:

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 this condition this way:

chr NUMBER

chr

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. Note that characters from 127 to 255 (inclusive) are by default not encoded in Unicode for backward compatibility reasons (but see the encoding manpage ).
For the reverse, use ord . See the perlunicode manpage and the encoding manpage 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 IO buffers are successfully flushed 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.
Example:

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''.

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.

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 relation:

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 (in other words, the crypt() is a one-way hash 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 C$plain, $crypted) eq $crypted>). This allows your code to work with the standard crypt and with more exotic implementations. In other words, do not assume anything about the returned string itself, or how many bytes in the encrypted string matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set [./0-9A-Za-z], and only the first eight bytes of the encrypted string mattered, but alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-UNIX platforms may produce different strings.
When choosing a new salt create a random two character string whose characters come from the set [./0-9A-Za-z] (like CHere's an example that makes sure that whoever runs this program knows their own password:

Of course, typing in your own password to whoever asks you for it is unwise.
The the crypt manpage 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.
If using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the string back to an eight-bit byte string before calling crypt() (on that copy). If that works, good. If not, crypt() dies with Wide character in crypt.

dbmclose HASH

[This function has been largely superseded by the untie function.]
Breaks the binding between a DBM file and a hash.

dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the tie function.]
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 DBM files. Example:

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() :

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 ``0'', 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(&func) to check whether subroutine &func has ever been defined. The return value is unaffected by any forward declarations of &foo. 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 the perlsub manpage .
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 a simple test for size:

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.
Examples:

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 tie d 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 %HASH and @ARRAY:

And so do these:

But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array element, hash slice, or array slice lookup:

die LIST

Outside an eval , prints the value of LIST to STDERR and exits with the current value of $! (errno). If $! is 0, exits with the value of C<<< ($? >> 8) >>> (backtick `command` status). If C<<< ($? >> 8) >>> 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.
Equivalent examples:

If the last element of LIST 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 $. . See ``$/'' and ``$.''.
Hint: sometimes appending ``, stopped'' to your message will cause it to make better sense when the string ``at foo line 123'' is appended. Suppose you are running script ``canasta''.

produce, respectively

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 ``\t...propagated''. This is useful for propagating exceptions:

If LIST is empty and $@ contains an object reference that has a PROPAGATE method, that method will be called with additional file and line number parameters. The return value replaces the value in $@ . ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>> were called.
If $@ is empty then the string ``Died'' 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. Here's an example:

Because perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See the overload manpage 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} 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

as the first line of the handler (see $^ S). 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 the perlsyn manpage for alternative strategies.

do SUBROUTINE(LIST)

A deprecated form of subroutine call. See the perlsub manpage .

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.

is just like

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 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 way:

dump LABEL

dump

This function causes an immediate core dump. See also the -u command-line switch in the perlrun manpage , 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. That's why you should now invoke it as CORE:: dump(), if you don't want to be warned against a possible typo.
If you're looking to use the dump manpage to speed up your program, consider generating bytecode or native C code as described in the perlcc manpage . If you're just trying to accelerate a CGI script, consider using the mod_perl extension to Apache, or the CPAN module, CGI::Fast. 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 following code will work:

The following prints out your environment like the printenv(1) program, only in a different order:

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 C<< <> >> operator. Since C<< <> >> isn't explicitly opened, as a normal filehandle is, an eof() before C<< <> >> has been used will cause @ARGV to be examined to determine if input is available. Similarly, an eof() after C<< <> >> has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV , will read input from STDIN; see ``I/O Operators''.
In a C<< 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 file. Examples:

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, or turn off warnings inside the BLOCK or EXPR using no warnings 'all' >. See warn , the perlvar manpage , the warnings manpage and the perllexwarn manpage .
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 still returned in $@ . Examples:

Due to the current arguably broken state of __DIE__ hooks, when using the eval{} form as an exception trap in libraries, you may wish not to trigger any __DIE__ hooks that user code may have installed. You can use the local $SIG{__DIE__} construct for this purpose, as shown in this example:

This is especially significant, given that __DIE__ hooks can call die again, which has the effect of changing their error messages:

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 remember what's being looked at when:

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 to avoid the warning:

If there is more than one argument in LIST, or if LIST is an array with more than one value, calls execvp(3) with the arguments in LIST. If there is only one scalar argument or an array with one element in it, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp, which is more efficient. Examples:

If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an ``indirect object'' (without a comma) in front of the LIST. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:

or, more directly,

When the arguments get executed via the system shell, results will be subject to its quirks and capabilities. See ```STRING`'' 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.

The first version, the one without the indirect object, ran the echo program, passing it ``surprise'' 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 the perlport manpage ). 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.

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 the perlsub manpage .

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:

Although the deepest nested array or hash will not spring into existence just because its existence was tested, any intervening ones will. Thus C<< $ref->{``A''} >> and C<< $ref->{``A''}->{``B''} >> will spring into existence due to the existence test for the $key element above. This happens anywhere the arrow operator is used, including even:

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'' 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.

exit EXPR

Evaluates EXPR and exits immediately with that value. Example:

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 the perlmod manpage 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

first to get the correct constant definitions. Argument processing and value return works just like ioctl below. For example:

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 ``0 but true'' 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 the same underlying descriptor:

(Filehandles connected to memory objects via new features of open may return undefined even though they are open.)

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 the perlport manpage , 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 that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to use LOCK_SH and requires that it be open with write intent to use LOCK_EX.
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.

open(MBOX, ``>>/usr/spool/mail/$ENV{'USER'}'') or die ``Can't open mailbox: $!''; lock(); print MBOX $msg,"\n\n"; unlock();

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 the perlport manpage ). 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 ``IGNORE''. See also the perlipc manpage 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 function. For example:

format Something = Test: