PERLFUNC
Section: Perl Programmers Reference Guide (1)Updated: 2009-08-12
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NAME
perlfunc - Perl builtin functionsDESCRIPTION
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. Commas should separate elements of the LIST.
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//", "qq//", "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//", "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", "say", "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
- "-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink", "rename", "rmdir", "stat", "symlink", "sysopen", "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 switch
-
"break", "continue", "given", "when", "default"
(These are only available if you enable the ``switch'' feature. See feature and ``Switch statements'' in perlsyn.)
- Keywords related to scoping
-
"caller", "import", "local", "my", "our", "state", "package",
"use"
("state" is only available if the ``state'' feature is enabled. See feature.)
- Miscellaneous functions
- "defined", "dump", "eval", "formline", "local", "my", "our", "reset", "scalar", "state", "undef", "wantarray"
- Functions for processes and process groups
- "alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx//", "setpgrp", "setpriority", "sleep", "system", "times", "wait", "waitpid"
- Keywords related to perl modules
- "do", "import", "no", "package", "require", "use"
- Keywords related to classes and object-orientation
- "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", "break", "chomp", "chr", "continue", "default",
"exists", "formline", "given", "glob", "import", "lc", "lcfirst",
"lock", "map", "my", "no", "our", "prototype", "qr//", "qw//", "qx//",
"readline", "readpipe", "ref", "sub"*, "sysopen", "tie", "tied", "uc",
"ucfirst", "untie", "use", "when"
* - "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", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp", "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 DIRHANDLE
- -X
-
A file test, where X is one of the letters listed below. This unary
operator takes one argument, either a filename, a filehandle, or a dirhandle,
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. The
operator may be any of:
-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 $_; # ignore specials #... }
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file: for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results than the bare stat() mode bits. When under the "use filetest 'access'" the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the "-x" and "-X" may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Note also that, due to the implementation of "use filetest 'access'", the "_" special filehandle won't cache the results of the file tests when this pragma is in effect. Read the documentation for the "filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($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 an "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 _;
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file test operators, in a way that "-f -w -x $file" is equivalent to "-x $file && -w _ && -f _". (This is only syntax fancy: if you use the return value of "-f $file" as an argument to another filetest operator, no special magic will happen.)
- 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 wallclock seconds has 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, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides ualarm(). You may also 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. See perlfaq8 for details.
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 "alarm\n" }; # NB: \n required alarm $timeout; $nread = sysread SOCKET, $buffer, $size; alarm 0; }; if ($@) { die unless $@ eq "alarm\n"; # propagate unexpected errors # timed out } else { # didn't }
For more information see perlipc.
- atan2 Y,X
-
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3) manpage for more information.
- 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, 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).
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. Also, people can set their I/O to be by default UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data, 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 a text file makes sense.
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, despite what may be implied in ``Programming Perl'' (the Camel) or elsewhere, ":raw" is not simply the inverse of ":crlf" --- other layers which would affect the binary nature of the stream are also disabled. See PerlIO, perlrun and the discussion about the PERLIO environment variable.
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 open.
The LAYER parameter of the binmode() 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...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(utf8)". ":utf8" just marks the data as UTF-8 without further checking, while ":encoding(utf8)" checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.
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 open. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. The ":encoding" also implicitly pushes on top of itself the ":utf8" layer because internally Perl will operate on UTF-8 encoded Unicode characters.
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 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 a derived class might inherit the function doing the blessing.
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. 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.
- break
-
Break out of a "given()" block.
This keyword is enabled by the ``switch'' feature: see feature for more information.
- 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
# 0 1 2 ($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
# 0 1 2 3 4 ($package, $filename, $line, $subroutine, $hasargs, # 5 6 7 8 9 10 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) = caller($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, $subroutine 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.
$hinthash is a reference to a hash containing the value of "%^H" when the caller was compiled, or "undef" if "%^H" was empty. Do not modify the values of this hash, as they are the actual values stored in the optree.
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 > 1". In particular, @DB::args might have information from the previous time "caller" was called.
- chdir EXPR
- chdir FILEHANDLE
- chdir DIRHANDLE
- chdir
-
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".
On systems that support fchdir, you might pass a file handle or directory handle as argument. On systems that don't support fchdir, passing handles produces a fatal error at run time.
- 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 be 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 # --w----r-T $mode = '0644'; chmod oct($mode), 'foo'; # this is better $mode = 0644; chmod $mode, 'foo'; # this is best
On systems that support fchmod, you might pass file handles among the files. On systems that don't support fchmod, passing file handles produces a fatal error at run time. The file handles must be passed as globs or references to be recognized. Barewords are considered file names.
open(my $fh, "<", "foo"); my $perm = (stat $fh)[2] & 07777; chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the Fcntl module:
use Fcntl ':mode'; chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; # This is identical to the chmod 0755 of the above example.
- 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 chomps $_. Example:
while (<>) { chomp; # avoid \n on last field @array = split(/:/); # ... }
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`); chomp($answer = <STDIN>);
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.
$cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames;
On systems that support fchown, you might pass file handles among the files. On systems that don't support fchown, passing file handles produces a fatal error at run time. The file handles must be passed as globs or references to be recognized. Barewords are considered file names.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: "; chomp($user = <STDIN>); print "Files: "; chomp($pattern = <STDIN>); ($login,$pass,$uid,$gid) = getpwnam($user) or die "$user not in passwd file"; @ary = glob($pattern); # expand filenames chown $uid, $gid, @ary;
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:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); $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 "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face.
Negative values give the Unicode replacement character (chr(0xfffd)), except under the bytes pragma, where low eight bits of the value (truncated to an integer) are used.
If NUMBER is omitted, uses $_.
For the reverse, use ``ord''.
Note that characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8 for backward compatibility reasons.
See perlunicode for more about Unicode.
- 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 $_.
- close FILEHANDLE
- close
-
Closes the file or pipe associated with the file handle, flushes the IO
buffers, and closes the system file descriptor. Returns true if those
operations have succeeded and if no error was reported by any PerlIO
layer. Closes the currently selected filehandle if the argument is
omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it, because "open" will close it for you. (See "open".) However, an explicit "close" on an input file resets the line counter ($.), 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 $? and "${^CHILD_ERROR_NATIVE}".
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:
open(OUTPUT, '|sort >foo') # pipe to sort or die "Can't start sort: $!"; #... # print stuff to output close OUTPUT # wait for sort to finish or warn $! ? "Error closing sort pipe: $!" : "Exit status $? from sort"; open(INPUT, 'foo') # get sort's results or die "Can't open 'foo' for input: $!";
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.
- 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
- continue
-
"continue" is 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) { ### redo always comes here do_something; } continue { ### next always comes here do_something_else; # then back the top to re-check EXPR } ### last always comes here
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.
If the ``switch'' feature is enabled, "continue" is also a function that will break out of the current "when" or "default" block, and fall through to the next case. See feature and ``Switch statements'' in perlsyn for more information.
- 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:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
- crypt PLAINTEXT,SALT
-
Creates a digest string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has not
been extirpated as a potential munition).
crypt() is a one-way hash function. The PLAINTEXT and SALT is turned into a short string, called a digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT from the hash. Small changes in the PLAINTEXT or SALT will result in large changes in the digest.
There is no decrypt function. This function isn't all that useful for cryptography (for that, look for Crypt modules on your nearby CPAN mirror) and the name ``crypt'' is a bit of a misnomer. Instead it is primarily used to check if two pieces of text are the same without having to transmit or store the text itself. An example is checking if a correct password is given. The digest of the password is stored, not the password itself. The user types in a password that is crypt()'d with the same salt as the stored digest. If the two digests match the password is correct.
When verifying an existing digest string you should use the digest as the salt (like "crypt($plain, $digest) eq $digest"). The SALT used to create the digest is visible as part of the digest. This ensures crypt() will hash the new string with the same salt as the digest. 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 digest 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 PLAINTEXT 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 "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this program knows their password:
$pwd = (getpwuid($<))[1]; system "stty -echo"; print "Password: "; chomp($word = <STDIN>); print "\n"; system "stty echo"; if (crypt($word, $pwd) ne $pwd) { die "Sorry...\n"; } else { print "ok\n"; }
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for hashing large quantities of data, not least of all because you can't get the information back. Look at the Digest module for more robust algorithms.
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:
# print out history file offsets dbmopen(%HIST,'/usr/lib/news/history',0666); while (($key,$val) = each %HIST) { print $key, ' = ', unpack('L',$val), "\n"; } dbmclose(%HIST);
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():
use DB_File; dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") or die "Can't open netscape history file: $!";
- 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 &func. 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 a simple test for size:
if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" }
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:
print if defined $switch{'D'}; print "$val\n" while defined($val = pop(@ary)); die "Can't readlink $sym: $!" unless defined($value = readlink $sym); sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } $debugging = 0 unless defined $debugging;
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
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact that it matched ``nothing''. It didn't really fail to match anything. 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 a list with the same number of elements as the number of elements for which deletion was attempted. Each element of that list consists of either the value of the element deleted, or the undefined value. In scalar context, this means that you get the value of the last element deleted (or the undefined value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33); $scalar = delete $hash{foo}; # $scalar is 11 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
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. Also, deleting array elements in the middle of an array will not shift the index of the elements after them down. Use splice() for that. See ``exists''.
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) { delete $HASH{$key}; } foreach $index (0 .. $#ARRAY) { delete $ARRAY[$index]; }
And so do these:
delete @HASH{keys %HASH}; delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH undef %HASH; # forget %HASH ever existed @ARRAY = (); # completely empty @ARRAY undef @ARRAY; # forget @ARRAY ever existed
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:
delete $ref->[$x][$y]{$key}; delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; delete $ref->[$x][$y][$index]; delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
- die LIST
-
Outside an "eval", prints the value of LIST to "STDERR" and
exits with the current value of $! (errno). If $! is 0,
exits with the value of "($? >> 8)" (backtick `command`
status). If "($? >> 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:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
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 $.. See ``$/'' in perlvar and ``$.'' in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended.
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:
eval { ... }; die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a "PROPAGATE" method, that method will be called with additional file and line number parameters. The return value replaces the value in<EMAIL_ADDRESS>i.e. as if "$@ = 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. Because $@ is a global variable, and eval() may be used within object implementations, care must be taken that analyzing the error object doesn't replace the reference in the global variable. The easiest solution is to make a local copy of the reference before doing other manipulations. Here's an example:
use Scalar::Util 'blessed'; eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; if (my $ev_err = $@) { if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { # handle Some::Module::Exception } else { # handle all other possible exceptions } }
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 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 the "while" or
"until" 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)
- This form of subroutine call is deprecated. See perlsub.
- do EXPR
-
Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error messages, searches the @INC directories, 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<EMAIL_ADDRESS>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:
# read in config files: system first, then user for $file ("/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") { unless ($return = do $file) { warn "couldn't parse $file: $@" if $@; warn "couldn't do $file: $!" unless defined $return; warn "couldn't run $file" unless $return; } }
- 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, mostly because it's very hard to convert a core file into an executable. That's why you should now invoke it as "CORE::dump()", if you don't want to be warned against a possible typo.
- 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. Since Perl 5.8.2 the ordering can be different even between different runs of Perl for security reasons (see ``Algorithmic Complexity Attacks'' in perlsec).
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:
while (($key, $value) = each %hash) { print $key, "\n"; delete $hash{$key}; # This is safe }
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) { print "$key=$value\n"; }
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 "ungetc"s 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. Similarly, an "eof()" after "<>" 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 perlop.
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 file. Examples:
# reset line numbering on each input file while (<>) { next if /^\s*#/; # skip comments print "$.\t$_"; } continue { close ARGV if eof; # Not eof()! } # insert dashes just before last line of last file while (<>) { if (eof()) { # check for end of last file print "--------------\n"; } print; last if eof(); # needed if we're reading from a terminal }
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
- eval
-
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, "eval" returns an undefined value in scalar context or an empty list in list context, 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<EMAIL_ADDRESS>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'', perlvar, warnings and perllexwarn.
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 you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.
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<EMAIL_ADDRESS>Examples:
# make divide-by-zero nonfatal eval { $answer = $a / $b; }; warn $@ if $@; # same thing, but less efficient eval '$answer = $a / $b'; warn $@ if $@; # a compile-time error eval { $answer = }; # WRONG # a run-time error eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in libraries does have some issues. Due to the current arguably broken state of "__DIE__" hooks, 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:
# a very private exception trap for divide-by-zero eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages { local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; eval { die "foo lives here" }; print $@ if $@; # prints "bar lives here" }
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:
eval $x; # CASE 1 eval "$x"; # CASE 2 eval '$x'; # CASE 3 eval { $x }; # CASE 4 eval "\$$x++"; # CASE 5 $$x++; # CASE 6
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.
The assignment to $@ occurs before restoration of localised variables, which means a temporary is required if you want to mask some but not all errors:
# alter $@ on nefarious repugnancy only { my $e; { local $@; # protect existing $@ eval { test_repugnancy() }; # $@ =~ /nefarious/ and die $@; # DOES NOT WORK $@ =~ /nefarious/ and $e = $@; } die $e if defined $e }
"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.
Note that as a very special case, an "eval ''" executed within the "DB" package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger.
- 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:
exec ('foo') or print STDERR "couldn't exec foo: $!"; { exec ('foo') }; print STDERR "couldn't exec foo: $!";
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:
exec '/bin/echo', 'Your arguments are: ', @ARGV; exec "sort $outfile | uniq";
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:
$shell = '/bin/csh'; 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 = ( "echo surprise" ); exec @args; # subject to shell escapes # if @args == 1 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 "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 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.
print "Exists\n" if exists $hash{$key}; print "Defined\n" if defined $hash{$key}; print "True\n" if $hash{$key}; print "Exists\n" if exists $array[$index]; print "Defined\n" if defined $array[$index]; print "True\n" if $array[$index];
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.
print "Exists\n" if exists &subroutine; print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { } if (exists $hash{A}{B}{$key}) { } if (exists $ref->{A}->{B}->[$ix]) { } if (exists $hash{A}{B}[$ix]) { } if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into existence just because its existence was tested, any intervening ones will. Thus "$ref->{"A"}" and "$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:
undef $ref; if (exists $ref->{"Some key"}) { } print $ref; # prints HASH(0x80d3d5c)
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.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK exists &sub(); # Error
- exit EXPR
- exit
-
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>; exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values for EXPR are 0 for success and 1 for error; other values are subject to interpretation depending on the environment in which the Perl program is running. For example, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap whatever error happened. Use "die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first, but these "END" routines may not themselves abort the exit. Likewise any object destructors that need to be called are called before the real exit. If this is a problem, you can call "POSIX:_exit($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($_)".
- fcntl FILEHANDLE,FUNCTION,SCALAR
-
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value return works just like "ioctl" below. For example:
use Fcntl; fcntl($filehandle, F_GETFL, $packed_return_buffer) or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". 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.
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 "Can't get flags for the socket: $!\n"; $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) or die "Can't set flags for the socket: $!\n";
- 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:
if (fileno(THIS) == fileno(THAT)) { print "THIS and THAT are dups\n"; }
(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 programs that do not also use "flock" may modify files locked with "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 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.

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 "IGNORE". 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" function. For
example:
format Something = Test: @<<<<<<<< @||||| @>>>>> $str, $%, '$' . int($num) . $str = "widget"; $num = $cost/$quantity; $~ = 'Something'; write;
See perlform for many details and examples.
- formline PICTURE,LIST
-
This is an internal function used by "format"s, 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. You could also read $^A
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 (in
the latter case $! is set). If FILEHANDLE is omitted, reads from
STDIN. This is not particularly efficient. However, it cannot be
used by itself to fetch single characters without waiting for the user
to hit enter. For that, try something more like:
if ($BSD_STYLE) { system "stty cbreak </dev/tty >/dev/tty 2>&1"; } else { system "stty", '-icanon', 'eol', "\001"; } $key = getc(STDIN); if ($BSD_STYLE) { system "stty -cbreak </dev/tty >/dev/tty 2>&1"; } else { system "stty", 'icanon', 'eol', '^@'; # ASCII null } print "\n";
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
-
This 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".
$login = getlogin || getpwuid($<) || "Kilroy";
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.
Note for Linux users: on Linux, the C functions "getpid()" and "getppid()" return different values from different threads. In order to be portable, this behavior is not reflected by the perl-level function "getppid()", that returns a consistent value across threads. If you want to call the underlying "getppid()", you may use the CPAN module "Linux::Pid".
- 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 follows:
($name,$passwd,$uid,$gid, $quota,$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 undefined value.) For example:
$uid = getpwnam($name); $name = getpwuid($num); $name = getpwent(); $gid = getgrnam($name); $name = getgrgid($num); $name = getgrent(); #etc.
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 that 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 $? if the function call fails. The @addrs value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket; $iaddr = inet_aton("127.1"); # or whatever address $name = gethostbyaddr($iaddr, AF_INET); # or going the other way $straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket; $packed_ip = gethostbyname("www.perl.org"); if (defined $packed_ip) { $ip_address = inet_ntoa($packed_ip); }
Make sure <gethostbyname()> is called in SCALAR context and that its return value is checked for definedness.
If you get tired of remembering which element of the return list contains which return value, by-name interfaces are provided in standard modules: "File::stat", "Net::hostent", "Net::netent", "Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime", and "User::grent". These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For example:
use File::stat; use User::pwent; $is_his = (stat($filename)->uid == pwent($whoever)->uid);
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 "Connect to %s [%s]\n", scalar gethostbyaddr($myaddr, AF_INET), inet_ntoa($myaddr);
- getsockopt SOCKET,LEVEL,OPTNAME
-
Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
Options may exist at multiple protocol levels depending on the socket
type, but at least the uppermost socket level SOL_SOCKET (defined in the
"Socket" module) will exist. To query options at another level the
protocol number of the appropriate protocol controlling the option
should be supplied. For example, to indicate that an option is to be
interpreted by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using getprotobyname.
The call returns a packed string representing the requested socket option, or "undef" if there is an error (the error reason will be in $!). What exactly is in the packed string depends in the LEVEL and OPTNAME, consult your system documentation for details. A very common case however is that the option is an integer, in which case the result will be a packed integer which you can decode using unpack with the "i" (or "I") format.
An example testing if Nagle's algorithm is turned on on a socket:
use Socket qw(:all); defined(my $tcp = getprotobyname("tcp")) or die "Could not determine the protocol number for tcp"; # my $tcp = IPPROTO_TCP; # Alternative my $packed = getsockopt($socket, $tcp, TCP_NODELAY) or die "Could not query TCP_NODELAY socket option: $!"; my $nodelay = unpack("I", $packed); print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
- glob EXPR
- glob
-
In list context, returns a (possibly empty) list of filename expansions on
the value of EXPR such as the standard Unix shell /bin/csh would do. In
scalar context, glob iterates through such filename expansions, returning
undef when the list is exhausted. This is the internal function
implementing the "<*.c>" operator, but you can use it directly. If
EXPR is omitted, $_ is used. The "<*.c>" operator is discussed in
more detail in ``I/O Operators'' in perlop.
Note that "glob" will split its arguments on whitespace, treating each segment as separate pattern. As such, "glob('*.c *.h')" would match all files with a .c or .h extension. The expression "glob('.* *')" would match all files in the current working directory.
Beginning with v5.6.0, this operator is implemented using the standard "File::Glob" extension. See File::Glob for details, including "bsd_glob" which does not treat whitespace as a pattern separator.
- gmtime EXPR
- gmtime
-
Works just like localtime but the returned values are
localized for the standard Greenwich time zone.
Note: when called in list context, $isdst, the last value returned by gmtime is always 0. There is no Daylight Saving Time in GMT.
See ``gmtime'' in perlport for portability concerns.
- goto LABEL
- goto EXPR
- goto &NAME
-
The "goto-LABEL" form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a "foreach" loop. It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to "sort".
It can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as "last" or "die". The author of Perl has never felt the
need to use this form of "goto" (in Perl, that is---C is another matter).
(The difference being that C does not offer named loops combined with
loop control. Perl does, and this replaces most structured uses of "goto"
in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be resolved dynamically. This allows for computed "goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" 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 exits the current subroutine (losing any changes set by local()) and immediately calls in its place the named subroutine using the current value of @_. 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 that 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.
If $_ is lexical in the scope where the "grep" appears (because it has been declared with "my $_") then, in addition to being locally aliased to the list elements, $_ keeps being lexical inside the block; i.e. it can't be seen from the outside, avoiding any potential side-effects.
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. Leading whitespace is not stripped, unlike oct(). To present something as hex, look into ``printf'', ``sprintf'', or ``unpack''.
- import LIST
- 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. POSITION before the beginning of the string or after its end is treated as if it were the beginning or the end, respectively. POSITION and the return value are based at 0 (or whatever you've set the $[ variable to---but don't do that). If the substring is not found, "index" returns one less than the base, ordinarily "-1".
- 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 -268.99999999999994315658 instead. Usually, the "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 "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/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 <sys/ioctl.h>. (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 as follows:
if OS returns: then Perl returns: -1 undefined value 0 string "0 but true" anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
$retval = ioctl(...) || -1; printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about improper numeric conversions.
- 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. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
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). Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see ``Algorithmic Complexity Attacks'' in perlsec).
As a side effect, calling keys() resets the HASH's internal iterator (see ``each''). In particular, calling keys() in void context resets the iterator with no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV; @values = values %ENV; while (@keys) { print pop(@keys), '=', pop(@values), "\n"; }
or how about sorted by key:
foreach $key (sort(keys %ENV)) { print $key, '=', $ENV{$key}, "\n"; }
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 values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { printf "%4d %s\n", $hash{$key}, $key; }
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, $child2; kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process, but the kill(2) system call will check whether it's possible to send a signal to it (that means, to be brief, that the process is owned by the same user, or we are the super-user). This is a useful way to check that a child process is alive (even if only as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes.
See ``Signals'' in perlipc for more details.
- last LABEL
- last
-
The "last" command is like the "break" statement in C (as used in
loops); it immediately exits the loop in question. If the LABEL is
omitted, the command refers to the innermost enclosing loop. The
"continue" block, if any, is not executed:
LINE: while (<STDIN>) { last LINE if /^$/; # exit when done with header #... }
"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 a 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 perlunicode for more details about locale and Unicode support.
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 and perlunicode for more
details about locale and Unicode support.
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 $_. Note that this cannot be used on
an entire array or hash to find out how many elements these have.
For that, use "scalar @array" and "scalar keys %hash" respectively.
Note the characters: if the EXPR is in Unicode, you will get the number of characters, not the number of bytes. To get the length of the internal string in bytes, use "bytes::length(EXPR)", see bytes. Note that the internal encoding is variable, and the number of bytes usually meaningless. To get the number of bytes that the string would have when encoded as UTF-8, use "length(Encoding::encode_utf8(EXPR))".
- link OLDFILE,NEWFILE
- Creates a new filename linked to the old filename. Returns true for success, false otherwise.
- listen SOCKET,QUEUESIZE
- Does the same thing that the listen system call does. Returns true if it succeeded, false otherwise. See the example in ``Sockets: Client/Server Communication'' in perlipc.
- local EXPR
-
You really probably want to be using "my" instead, because "local" isn't
what most people think of as ``local''. See
``Private Variables via my()'' in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See ``Temporary Values via local()'' in perlsub for details, including issues with tied arrays and hashes.
- localtime EXPR
- localtime
-
Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone. Typically used as
follows:
# 0 1 2 3 4 5 6 7 8 ($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 $mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December. This makes it easy to get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); print "$abbr[$mon] $mday"; # $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just the last two digits of the year. That is, $year is 123 in year 2023. The proper way to get a complete 4-digit year is simply:
$year += 1900;
Otherwise you create non-Y2K-compliant programs---and you wouldn't want to do that, would you?
To get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
$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 Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current time (as returned by time(3)).
In scalar context, "localtime()" returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a Perl builtin. For GMT instead of local time use the ``gmtime'' builtin. See also the "Time::Local" module (to convert the second, minutes, hours, ... back to the integer value returned by time()), and the POSIX module's strftime(3) and mktime(3) functions.
To get somewhat similar but locale dependent date strings, set up your locale environment variables appropriately (please see perllocale) and try for example:
use POSIX qw(strftime); $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; # or for GMT formatted appropriately for your locale: $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
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.
See ``localtime'' in perlport for portability concerns.
The Time::gmtime and Time::localtime modules provides a convenient, by-name access mechanism to the gmtime() and localtime() functions, respectively.
For a comprehensive date and time representation look at the DateTime module on CPAN.
- lock THING
-
This function places an advisory lock on a shared variable, or referenced
object contained in THING until the lock goes out of scope.
lock() is a ``weak keyword'' : this means that if you've defined a function by this name (before any calls to it), that function will be called instead. (However, if you've said "use threads", lock() is always a keyword.) See threads.
- 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 N. For example:
sub log10 { my $n = shift; return log($n)/log(10); }
See also ``exp'' for the inverse operation.
- 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. For much more detailed
information, please see the documentation for "stat".
If EXPR is omitted, stats $_.
- m//
- The match operator. See ``Regexp Quote-Like Operators'' in perlop.
- map BLOCK LIST
- map EXPR,LIST
-
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value composed of the
results of each such evaluation. In scalar context, returns the
total number of elements so generated. Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero, one, or
more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
%hash = (); foreach (@array) { $hash{get_a_key_for($_)} = $_; }
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.
If $_ is lexical in the scope where the "map" appears (because it has been declared with "my $_"), then, in addition to being locally aliased to the list elements, $_ keeps being lexical inside the block; that is, it can't be seen from the outside, avoiding any potential side-effects.
"{" starts both hash references and blocks, so "map { ..." could be either the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look ahead for the closing "}" it has to take a guess at which its dealing with based what it finds just after the "{". Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the "}" and encounters the missing (or unexpected) comma. The syntax error will be reported close to the "}" but you'll need to change something near the "{" such as using a unary "+" to give perl some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right %hash = map { ("\L$_", 1) } @array # this also works %hash = map { lc($_), 1 } @array # as does this. %hash = map +( lc($_), 1 ), @array # this is EXPR and works! %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only 1 entry.
- mkdir FILENAME,MASK
- mkdir FILENAME
- mkdir
-
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. If omitted, FILENAME defaults
to $_.
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.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.
In order to recursively create a directory structure look at the "mkpath" function of the File::Path module.
- 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 that will hold the returned "msqid_ds" structure. Returns like "ioctl": the undefined value for error, "0 but true" 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. This packing may be opened with "unpack("l! a*")". Taints the variable. Returns true if successful, or false if there is an error. See also ``SysV IPC'' in perlipc, "IPC::SysV", and "IPC::SysV::Msg" documentation.
- msgsnd ID,MSG,FLAGS
- Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the native long integer message type, and be followed by the length of the actual message, and finally the message itself. This kind of packing can be achieved with "pack("l! a*", $type, $message)". Returns true if successful, or false if there is an error. See also "IPC::SysV" and "IPC::SysV::Msg" documentation.
- my EXPR
- my TYPE EXPR
- my EXPR : ATTRS
- my TYPE EXPR : ATTRS
-
A "my" declares the listed variables to be local (lexically) to the
enclosing block, file, or "eval". If more than one value is listed,
the list must be placed in parentheses.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See ``Private Variables via my()'' in perlsub for details, and fields, attributes, and Attribute::Handlers.
- next LABEL
- next
-
The "next" command is like the "continue" statement in C; it starts
the next iteration of the loop:
LINE: while (<STDIN>) { next LINE if /^#/; # discard comments #... }
Note that if there were a "continue" block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop.
"next" 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 "next" will exit such a block early.
See also ``continue'' for an illustration of how "last", "next", and "redo" work.
- no Module VERSION LIST
- no Module VERSION
- no Module LIST
- no Module
- no VERSION
- See the "use" function, of which "no" is the opposite.
- oct EXPR
- oct
-
Interprets EXPR as an octal string and returns the corresponding
value. (If EXPR happens to start off with "0x", interprets it as a
hex string. If EXPR starts off with "0b", it is interpreted as a
binary string. Leading whitespace is ignored in all three cases.)
The following will handle decimal, binary, octal, and hex in the standard
Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf() or printf():
$perms = (stat("filename"))[2] & 07777; $oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string such as 644 needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.)
- open FILEHANDLE,EXPR
- open FILEHANDLE,MODE,EXPR
- open FILEHANDLE,MODE,EXPR,LIST
- open FILEHANDLE,MODE,REFERENCE
- open FILEHANDLE
-
Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.
Simple examples to open a file for reading:
open(my $fh, '<', "input.txt") or die $!;
and for writing:
open(my $fh, '>', "output.txt") or die $!;
(The following is a comprehensive reference to open(): for a gentler introduction you may consider perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array or hash element) the variable is assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. (This is considered a symbolic reference, so "use strict 'refs'" should not be in effect.)
If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variables---those declared with "my"--will not work for this purpose; so if you're using "my", specify EXPR in your call to open.)
If three or more arguments are specified then the mode of opening and the file name are separate. If MODE is '<' or nothing, the file is opened for input. If MODE is '>', the file is truncated and opened for output, being created if necessary. If MODE is '>>', the file is opened for appending, again being created if necessary.
You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always preferred for read/write updates---the '+>' mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records. See the -i switch in perlrun for a better approach. The file is created with permissions of 0666 modified by the process' "umask" value.
These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
In the 2-arguments (and 1-argument) form of the call the mode and filename should be concatenated (in this order), possibly separated by spaces. It is possible to omit the mode in these forms if the mode is '<'.
If the filename begins with '|', the filename is interpreted as a command to which output is to be piped, and if the filename ends with a '|', the filename is interpreted as a command which pipes output to us. See ``Using open() for IPC'' in perlipc for more examples of this. (You are not allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and ``Bidirectional Communication with Another Process'' in perlipc for alternatives.)
For three or more arguments if MODE is '|-', the filename is interpreted as a command to which output is to be piped, and if MODE is '-|', the filename is interpreted as a command which pipes output to us. In the 2-arguments (and 1-argument) form one should replace dash ('-') with the command. See ``Using open() for IPC'' in perlipc for more examples of this. (You are not allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and ``Bidirectional Communication'' in perlipc for alternatives.)
In the three-or-more argument form of pipe opens, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of "open" with more than three arguments for non-pipe modes is not yet specified. Experimental ``layers'' may give extra LIST arguments meaning.
In the 2-arguments (and 1-argument) form opening '-' opens STDIN and opening '>-' opens STDOUT.
You may use the three-argument form of open to specify IO ``layers'' (sometimes also referred to as ``disciplines'') to be applied to the handle that affect how the input and output are processed (see open and PerlIO for more details). For example
open(my $fh, "<:encoding(UTF-8)", "file")
will open the UTF-8 encoded file containing Unicode characters, see perluniintro. Note that if layers are specified in the three-arg form then default layers stored in ${^OPEN} (see perlvar; usually set by the open pragma or the switch -CioD) are ignored.
Open returns nonzero upon success, the undefined value otherwise. If the "open" involved a pipe, the return value happens to be the pid of the subprocess.
If you're running Perl on a system that distinguishes between text files and binary files, then you should check out ``binmode'' for tips for dealing with this. The key distinction between systems that need "binmode" and those that don't is their text file formats. Systems like Unix, Mac OS, and Plan 9, which delimit lines with a single character, and which encode that character in C as "\n", do not need "binmode". The rest need it.
When opening a file, it's usually a bad idea to continue normal execution if the request failed, so "open" is frequently used in connection with "die". Even if "die" won't do what you want (say, in a CGI script, where you want to make a nicely formatted error message (but there are modules that can help with that problem)) you should always check the return value from opening a file. The infrequent exception is when working with an unopened filehandle is actually what you want to do.
As a special case the 3-arg form with a read/write mode and the third argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also using ``+<'' works for symmetry, but you really should consider writing something to the temporary file first. You will need to seek() to do the reading.
Since v5.8.0, perl has built using PerlIO by default. Unless you've changed this (i.e. Configure -Uuseperlio), you can open file handles to ``in memory'' files held in Perl scalars via:
open($fh, '>', \$variable) || ..
Though if you try to re-open "STDOUT" or "STDERR" as an ``in memory'' file, you have to close it first:
close STDOUT; open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
Examples:
$ARTICLE = 100; open ARTICLE or die "Can't find article $ARTICLE: $!\n"; while (<ARTICLE>) {... open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) # if the open fails, output is discarded open(my $dbase, '+<', 'dbase.mine') # open for update or die "Can't open 'dbase.mine' for update: $!"; open(my $dbase, '+<dbase.mine') # ditto or die "Can't open 'dbase.mine' for update: $!"; open(ARTICLE, '-|', "caesar <$article") # decrypt article or die "Can't start caesar: $!"; open(ARTICLE, "caesar <$article |") # ditto or die "Can't start caesar: $!"; open(EXTRACT, "|sort >Tmp$$") # $$ is our process id or die "Can't start sort: $!"; # in memory files open(MEMORY,'>', \$var) or die "Can't open memory file: $!"; print MEMORY "foo!\n"; # output will end up in $var # process argument list of files along with any includes foreach $file (@ARGV) { process($file, 'fh00'); } sub process { my($filename, $input) = @_; $input++; # this is a string increment unless (open($input, $filename)) { print STDERR "Can't open $filename: $!\n"; return; } local $_; while (<$input>) { # note use of indirection if (/^#include "(.*)"/) { process($1, $input); next; } #... # whatever } }
See perliol for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify an EXPR beginning with '>&', in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>", and "+<". The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the 3-arg form then you can pass either a number, the name of a filehandle or the normal ``reference to a glob''.
Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using various methods:
#!/usr/bin/perl open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!"; open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; select STDERR; $| = 1; # make unbuffered select STDOUT; $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; print STDOUT "stdout 2\n"; print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then Perl will do an equivalent of C's "fdopen" of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using flock(). If you do just "open(A, '>>&B')", the filehandle A will not have the same file descriptor as B, and therefore flock(A) will not flock(B), and vice versa. But with "open(A, '>>&=B')" the filehandles will share the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl will be using the standard C libraries' fdopen() to implement the ``='' functionality. On many UNIX systems fdopen() fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is most often the default.
You can see whether Perl has been compiled with PerlIO or not by running "perl -V" and looking for "useperlio=" line. If "useperlio" is "define", you have PerlIO, otherwise you don't.
If you open a pipe on the command '-', i.e., either '|-' or '-|' with 2-arguments (or 1-argument) form of open(), then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use "defined($pid)" to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened---i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'"); open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); open(FOO, "cat -n '$file'|"); open(FOO, '-|', "cat -n '$file'"); open(FOO, '-|') || exec 'cat', '-n', $file; open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as ``list form'', which is not yet supported on all platforms. A good rule of thumb is that if your platform has true "fork()" (in other words, if your platform is UNIX) you can use the list form.
See ``Safe Pipe Opens'' in perlipc for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, 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.
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.
Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $? and "${^CHILD_ERROR_NATIVE}".
The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as ``magic open'', can often be used to good effect. A user could specify a filename of ``rsh cat file |'', or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#; open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form "rsh cat file |", but will not work on a filename which happens to have a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a ``real'' C "open" (see open(2) on your system), then you should use the "sysopen" function, which involves no such magic (but may use subtly different filemodes than Perl open(), which is mapped to C fopen()). This is another way to protect your filenames from interpretation. For example:
use IO::Handle; sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) or die "sysopen $path: $!"; $oldfh = select(HANDLE); $| = 1; select($oldfh); print HANDLE "stuff $$\n"; seek(HANDLE, 0, 0); print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package (or one of its subclasses, such as "IO::File" or "IO::Socket"), you can generate anonymous filehandles that have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope:
use IO::File; #... sub read_myfile_munged { my $ALL = shift; my $handle = IO::File->new; open($handle, "myfile") or die "myfile: $!"; $first = <$handle> or return (); # Automatically closed here. mung $first or die "mung failed"; # Or here. return $first, <$handle> if $ALL; # Or here. $first; # Or here. }
See ``seek'' for some details about mixing reading and writing.
- opendir DIRHANDLE,EXPR
-
Opens a directory named EXPR for processing by "readdir", "telldir",
"seekdir", "rewinddir", and "closedir". Returns true if successful.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
scalar variable (or array or hash element), the variable is assigned a
reference to a new anonymous dirhandle.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.
See example at "readdir".
- ord EXPR
- ord
-
Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
or Unicode) value of the first character of EXPR. If EXPR is omitted,
uses $_.
For the reverse, see ``chr''. See perlunicode for more about Unicode.
- our EXPR
- our TYPE EXPR
- our EXPR : ATTRS
- our TYPE EXPR : ATTRS
-
"our" associates a simple name with a package variable in the current
package for use within the current scope. When "use strict 'vars'" is in
effect, "our" lets you use declared global variables without qualifying
them with package names, within the lexical scope of the "our" declaration.
In this way "our" differs from "use vars", which is package scoped.
Unlike "my", which both allocates storage for a variable and associates a simple name with that storage for use within the current scope, "our" associates a simple name with a package variable in the current package, for use within the current scope. In other words, "our" has the same scoping rules as "my", but does not necessarily create a variable.
If more than one value is listed, the list must be placed in parentheses.
our $foo; our($bar, $baz);
An "our" declaration declares a global variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds:
package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the same lexical scope are allowed if they are in different packages. If they happen to be in the same package, Perl will emit warnings if you have asked for them, just like multiple "my" declarations. Unlike a second "my" declaration, which will bind the name to a fresh variable, a second "our" declaration in the same package, in the same scope, is merely redundant.
use warnings; package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; our $bar = 30; # declares $Bar::bar for rest of lexical scope print $bar; # prints 30 our $bar; # emits warning but has no other effect print $bar; # still prints 30
An "our" declaration may also have a list of attributes associated with it.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of "fields" pragma, and attributes are handled using the "attributes" pragma, or starting from Perl 5.8.0 also via the "Attribute::Handlers" module. See ``Private Variables via my()'' in perlsub for details, and fields, attributes, and Attribute::Handlers.
- pack TEMPLATE,LIST
-
Takes a LIST of values and converts it into a string using the rules
given by the TEMPLATE. The resulting string is the concatenation of
the converted values. Typically, each converted value looks
like its machine-level representation. For example, on 32-bit machines
an integer may be represented by a sequence of 4 bytes that will be
converted to a sequence of 4 characters.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows: