PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in(1,8) this section can serve as terms in(1,8) an expression.
They fall into two major categories: list operators and named(5,8) unary
operators. These differ in(1,8) their precedence relationship with a fol-
lowing comma. (See the precedence table in(1,8) 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 argu-
ments followed by a list, whereas gethostbyname() has four scalar argu-
ments.
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(1,8)
the list as if(3,n) each individual element were interpolated at that point
in(1,8) the list, forming a longer single-dimensional list value. Elements
of the LIST should be separated by commas.
Any function in(1,8) 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 opera-
tor 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(1,n) 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(1,8) 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(1,2,n)" and
"endpwent". For example, "time(1,2,n)+86_400" always means "time(1,2,n)() + 86_400".
For functions that can be used in(1,8) either a scalar or list context, non-
abortive failure is generally indicated in(1,8) a scalar context by return-
ing the undefined value, and in(1,8) 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(1,8) list context to its behavior in(1,8) scalar
context, or vice versa. It might do two totally different things.
Each operator and function decides which sort(1,3) of value it would be most
appropriate to return in(1,8) scalar context. Some operators return the
length of the list that would have been returned in(1,8) list context. Some
operators return the first value in(1,8) the list. Some operators return
the last value in(1,8) the list. Some operators return a count of success-
ful operations. In general, they do what you want, unless you want
consistency.
A named(5,8) array in(1,8) scalar context is quite different from what would at
first glance appear to be a list in(1,8) scalar context. You can't get a
list like "(1,2,3)" into being in(1,8) scalar context, because the compiler
knows the context at compile time. It would generate the scalar comma
operator there, not the list construction version(1,3,5) of the comma. That
means it was never a list to start with.
In general, functions in(1,8) Perl that serve as wrappers for system calls
of the same name (like chown(1,2)(2), fork(2), closedir(2), etc.) all return
true when they succeed and "undef" otherwise, as is usually mentioned
in(1,8) 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(7,n,1 builtins) the special $! vari-
able on failure. Other functions do not, except accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look(1,8,3 Search::Dict) like functions,
like some keywords and named(5,8) operators) arranged by category. Some
functions appear in(1,8) more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
"length", "oct", "ord", "pack(3,n,n pack-old)", "q/STRING/", "qq/STRING/",
"reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst",
"y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split(1,n)", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand(1,3)",
"sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join(1,n)", "map", "qw/STRING/", "reverse", "sort(1,3)", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close(2,7,n)", "closedir", "dbmclose", "dbmopen", "die",
"eof", "fileno", "flock(1,2)", "format", "getc", "print", "printf(1,3,1 builtins)",
"read(2,n,1 builtins)", "readdir(2,3)", "rewinddir", "seek", "seekdir", "select(2,7,2 select_tut)",
"syscall", "sysread", "sysseek", "syswrite", "tell", "telldir",
"truncate(2,7)", "warn", "write(1,2)"
Functions for fixed length data or records
"pack(3,n,n pack-old)", "read(2,n,1 builtins)", "syscall", "sysread", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod(1,2)", "chown(1,2)", "chroot(1,2)", "fcntl", "glob(1,3,7,n)",
"ioctl", "link(1,2)", "lstat", "mkdir(1,2)", "open(2,3,n)", "opendir", "readlink(1,2)",
"rename(1,2,n)", "rmdir(1,2)", "stat(1,2)", "symlink", "sysopen", "umask", "unlink(1,2)",
"utime"
Keywords related to the control flow of your perl program
"caller", "continue", "die", "do", "dump", "eval", "exit(3,n,1 builtins)", "goto",
"last", "next", "redo", "return", "sub", "wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our",
"reset(1,7,1 tput)", "scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm(1,2)", "exec(3,n,1 builtins)", "fork", "getpgrp", "getppid", "getpriority",
"kill(1,2,1 builtins)", "pipe(2,8)", "qx/STRING/", "setpgrp", "setpriority", "sleep(1,3)",
"system", "times", "wait", "waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
"untie", "use"
Low-level socket(2,7,n) functions
"accept(2,8)", "bind(2,n,1 builtins)", "connect", "getpeername(1,2)", "getsockname", "get-
sockopt", "listen(1,2,7)", "recv", "send(2,n)", "setsockopt", "shutdown(2,8)",
"socket(2,7,n)", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget",
"semop", "shmctl", "shmget", "shmread", "shmwrite"
Fetching user and group info(1,5,n)
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
"getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam", "getp-
wuid", "setgrent", "setpwent"
Fetching network info(1,5,n)
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
"gethostent", "getnetbyaddr", "getnetbyname", "getnetent", "getpro-
tobyname", "getprotobynumber", "getprotoent", "getservbyname",
"getservbyport", "getservent", "sethostent", "setnetent", "setpro-
toent", "setservent"
Time-related functions
"gmtime", "localtime", "time(1,2,n)", "times"
Functions new in(1,8) perl5
"abs", "bless", "chomp", "chr", "exists", "formline", "glob(1,3,7,n)",
"import", "lc", "lcfirst", "map", "my", "no", "our", "prototype",
"qx", "qw", "readline", "readpipe", "ref", "sub*", "sysopen",
"tie", "tied", "uc", "ucfirst", "untie", "use"
* - "sub" was a keyword in(1,8) perl4, but in(1,8) perl5 it is an operator,
which can be used in(1,8) expressions.
Functions obsoleted in(1,8) perl5
"dbmclose", "dbmopen"
Portability
Perl was born in(1,8) Unix and can therefore access(2,5) 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(1,2)", "chown(1,2)", "chroot(1,2)", "crypt", "dbmclose",
"dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endpro-
toent", "endpwent", "endservent", "exec(3,n,1 builtins)", "fcntl", "flock(1,2)", "fork",
"getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin",
"getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getprgp",
"getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpw-
nam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob(1,3,7,n)",
"ioctl", "kill(1,2,1 builtins)", "link(1,2)", "lstat", "msgctl", "msgget", "msgrcv",
"msgsnd", "open(2,3,n)", "pipe(2,8)", "readlink(1,2)", "rename(1,2,n)", "select(2,7,2 select_tut)", "semctl",
"semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
"setpriority", "setprotoent", "setpwent", "setservent", "setsockopt",
"shmctl", "shmget", "shmread", "shmwrite", "socket(2,7,n)", "socketpair",
"stat(1,2)", "symlink", "syscall", "sysopen", "system", "times", "truncate(2,7)",
"umask", "unlink(1,2)", "utime", "wait", "waitpid"
For more information about the portability of these functions, see
perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file(1,n) test, where X is one of the letters listed below. This
unary operator takes one argument, either a filename or a file-
handle, and tests the associated file(1,n) to see if(3,n) 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(3,n)
the file(1,n) doesn't exist. Despite the funny names, precedence is
the same as any other named(5,8) unary operator, and the argument
may be parenthesized like any other unary operator. The opera-
tor 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(1,8) bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named(5,8) pipe(2,8) (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(1,n) (heuristic guess).
-B File is a "binary" file(1,n) (opposite of -T).
-M Script start time(1,2,n) minus file(1,n) modification time(1,2,n), in(1,8) days.
-A Same for access(2,5) time.
-C Same for inode change time(1,2,n) (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file(1,n) permission operators "-r", "-R",
"-w", "-W", "-x", and "-X" is by default based solely on the
mode of the file(1,n) and the uids and gids of the user. There may
be other reasons you can't actually read(2,n,1 builtins), write(1,2), or execute the
file. Such reasons may be for example network filesystem
access(2,5) controls, ACLs (access(2,5) control lists), read-only
filesystems, and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the
"-r", "-R", "-w", and "-W" tests always return 1, and "-x" and
"-X" return 1 if(3,n) any execute bit is set(7,n,1 builtins) in(1,8) the mode. Scripts
run by the superuser may thus need to do a stat(1,2)() to determine
the actual mode of the file(1,n), or temporarily set(7,n,1 builtins) their effective
uid to something else.
If you are using ACLs, there is a pragma called "filetest" that
may produce more accurate results than the bare stat(1,2)() mode
bits. When under the "use filetest 'access(2,5)'" the above-men-
tioned filetests will test whether the permission can (not) be
granted using the access(2,5)() family of system calls. Also note
that the "-x" and "-X" may under this pragma return true even
if(3,n) there are no execute permission bits set(7,n,1 builtins) (nor any extra exe-
cute permission ACLs). This strangeness is due to the underly-
ing system calls' definitions. Read the documentation for the
"filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying
"-exp($foo)" still works as expected, however--only single let-
ters following a minus are interpreted as file(1,n) tests.
The "-T" and "-B" switches work as follows. The first block or
so of the file(1,n) 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(1,n), other-
wise it's a "-T" file. Also, any file(1,n) containing null in(1,8) 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(1,n), or a file(1,n) at EOF when testing a filehandle. Because you
have to read(2,n,1 builtins) a file(1,n) to do the "-T" test, on most occasions you
want to use a "-f" against the file(1,n) first, as in(1,8) "next unless
-f $file(1,n) && -T $file(1,n)".
If any of the file(1,n) tests (or either the "stat(1,2)" or "lstat" oper-
ators) are given the special filehandle consisting of a soli-
tary underline, then the stat(1,2) structure of the previous file(1,n)
test (or stat(1,2) 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(1,8) the stat(1,2) structure for the sym-
bolic link(1,2), not the real file.) (Also, if(3,n) the stat(1,2) buffer was
filled by a "lstat" call, "-T" and "-B" will reset(1,7,1 tput) it with the
results of "stat(1,2) _"). Example:
print "Can do.\n" if(3,n) -r $a || -w _ || -x _;
stat(1,2)($filename);
print "Readable\n" if(3,n) -r _;
print "Writable\n" if(3,n) -w _;
print "Executable\n" if(3,n) -x _;
print "Setuid\n" if(3,n) -u _;
print "Setgid\n" if(3,n) -g _;
print "Sticky\n" if(3,n) -k _;
print "Text\n" if(3,n) -T _;
print "Binary\n" if(3,n) -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omit-
ted, uses $_.
accept(2,8) NEWSOCKET,GENERICSOCKET
Accepts an incoming socket(2,7,n) connect, just as the accept(2,8)(2) sys-
tem call does. Returns the packed address if(3,n) it succeeded,
false otherwise. See the example in(1,8) "Sockets: Client/Server
Communication" in(1,8) perlipc.
On systems that support a close-on-exec flag on files, the flag
will be set(7,n,1 builtins) for the newly opened file(1,n) descriptor, as determined
by the value of $^F. See "$^F" in(1,8) perlvar.
alarm(1,2) SECONDS
alarm(1,2) Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds have elapsed. If SECONDS
is not specified, the value stored in(1,8) $_ is used. (On some
machines, unfortunately, the elapsed time(1,2,n) may be up to one sec-
ond less(1,3) or more than you specified because of how seconds are
counted, and process scheduling may delay the delivery of the
signal(2,7) 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(1,2,n) remaining on the previous timer.
For delays of finer granularity than one second, you may use
Perl's four-argument version(1,3,5) of select(2,7,2 select_tut)() leaving the first
three arguments undefined, or you might be able to use the
"syscall" interface to access(2,5) setitimer(2) if(3,n) your system sup-
ports it. The Time::HiRes module (from CPAN, and starting from
Perl 5.8 part of the standard distribution) may also prove use-
ful.
It is usually a mistake to intermix "alarm(1,2)" and "sleep(1,3)" calls.
("sleep(1,3)" may be internally implemented in(1,8) your system with
"alarm(1,2)")
If you want to use "alarm(1,2)" to time(1,2,n) out a system call you need
to use an "eval"/"die" pair. You can't rely on the alarm(1,2) caus-
ing the system call to fail with $! set(7,n,1 builtins) to "EINTR" because Perl
sets up signal(2,7) handlers to restart system calls on some sys-
tems. Using "eval"/"die" always works, modulo the caveats
given in(1,8) "Signals" in(1,8) perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm(1,2)\n" }; # NB: \n required
alarm(1,2) $timeout(1,3x,3x cbreak);
$nread = sysread SOCKET, $buffer, $size;
alarm(1,2) 0;
};
if(3,n) ($@) {
die unless $@ eq "alarm(1,2)\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in(1,8) the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan"
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
bind(2,n,1 builtins) SOCKET,NAME
Binds a network address to a socket(2,7,n), just as the bind(2,n,1 builtins) system
call does. Returns true if(3,n) it succeeded, false otherwise.
NAME should be a packed address of the appropriate type for the
socket. See the examples in(1,8) "Sockets: Client/Server Communica-
tion" in(1,8) perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read(2,n,1 builtins) or written in(1,8) "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(1,8) 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(7,n,1 builtins) 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(3,n), but may contain mul-
tiple directives. The directives alter the behaviour of the
file(1,n) handle. When LAYER is present using binmode on text file(1,n)
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(1,8) "Programming Perl" (the Camel) or elsewhere, ":raw"
is not the simply inverse of ":crlf" -- other layers which
would affect binary nature of the stream are also disabled. See
PerlIO, 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(2,3,n)" pragma can
be used to establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as
"DISCIPLINE" in(1,8) "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(1,3,5) of Perl therefore refers to "layers" rather than to
"disciplines". Now back to the regularly scheduled documenta-
tion...
To mark FILEHANDLE as UTF-8, use ":utf8".
In general, binmode() should be called after open(2,3,n)() but before
any I/O is done on the filehandle. Calling binmode() will nor-
mally flush(8,n) any pending buffered output data (and perhaps pend-
ing input data) on the handle. An exception to this is the
":encoding" layer that changes the default character encoding(3,n)
of the handle, see open. The ":encoding" layer sometimes needs
to be called in(1,8) mid-stream, and it doesn't flush(8,n) 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(1,n) representation matches the internal represen-
tation, 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(1,8) the external representa-
tion 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-Win-
dows your program sees a "\n" as a simple "\cJ", but what's
stored in(1,8) text files are the two characters "\cM\cJ". That
means that, if(3,n) you don't use binmode() on these systems,
"\cM\cJ" sequences on disk will be converted to "\n" on input,
and any "\n" in(1,8) 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(3,n) your binary data contains "\cZ", the I/O subsystem will
regard it as the end of the file(1,n), unless you use binmode().
binmode() is not only important for readline() and print()
operations, but also when using read(2,n,1 builtins)(), seek(), sysread(),
syswrite() and tell() (see perlport for more details). See the
$/ and "$\" variables in(1,8) perlvar for how to manually set(7,n,1 builtins) 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(1,8) the CLASSNAME package. If CLASSNAME is omitted,
the current package is used. Because a "bless" is often the
last thing in(1,8) a constructor, it returns the reference for con-
venience. Always use the two-argument version(1,3,5) if(3,n) the function
doing the blessing might be inherited by a derived class. See
perltoot and perlobj for more about the blessing (and bless-
ings) of objects.
Consider always blessing objects in(1,8) CLASSNAMEs that are mixed
case. Namespaces with all lowercase names are considered
reserved for Perl pragmata. Builtin types have all uppercase
names, so to prevent confusion, you may wish to avoid such
package names as well. Make sure that CLASSNAME is a true
value.
See "Perl Modules" in(1,8) perlmod.
caller EXPR
caller Returns the context of the current subroutine call. In scalar
context, returns the caller's package name if(3,n) there is a
caller, that is, if(3,n) we're in(1,8) a subroutine or "eval" or
"require", and the undefined value otherwise. In list context,
returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger
uses to print a stack trace. The value of EXPR indicates how
many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if(3,n) the frame is not a subrou-
tine call, but an "eval". In such a case additional elements
$evaltext and $is_require are set: $is_require is true if(3,n) the
frame is created by a "require" or "use" statement, $evaltext
contains the text of the "eval EXPR" statement. In particular,
for an "eval BLOCK" statement, $filename is "(eval)", but
$evaltext is undefined. (Note also that each "use" statement
creates a "require" frame inside an "eval EXPR" frame.) $sub-
routine may also be "(unknown)" if(3,n) this particular subroutine
happens to have been deleted from the symbol table. $hasargs
is true if(3,n) a new instance of @_ was set(7,n,1 builtins) up for the frame.
$hints and $bitmask contain pragmatic hints that the caller was
compiled with. The $hints and $bitmask values are subject to
change between versions of Perl, and are not meant for external
use.
Furthermore, when called from within the DB package, caller
returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was
invoked.
Be aware that the optimizer might have optimized call frames
away before "caller" had a chance to get the information. That
means that caller(N) might not return information about the
call frame you expect it do, for "N > 1". In particular,
@DB::args might have information from the previous time(1,2,n)
"caller" was called.
chdir EXPR
Changes the working directory to EXPR, if(3,n) possible. If EXPR is
omitted, changes to the directory specified by $ENV{HOME}, if(3,n)
set(7,n,1 builtins); if(3,n) not, changes to the directory specified by
$ENV{LOGDIR}. (Under VMS, the variable $ENV{SYS$LOGIN} is also
checked, and used if(3,n) it is set.) If neither is set(7,n,1 builtins), "chdir"
does nothing. It returns true upon success, false otherwise.
See the example under "die".
chmod(1,2) LIST
Changes the permissions of a list of files. The first element
of the list must be the numerical mode, which should probably
be an octal number, and which definitely should not a string(3,n) of
octal digits: 0644 is okay, '0644' is not. Returns the number
of files successfully changed. See also "oct", if(3,n) all you have
is a string.
$cnt = chmod(1,2) 0755, 'foo', 'bar';
chmod(1,2) 0755, @executables;
$mode = '0644'; chmod(1,2) $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod(1,2) oct($mode), 'foo'; # this is better
$mode = 0644; chmod(1,2) $mode, 'foo'; # this is best
You can also import the symbolic "S_I*" constants from the
Fcntl module:
use Fcntl ':mode';
chmod(1,2) S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod(1,2) 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version(1,3,5) of "chop" removes any trailing string(3,n) that
corresponds to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in(1,8) the "English" module). It returns
the total number of characters removed from all its arguments.
It's often used to remove the newline from the end of an input
record when you're worried that the final record may be missing
its newline. When in(1,8) paragraph mode ("$/ = """), it removes
all trailing newlines from the string. When in(1,8) slurp mode ("$/
= undef") or fixed-length record mode ($/ is a reference to an
integer or the like, see perlvar) chomp() won't remove any-
thing. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(1,n)(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its
keys.
You can actually chomp anything that's an lvalue, including an
assignment:
chomp($cwd = `pwd(1,n,1 builtins)`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total
number of characters removed is returned.
If the "encoding(3,n)" pragma is in(1,8) scope then the lengths returned
are calculated from the length of $/ in(1,8) Unicode characters,
which is not always the same as the length of $/ in(1,8) the native
encoding.
Note that parentheses are necessary when you're chomping any-
thing that is not a simple variable. This is because "chomp
$cwd = `pwd(1,n,1 builtins)`;" is interpreted as "(chomp $cwd) = `pwd(1,n,1 builtins)`;",
rather than as "chomp( $cwd = `pwd(1,n,1 builtins)` )" which you might expect.
Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b"
rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string(3,n) and returns the char-
acter chopped. It is much more efficient than "s/.$//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(3,n), 0, -1)".
See also "chomp".
chown(1,2) 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(1,8)
that order. A value of -1 in(1,8) either position is interpreted by
most systems to leave that value unchanged. Returns the number
of files successfully changed.
$cnt = chown(1,2) $uid, $gid, 'foo', 'bar';
chown(1,2) $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in(1,8) the passwd(1,5)
file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login(1,3,5),$pass,$uid,$gid) = getpwnam($user)
or die "$user not in(1,8) passwd(1,5) file(1,n)";
@ary = glob(1,3,7,n)($pattern); # expand filenames
chown(1,2) $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of
the file(1,n) 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(1,8) the charac-
ter set. For example, "chr(65)" is "A" in(1,8) either ASCII or Uni-
code, and chr(0x263a) is a Unicode smiley face. Note that
characters from 128 to 255 (inclusive) are by default not
encoded in(1,8) UTF-8 Unicode for backward compatibility reasons
(but see encoding(3,n)).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is masked to the
low eight bits.
See perlunicode and encoding(3,n) for more about Unicode.
chroot(1,2) FILENAME
chroot(1,2) This function works like the system call by the same name: it
makes the named(5,8) directory the new root directory for all fur-
ther pathnames that begin with a "/" by your process and all
its children. (It doesn't change your current working direc-
tory, which is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted, does a
"chroot(1,2)" to $_.
close(2,7,n) FILEHANDLE
close(2,7,n) Closes the file(1,n) or pipe(2,8) associated with the file(1,n) handle,
returning true only if(3,n) IO buffers are successfully flushed and
closes the system file(1,n) descriptor. Closes the currently
selected filehandle if(3,n) the argument is omitted.
You don't have to close(2,7,n) FILEHANDLE if(3,n) you are immediately going
to do another "open(2,3,n)" on it, because "open(2,3,n)" will close(2,7,n) it for
you. (See "open(2,3,n)".) However, an explicit "close(2,7,n)" on an input
file(1,n) resets the line counter ($.), while the implicit close(2,7,n)
done by "open(2,3,n)" does not.
If the file(1,n) handle came from a piped open(2,3,n), "close(2,7,n)" will addi-
tionally return false if(3,n) one of the other system calls involved
fails, or if(3,n) the program exits with non-zero status. (If the
only problem was that the program exited non-zero, $! will be
set(7,n,1 builtins) to 0.) Closing a pipe(2,8) also waits for the process executing
on the pipe(2,8) to complete, in(1,8) case you want to look(1,8,3 Search::Dict) at the output
of the pipe(2,8) afterwards, and implicitly puts(3,n) the exit(3,n,1 builtins) status
value of that command into $?.
Prematurely closing the read(2,n,1 builtins) end of a pipe(2,8) (i.e. before the
process writing to it at the other end has closed it) will
result in(1,8) a SIGPIPE being delivered to the writer. If the
other end can't handle that, be sure to read(2,n,1 builtins) all the data
before closing the pipe.
Example:
open(2,3,n)(OUTPUT, '|sort(1,3) >foo') # pipe(2,8) to sort(1,3)
or die "Can't start sort: $!";
#... # print stuff to output
close(2,7,n) OUTPUT # wait for sort(1,3) to finish
or warn $! ? "Error closing sort(1,3) pipe: $!"
: "Exit status $? from sort(1,3)";
open(2,3,n)(INPUT, 'foo') # get sort(1,3)'s results
or die "Can't open(2,3,n) '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(2,7,n), just as the connect
system call does. Returns true if(3,n) it succeeded, false other-
wise. NAME should be a packed address of the appropriate type
for the socket. See the examples in(1,8) "Sockets: Client/Server
Communication" in(1,8) perlipc.
continue BLOCK
Actually a flow control statement rather than a function. If
there is a "continue" BLOCK attached to a BLOCK (typically in(1,8) 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(1,8) 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" state-
ment).
"last", "next", or "redo" may appear within a "continue" block.
"last" and "redo" will behave as if(3,n) they had been executed
within the main block. So will "next", but since it will exe-
cute 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.
cos EXPR
cos Returns the cosine of EXPR (expressed in(1,8) 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]), $_[0] ) }
crypt PLAINTEXT,SALT
Encrypts a string(3,n) exactly like the crypt(3) function in(1,8) the C
library (assuming that you actually have a version(1,3,5) there that
has not been extirpated as a potential munition). This can
prove useful for checking the password file(1,n) for lousy pass-
words, amongst other things. Only the guys wearing white hats
should do this.
Note that crypt is intended to be a one-way function, much like
breaking eggs to make an omelette. There is no (known) corre-
sponding decrypt function (in(1,8) other words, the crypt() is a
one-way hash function). As a result, this function isn't all
that useful for cryptography. (For that, see your nearby CPAN
mirror.)
When verifying an existing encrypted string(3,n) you should use the
encrypted text as the salt (like "crypt($plain, $crypted) eq
$crypted"). This allows your code to work with the standard
crypt and with more exotic implementations. In other words, do
not assume anything about the returned string(3,n) itself, or how
many bytes in(1,8) the encrypted string(3,n) matter.
Traditionally the result is a string(3,n) of 13 bytes: two first
bytes of the salt, followed by 11 bytes from the set(7,n,1 builtins)
"[./0-9A-Za-z]", and only the first eight bytes of the
encrypted string(3,n) mattered, but alternative hashing schemes
(like MD5), higher level security schemes (like C2), and imple-
mentations on non-UNIX platforms may produce different strings.
When choosing a new salt create a random(3,4,6) two character string(3,n)
whose characters come from the set(7,n,1 builtins) "[./0-9A-Za-z]" (like "join(1,n)
'', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand(1,3) 64, rand(1,3) 64]").
This set(7,n,1 builtins) of characters is just a recommendation; the characters
allowed in(1,8) 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 pro-
gram knows their own password:
$pwd(1,n,1 builtins) = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo(1,3x,1 builtins)";
if(3,n) (crypt($word, $pwd(1,n,1 builtins)) ne $pwd(1,n,1 builtins)) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in(1,8) your own password to whoever asks you for
it is unwise.
The crypt function is unsuitable for encrypting large quanti-
ties of data, not least of all because you can't get the infor-
mation back. Look at the by-module/Crypt and by-module/PGP
directories on your favorite CPAN mirror for a slew of poten-
tially useful modules.
If using crypt() on a Unicode string(3,n) (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(3,n))
the string(3,n) back to an eight-bit byte string(3,n) before calling
crypt() (on that copy). If that works, good. If not, crypt()
dies with "Wide character in(1,8) crypt".
dbmclose HASH
[This function has been largely superseded by the "untie" func-
tion.]
Breaks the binding between a DBM file(1,n) and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" func-
tion.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB
file(1,n) to a hash. HASH is the name of the hash. (Unlike normal
"open(2,3,n)", 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(3,n) any). If the database does not
exist, it is created with protection specified by MASK (as mod-
ified by the "umask"). If your system supports only the older
DBM functions, you may perform only one "dbmopen" in(1,8) your pro-
gram. In older versions of Perl, if(3,n) your system had neither
DBM nor ndbm, calling "dbmopen" produced a fatal error(8,n); it now
falls back to sdbm(3).
If you don't have write(1,2) access(2,5) to the DBM file(1,n), you can only
read(2,n,1 builtins) hash variables, not set(7,n,1 builtins) them. If you want to test whether
you can write(1,2), either use file(1,n) 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(1,3,n,1 builtins) file(1,n) offsets
dbmopen(%HIST,'/usr/lib/news/history(1,3,n,1 builtins)',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(2,3,n) netscape history(1,3,n,1 builtins) 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(1,n), system error(8,n), uninitialized variable, and other excep-
tional conditions. This function allows you to distinguish
"undef" from other values. (A simple Boolean test will not
distinguish among "undef", zero, the empty string(3,n), 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(1,2,n) that it is called -- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is depre-
cated. It used to report whether memory for that aggregate has
ever been allocated. This behavior may disappear in(1,8) future
versions of Perl. You should instead use a simple test for
size:
if(3,n) (@an_array) { print "has array elements\n" }
if(3,n) (%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(1,8) the hash. Use "exists"
for the latter purpose.
Examples:
print if(3,n) defined $switch(1,n){'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink(1,2) $sym: $!"
unless defined($value = readlink(1,2) $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are sur-
prised to discover(1,3,5) that the number 0 and "" (the zero-length
string(3,n)) are, in(1,8) fact, defined values. For example, if(3,n) you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact
that it matched "nothing". But it didn't really match noth-
ing--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 ele-
ment, hash slice, or array slice, deletes the specified ele-
ment(s) from the hash or array. In the case of an array, if(3,n)
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(3,n) 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(3,n) 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(1,n) deletes the entry from the DBM file.
Deleting from a "tie"d hash or array may not necessarily return
anything.
Deleting an array element effectively returns that position of
the array to its initial, uninitialized state. Subsequently
testing for the same element with exists() will return false.
Note that deleting array elements in(1,8) the middle of an array
will not shift the index of the ones after them down--use
splice() for that. See "exists".
The following (inefficiently) deletes all the values of %HASH
and @ARRAY:
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(8,n) message is stuffed into $@ and the "eval" is terminated
with the undefined value. This makes "die" the way to raise(3,n) 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(1,8) a newline, the cur-
rent script line number and input line number (if(3,n) 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(1,8) effect, and is also avail-
able as the special variable $.. See "$/" in(1,8) perlvar and "$."
in(1,8) perlvar.
Hint: sometimes appending ", stopped" to your message will
cause it to make better sense when the string(3,n) "at foo line 123"
is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(3,n,1 builtins)(), 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(1,n) and line number parameters. The return value replaces the
value in(1,8) $@. ie. as if(3,n) "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string(3,n) "Died" is used.
die() can also be called with a reference argument. If this
happens to be trapped within an eval(), $@ contains the refer-
ence. This behavior permits a more elaborate exception han-
dling implementation using objects that maintain arbitrary
state about the nature of the exception. Such a scheme is
sometimes preferable to matching particular string(3,n) values of $@
using regular expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if(3,n) ($@) {
if(3,n) (ref($@) && UNIVERSAL::isa($@,"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 oper-
ations 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 $SIG{__DIE__} hook. The associ-
ated handler will be called with the error(8,n) text and can change
the error(8,n) message, if(3,n) it sees fit, by calling "die" again. See
"$SIG{expr(1,3,n)}" in(1,8) perlvar for details on setting %SIG entries,
and "eval BLOCK" for some examples. Although this feature was
meant to be run only right before your program was to exit(3,n,1 builtins),
this is not currently the case--the $SIG{__DIE__} hook is cur-
rently called even inside eval()ed blocks/strings! If one
wants the hook to do nothing in(1,8) such situations, put
die @_ if(3,n) $^S;
as the first line of the handler (see "$^S" in(1,8) perlvar).
Because this promotes strange action at a distance, this coun-
terintuitive behavior may be fixed in(1,8) a future release.
do BLOCK
Not really a function. Returns the value of the last command
in(1,8) the sequence of commands indicated by BLOCK. When modified
by a loop modifier, executes the BLOCK once before testing the
loop condition. (On other statements the loop modifiers test
the conditional first.)
"do BLOCK" does not count as a loop, so the loop control state-
ments "next", "last", or "redo" cannot be used to leave or
restart the block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents
of the file(1,n) as a Perl script. Its primary use is to include
subroutines from a Perl subroutine library.
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(8,n) messages, searches the @INC
libraries, and updates %INC if(3,n) the file(1,n) is found. See "Prede-
fined Names" in(1,8) perlvar for these variables. It also differs
in(1,8) that code evaluated with "do FILENAME" cannot see lexicals
in(1,8) the enclosing scope; "eval STRING" does. It's the same,
however, in(1,8) that it does reparse the file(1,n) every time(1,2,n) you call
it, so you probably don't want to do this inside a loop.
If "do" cannot read(2,n,1 builtins) the file(1,n), it returns undef and sets $! to
the error. If "do" can read(2,n,1 builtins) the file(1,n) but cannot compile it, it
returns undef and sets an error(8,n) message in(1,8) $@. If the file(1,n) 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(8,n)
checking and raise(3,n) an exception if(3,n) there's a problem.
You might like to use "do" to read(2,n,1 builtins) in(1,8) a program configuration
file. Manual error(8,n) checking can be done this way:
# read(2,n,1 builtins) in(1,8) config(1,5) files: system first, then user
for $file(1,n) ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file(1,n)) {
warn "couldn't parse $file: $@" if(3,n) $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file(1,n)" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See also the -u
command-line switch(1,n) in(1,8) 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(1,2,n) of the dump will not be
open(2,3,n) any more when the program is reincarnated, with possible
resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very
hard to convert a core file(1,n) into an executable, and because the
real compiler backends for generating portable bytecode and
compilable C code have superseded it. That's why you should
now invoke it as "CORE::dump()", if(3,n) you don't want to be warned
against a possible typo.
If you're looking to use dump to speed up your program, con-
sider generating bytecode or native C code as described in(1,8)
perlcc. If you're just trying to accelerate a CGI script, con-
sider using the "mod_perl" extension to Apache, or the CPAN
module, CGI::Fast. You might also consider autoloading or
selfloading, which at least make your program appear to run
faster.
each HASH
When called in(1,8) list context, returns a 2-element list consist-
ing of the key and value for the next element of a hash, so
that you can iterate over it. When called in(1,8) scalar context,
returns only the key for the next element in(1,8) the hash.
Entries are returned in(1,8) an apparently random(3,4,6) order. The actual
random(3,4,6) order is subject to change in(1,8) future versions of perl,
but it is guaranteed to be in(1,8) the same order as either the
"keys" or "values" function would produce on the same (unmodi-
fied) hash. Since Perl 5.8.1 the ordering is different even
between different runs of Perl for security reasons (see "Algo-
rithmic Complexity Attacks" in(1,8) perlsec).
When the hash is entirely read(2,n,1 builtins), a null array is returned in(1,8)
list context (which when assigned produces a false (0) value),
and "undef" in(1,8) 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" func-
tion calls in(1,8) the program; it can be reset(1,7,1 tput) 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(1,8) a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort(1,3)".
eof FILEHANDLE
eof ()
eof Returns 1 if(3,n) the next read(2,n,1 builtins) on FILEHANDLE will return end of
file(1,n), or if(3,n) 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(1,8) an interactive context.) Do not read(2,n,1 builtins)
from a terminal file(1,n) (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(3,n) you do.
An "eof" without an argument uses the last file(1,n) read. Using
"eof()" with empty parentheses is very different. It refers to
the pseudo file(1,n) 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(3,n) input is available. Similarly, an "eof()" after "<>" has
returned end-of-file will assume you are processing another
@ARGV list, and if(3,n) you haven't set(7,n,1 builtins) @ARGV, will read(2,n,1 builtins) input from
"STDIN"; see "I/O Operators" in(1,8) perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to
detect the end of each file(1,n), "eof()" will only detect the end
of the last file. Examples:
# reset(1,7,1 tput) line numbering on each input file(1,n)
while (<>) {
next if(3,n) /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close(2,7,n) ARGV if(3,n) eof; # Not eof()!
}
# insert dashes just before last line of last file(1,n)
while (<>) {
if(3,n) (eof()) { # check for end of last file(1,n)
print "--------------\n";
}
print;
last if(3,n) eof(); # needed if(3,n) we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in(1,8) Perl,
because the input operators typically return "undef" when they
run out of data, or if(3,n) there was an error.
eval EXPR
eval BLOCK
In the first form, the return value of EXPR is parsed and exe-
cuted as if(3,n) it were a little Perl program. The value of the
expression (which is itself determined within scalar context)
is first parsed, and if(3,n) there weren't any errors, executed in(1,8)
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(1,2,n) the eval
executes. If EXPR is omitted, evaluates $_. This form is typ-
ically 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(1,2,n) 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(3,n) 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 state-
ment may be also used, just as with subroutines. The expres-
sion providing the return value is evaluated in(1,8) 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(8,n) or runtime error(8,n), or a "die" state-
ment is executed, an undefined value is returned by "eval", and
$@ is set(7,n,