of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
-Elements of the LIST should be separated by commas.
+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
C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
C<umask>, C<unlink>, C<utime>
-=item Keywords related to the control flow of your perl program
+=item Keywords related to the control flow of your Perl program
C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
-C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
+C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
The C<-T> and C<-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 C<-B> file, otherwise it's a C<-T> file. Also, any file
+are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
containing null in the first block is considered a binary file. If C<-T>
or C<-B> is used on a filehandle, the current IO buffer is examined
rather than the first block. Both C<-T> and C<-B> return true on a null
a system call. (This doesn't work with C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
symbolic link, not the real file.) (Also, if the stat buffer was filled by
-a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
+an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
Example:
print "Can do.\n" if -r $a || -w _ || -x _;
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
test operators, in a way that C<-f -w -x $file> is equivalent to
-C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
+C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
the return value of C<-f $file> as an argument to another filetest
operator, no special magic will happen.)
=item alarm
Arranges to have a SIGALRM delivered to this process after the
-specified number of wallclock seconds have elapsed. If SECONDS is not
+specified number of wallclock seconds has elapsed. If SECONDS is not
specified, the value stored in C<$_> 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
in the CLASSNAME package. If CLASSNAME is omitted, the current package
is used. Because a C<bless> is often the last thing in a constructor,
it returns the reference for convenience. Always use the two-argument
-version if the function doing the blessing might be inherited by a
-derived class. See L<perltoot> and L<perlobj> for more about the blessing
-(and blessings) of objects.
+version if a derived class might inherit the function doing the blessing.
+See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
+of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
-Perl pragmata. Builtin types have all uppercase names, so to prevent
+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.
=item chdir EXPR
+=item chdir FILEHANDLE
+
+=item chdir DIRHANDLE
+
+=item chdir
+
Changes the working directory to EXPR, if possible. If EXPR is omitted,
changes to the directory specified by C<$ENV{HOME}>, if set; if not,
changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
neither is set, C<chdir> does nothing. It returns true upon success,
false otherwise. See the example under C<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.
+
=item 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 I<not> a string of octal digits:
+number, and which definitely should I<not> be a string of octal digits:
C<0644> is okay, C<'0644'> is not. Returns the number of files
successfully changed. See also L</oct>, if all you have is a string.
$mode = '0644'; chmod oct($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.
+
+ open(my $fh, "<", "foo");
+ my $perm = (stat $fh)[2] & 07777;
+ chmod($perm | 0600, $fh);
+
You can also import the symbolic C<S_I*> constants from the Fcntl
module:
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
+On systems that support fchown, you might pass file handles among the
+files. On systems that don't support fchown, passing file handles
+produces a fatal error at run time.
+
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
backward compatibility reasons (but see L<encoding>).
+Negative values give the Unicode replacement character (chr(0xfffd)),
+except under the L</bytes> pragma, where low eight bits of the value
+(truncated to an integer) are used.
+
If NUMBER is omitted, uses C<$_>.
For the reverse, use L</ord>.
program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
also waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards, and
-implicitly puts the exit status value of that command into C<$?>.
+implicitly puts the exit status value of that command into C<$?> and
+C<${^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
=item continue BLOCK
-Actually a flow control statement rather than a function. If there is a
-C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
+C<continue> is actually a flow control statement rather than a function. If
+there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C. Thus
it can be used to increment a loop variable, even when the loop has been
=item crypt PLAINTEXT,SALT
-Encrypts a string exactly like the crypt(3) function in the C library
-(assuming that you actually have a version there that has not been
-extirpated as a potential munition). This can prove useful for checking
-the password file for lousy passwords, amongst other things. Only the
-guys wearing white hats should do this.
-
-Note that L<crypt|/crypt> is intended to be a one-way function, much like
-breaking eggs to make an omelette. There is no (known) corresponding
-decrypt function (in other words, the crypt() is a one-way hash
-function). As a result, this function isn't all that useful for
-cryptography. (For that, see your nearby CPAN mirror.)
-
-When verifying an existing encrypted string you should use the
-encrypted text as the salt (like C<crypt($plain, $crypted) eq
-$crypted>). This allows your code to work with the standard L<crypt|/crypt>
-and with more exotic implementations. In other words, do not assume
-anything about the returned string itself, or how many bytes in
-the encrypted string matter.
+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 munitions).
+
+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 F<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 C<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 L<crypt|/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 C<[./0-9A-Za-z]>, and only
-the first eight bytes of the encrypted string mattered, but
-alternative hashing schemes (like MD5), higher level security schemes
-(like C2), and implementations on non-UNIX platforms may produce
-different strings.
+the first eight bytes of the digest string mattered, but alternative
+hashing schemes (like MD5), higher level security schemes (like C2),
+and implementations on non-UNIX platforms may produce different
+strings.
When choosing a new salt create a random two character string whose
characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
restrict what salts C<crypt()> accepts.
Here's an example that makes sure that whoever runs this program knows
-their own password:
+their password:
$pwd = (getpwuid($<))[1];
Of course, typing in your own password to whoever asks you
for it is unwise.
-The L<crypt|/crypt> function is unsuitable for encrypting large quantities
+The L<crypt|/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 F<by-module/Crypt> and F<by-module/PGP> directories
-on your favorite CPAN mirror for a slew of potentially useful
-modules.
+back. Look at the L<Digest> module for more robust algorithms.
If using crypt() on a Unicode string (which I<potentially> has
characters with codepoints above 255), Perl tries to make sense
"ab" =~ /a(.*)b/;
The pattern match succeeds, and C<$1> is defined, despite the fact that it
-matched "nothing". But it didn't really match nothing--rather, it
+matched "nothing". It didn't really fail to match anything. Rather, it
matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
Deleting an array element effectively returns that position of the array
to its initial, uninitialized state. Subsequently testing for the same
-element with exists() will return false. Note that deleting array
-elements in the middle of an array will not shift the index of the ones
-after them down--use splice() for that. See L</exists>.
+element with exists() will return false. Also, deleting array elements
+in the middle of an array will not shift the index of the elements
+after them down. Use splice() for that. See L</exists>.
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
If LIST is empty and C<$@> contains an object reference that has a
C<PROPAGATE> method, that method will be called with additional file
and line number parameters. The return value replaces the value in
-C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
+C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
were called.
If C<$@> is empty then the string C<"Died"> is used.
is sometimes preferable to matching particular string values of $@ using
regular expressions. Here's an example:
+ use Scalar::Util 'blessed';
+
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
- if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
+ if (blessed($@) && $@->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
handler will be called with the error text and can change the error
message, if it sees fit, by calling C<die> again. See
L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
-L<"eval BLOCK"> for some examples. Although this feature was meant
+L<"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 C<$SIG{__DIE__}> hook is currently called
even inside eval()ed blocks/strings! If one wants the hook to do
=item do BLOCK
Not really a function. Returns the value of the last command in the
-sequence of commands indicated by BLOCK. When modified by a loop
-modifier, executes the BLOCK once before testing the loop condition.
-(On other statements the loop modifiers test the conditional first.)
+sequence of commands indicated by BLOCK. When modified by the C<while> or
+C<until> loop modifier, executes the BLOCK once before testing the loop
+condition. (On other statements the loop modifiers test the conditional
+first.)
C<do BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
=item do SUBROUTINE(LIST)
-
A deprecated form of subroutine call. See L<perlsub>.
+
This form of subroutine call is deprecated. See L<perlsub>.
=item do EXPR
Uses the value of EXPR as a filename and executes the contents of the
-file as a Perl script. Its primary use is to include subroutines
-from a Perl subroutine library.
+file as a Perl script.
do 'stat.pl';
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current
-filename for error messages, searches the @INC libraries, and updates
+filename for error messages, searches the @INC directories, and updates
C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
variables. It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
=item eval BLOCK
+=item 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.
+afterwards. Note that the value is parsed every time the C<eval> executes.
If EXPR is omitted, evaluates C<$_>. 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
+same time the code surrounding the C<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
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 L</wantarray> for more on how the evaluation context can be determined.
+in void, scalar, or list context, depending on the context of the C<eval>
+itself. See L</wantarray> for more on how the evaluation context can be
+determined.
If there is a syntax error or runtime error, or a C<die> statement is
executed, an undefined value is returned by C<eval>, and C<$@> is set to the
# a run-time error
eval '$answer ='; # sets $@
-Due to the current arguably broken state of C<__DIE__> hooks, when using
-the C<eval{}> form as an exception trap in libraries, you may wish not
-to trigger any C<__DIE__> hooks that user code may have installed.
+Using the C<eval{}> form as an exception trap in libraries does have some
+issues. Due to the current arguably broken state of C<__DIE__> hooks, you
+may wish not to trigger any C<__DIE__> hooks that user code may have installed.
You can use the C<local $SIG{__DIE__}> construct for this purpose,
as shown in this example:
=item exit EXPR
+=item exit
+
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
B<merely advisory>. Such discretionary locks are more flexible, but offer
-fewer guarantees. This means that files locked with C<flock> may be
-modified by programs that do not also use C<flock>. See L<perlport>,
+fewer guarantees. This means that programs that do not also use C<flock>
+may modify files locked with C<flock>. See L<perlport>,
your port's specific documentation, or your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as always feel perfectly
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
Eventually, when a C<write> is done, the contents of
-C<$^A> are written to some filehandle, but you could also read C<$^A>
-yourself and then set C<$^A> back to C<"">. Note that a format typically
+C<$^A> are written to some filehandle. You could also read C<$^A>
+and then set C<$^A> back to C<"">. Note that a format typically
does one C<formline> per line of form, but the C<formline> function itself
doesn't care how many newlines are embedded in the PICTURE. This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
=item getlogin
-Implements the C library function of the same name, which on most
+This implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any. If null,
use C<getpwuid>.
files are only supported if your vendor has implemented them in the
intuitive fashion that calling the regular C library routines gets the
shadow versions if you're running under privilege or if there exists
-the shadow(3) functions as found in System V ( this includes Solaris
-and Linux.) Those systems which implement a proprietary shadow password
+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 $members value returned by I<getgr*()> is a space separated list of
Internet domain, each address is four bytes long and you can unpack it
by saying something like:
- ($a,$b,$c,$d) = unpack('C4',$addr[0]);
+ ($a,$b,$c,$d) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
=item getsockopt SOCKET,LEVEL,OPTNAME
-Returns the socket option requested, or undef if there is an error.
+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
+C<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 C<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 C<i> (or C<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";
+
=item glob EXPR
=item gmtime EXPR
+=item gmtime
+
Converts a time as returned by the time function to an 8-element list
with the time localized for the standard Greenwich time zone.
Typically used as follows:
the year, in the range C<0..364> (or C<0..365> in leap years.)
Note that the $year element is I<not> simply the last two digits of
-the year. If you assume it is, then you create non-Y2K-compliant
+the year. If you assume it is then you create non-Y2K-compliant
programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
instead a Perl builtin. To get somewhat similar but locale dependent date
strings, see the example in L</localtime>.
+See L<perlport/gmtime> for portability concerns.
+
=item goto LABEL
=item goto EXPR
routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable
-containing a code reference, or a block which evaluates to a code
+containing a code reference, or a block that evaluates to a code
reference.
=item grep BLOCK LIST
This is usually something to be avoided when writing clear code.
If C<$_> is lexical in the scope where the C<grep> appears (because it has
-been declared with C<my $_>) then, in addition the be locally aliased to
+been declared with C<my $_>) then, in addition to being locally aliased to
the list elements, C<$_> keeps being lexical inside the block; i.e. it
can't be seen from the outside, avoiding any potential side-effects.
=item 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
+(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
L</oct>.) If EXPR is omitted, uses C<$_>.
print hex '0xAf'; # prints '175'
Hex strings may only represent integers. Strings that would cause
integer overflow trigger a warning. Leading whitespace is not stripped,
-unlike oct().
+unlike oct(). To present something as hex, look into L</printf>,
+L</sprintf>, or L</unpack>.
-=item import
+=item import LIST
There is no builtin C<import> function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
or after POSITION. If POSITION is omitted, starts searching from the
beginning of the string. The return value is based at C<0> (or whatever
you've set the C<$[> variable to--but don't do that). If the substring
-is not found, returns one less than the base, ordinarily C<-1>.
+is not found, C<index> returns one less than the base, ordinarily C<-1>.
=item int EXPR
Implements the ioctl(2) function. You'll probably first have to say
- require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
+ require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
-to get the correct function definitions. If F<ioctl.ph> doesn't
+to get the correct function definitions. If F<sys/ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<< <sys/ioctl.h> >>.
(There is a Perl script called B<h2ph> that comes with the Perl kit that
Perl for security reasons (see L<perlsec/"Algorithmic Complexity
Attacks">).
-As a side effect, calling keys() resets the HASH's internal iterator,
-see L</each>. (In particular, calling keys() in void context resets
-the iterator with no other overhead.)
+As a side effect, calling keys() resets the HASH's internal iterator
+(see L</each>). In particular, calling keys() in void context resets
+the iterator with no other overhead.
Here is yet another way to print your environment:
=item localtime EXPR
+=item localtime
+
Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone. Typically used as
follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
- localtime(time);
+ 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 C<0..11> with 0 indicating January and 11
-indicating December. $year is the number of years since 1900. That
-is, $year is C<123> in year 2023. $wday is the day of the week, with
-0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
-the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
-is true if the specified time occurs during daylight savings time,
-false otherwise.
+tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
+of the specified time.
-Note that the $year element is I<not> simply the last two digits of
-the year. If you assume it is, then you create non-Y2K-compliant
-programs--and you wouldn't want to do that, would you?
+C<$mday> is the day of the month, and C<$mon> is the month itself, in
+the range C<0..11> with 0 indicating January and 11 indicating December.
+This makes it easy to get a month name from a list:
-The proper way to get a complete 4-digit year is simply:
+ my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
+ print "$abbr[$mon] $mday";
+ # $mon=9, $mday=18 gives "Oct 18"
- $year += 1900;
+C<$year> is the number of years since 1900, not just the last two digits
+of the year. That is, C<$year> is C<123> in year 2023. The proper way
+to get a complete 4-digit year is simply:
-And to get the last two digits of the year (e.g., '01' in 2001) do:
+ $year += 1900;
- $year = sprintf("%02d", $year % 100);
+To get the last two digits of the year (e.g., '01' in 2001) do:
+
+ $year = sprintf("%02d", $year % 100);
+
+C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
+Wednesday. C<$yday> is the day of the year, in the range C<0..364>
+(or C<0..365> in leap years.)
+
+C<$isdst> is true if the specified time occurs during Daylight Saving
+Time, false otherwise.
If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
+See L<perlport/localtime> for portability concerns.
+
=item lock THING
This function places an advisory lock on a shared variable, or referenced
the original list for which the BLOCK or EXPR evaluates to true.
If C<$_> is lexical in the scope where the C<map> appears (because it has
-been declared with C<my $_>) then, in addition the be locally aliased to
+been declared with C<my $_>) then, in addition to being locally aliased to
the list elements, C<$_> keeps being lexical inside the block; i.e. it
can't be seen from the outside, avoiding any potential side-effects.
=item mkdir FILENAME
+=item mkdir
+
Creates the directory specified by FILENAME, with permissions
specified by MASK (as modified by C<umask>). If it succeeds it
returns true, otherwise it returns false and sets C<$!> (errno).
-If omitted, MASK defaults to 0777.
+If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
+to C<$_>.
In general, it is better to create directories with permissive MASK,
and let the user modify that with their C<umask>, than it is to supply
use IPC::SysV;
first to get the correct constant definitions. If CMD is C<IPC_STAT>,
-then ARG must be a variable which will hold the returned C<msqid_ds>
+then ARG must be a variable that will hold the returned C<msqid_ds>
structure. Returns like C<ioctl>: the undefined value for error,
C<"0 but true"> for zero, or the actual return value otherwise. See also
L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
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
+As a special case the 3-arg form with a read/write mode and the third
argument being C<undef>:
open(TMP, "+>", undef) or die ...
to the temporary file first. You will need to seek() to do the
reading.
-File handles can be opened to "in memory" files held in Perl scalars via:
+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) || ..
}
}
+See L<perliol> for detailed info on PerlIO.
+
You may also, in the Bourne shell tradition, specify an EXPR beginning
with C<< '>&' >>, in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
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
+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 C<STDOUT> and
of $^F. See L<perlvar/$^F>.
Closing any piped filehandle causes the parent process to wait for the
-child to finish, and returns the status value in C<$?>.
+child to finish, and returns the status value in C<$?> and
+C<${^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
=item our TYPE EXPR : ATTRS
-An C<our> declares the listed variables to be valid globals within
-the enclosing block, file, or C<eval>. That is, it has the same
-scoping rules as a "my" declaration, but does not create a local
-variable. If more than one value is listed, the list must be placed
-in parentheses. The C<our> declaration has no semantic effect unless
-"use strict vars" is in effect, in which case it lets you use the
-declared global variable without qualifying it with a package name.
-(But only within the lexical scope of the C<our> declaration. In this
-it differs from "use vars", which is package scoped.)
+C<our> associates a simple name with a package variable in the current
+package for use within the current scope. When C<use strict 'vars'> is in
+effect, C<our> lets you use declared global variables without qualifying
+them with package names, within the lexical scope of the C<our> declaration.
+In this way C<our> differs from C<use vars>, which is package scoped.
+
+Unlike C<my>, which both allocates storage for a variable and associates
+a simple name with that storage for use within the current scope, C<our>
+associates a simple name with a package variable in the current package,
+for use within the current scope. In other words, C<our> has the same
+scoping rules as C<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 C<our> declaration declares a global variable that will be visible
across its entire lexical scope, even across package boundaries. The
$bar = 20;
package Bar;
- print $bar; # prints 20
+ print $bar; # prints 20, as it refers to $Foo::bar
-Multiple C<our> declarations in the same lexical scope are allowed
-if they are in different packages. If they happened to be in the same
-package, Perl will emit warnings if you have asked for them.
+Multiple C<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 C<my> declarations. Unlike a second
+C<my> declaration, which will bind the name to a fresh variable, a
+second C<our> declaration in the same package, in the same scope, is
+merely redundant.
use warnings;
package Foo;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
- our $bar; # emits warning
+ our $bar; # emits warning but has no other effect
+ print $bar; # still prints 30
An C<our> declaration may also have a list of attributes associated
with it.
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
-a converted integer may be represented by a sequence of 4 bytes.
+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:
H A hex string (high nybble first).
c A signed char (8-bit) value.
- C An unsigned char value. Only does bytes. See U for Unicode.
+ C An unsigned C char (octet) even under Unicode. Should normally not
+ be used. See U and W instead.
+ W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
U A Unicode character number. Encodes to UTF-8 internally
(or UTF-EBCDIC in EBCDIC platforms).
- w A BER compressed integer. Its bytes represent an unsigned
- integer in base 128, most significant digit first, with as
- few digits as possible. Bit eight (the high bit) is set
- on each byte except the last.
+ w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
+ details). Its bytes represent an unsigned integer in base 128,
+ most significant digit first, with as few digits as possible. Bit
+ eight (the high bit) is set on each byte except the last.
x A null byte.
X Back up a byte.
- @ Null fill to absolute position, counted from the start of
- the innermost ()-group.
+ @ Null fill or truncate to absolute position, counted from the
+ start of the innermost ()-group.
+ . Null fill or truncate to absolute position specified by value.
( Start of a ()-group.
-Some letters in the TEMPLATE may optionally be followed by one or
-more of these modifiers (the second column lists the letters for
-which the modifier is valid):
+One or more of the modifiers below may optionally follow some letters in the
+TEMPLATE (the second column lists the letters for which the modifier is
+valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
nNvV Treat integers as signed instead of unsigned.
+ @. Specify position as byte offset in the internal
+ representation of the packed string. Efficient but
+ dangerous.
+
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
Each letter may optionally be followed by a number giving a repeat
count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
-C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
-many values from the LIST. A C<*> for the repeat count means to use
-however many items are left, except for C<@>, C<x>, C<X>, where it is
-equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
-is the same). A numeric repeat count may optionally be enclosed in
-brackets, as in C<pack 'C[80]', @arr>.
+C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
+that many values from the LIST. A C<*> for the repeat count means to
+use however many items are left, except for C<@>, C<x>, C<X>, where it
+is equivalent to C<0>, for <.> where it means relative to string start
+and C<u>, where it is equivalent to 1 (or 45, which is the same).
+A numeric repeat count may optionally be enclosed in brackets, as in
+C<pack 'C[80]', @arr>.
One can replace the numeric repeat count by a template enclosed in brackets;
then the packed length of this template in bytes is used as a count.
byte (so the packed result will be one longer than the byte C<length>
of the item).
+When used with C<@>, the repeat count represents an offset from the start
+of the innermost () group.
+
+When used with C<.>, the repeat count is used to determine the starting
+position from where the value offset is calculated. If the repeat count
+is 0, it's relative to the current position. If the repeat count is C<*>,
+the offset is relative to the start of the packed string. And if its an
+integer C<n> the offset is relative to the start of the n-th innermost
+() group (or the start of the string if C<n> is bigger then the group
+level).
+
The repeat count for C<u> is interpreted as the maximal number of bytes
-to encode per line of output, with 0 and 1 replaced by 45.
+to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
+count should not be more than 65.
=item *
The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as necessary. When
-unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
-after the first null, and C<a> returns data verbatim. When packing,
-C<a>, and C<Z> are equivalent.
+unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
+after the first null, and C<a> returns data verbatim.
If the value-to-pack is too long, it is truncated. If too long and an
explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
-by a null byte. Thus C<Z> always packs a trailing null byte under
-all circumstances.
+by a null byte. Thus C<Z> always packs a trailing null (except when the
+count is 0).
=item *
Likewise, the C<b> and C<B> fields pack a string that many bits long.
-Each byte of the input field of pack() generates 1 bit of the result.
+Each character of the input field of pack() generates 1 bit of the result.
Each result bit is based on the least-significant bit of the corresponding
-input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
-C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
+input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
+and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
Starting from the beginning of the input string of pack(), each 8-tuple
-of bytes is converted to 1 byte of output. With format C<b>
-the first byte of the 8-tuple determines the least-significant bit of a
-byte, and with format C<B> it determines the most-significant bit of
-a byte.
+of characters is converted to 1 character of output. With format C<b>
+the first character of the 8-tuple determines the least-significant bit of a
+character, and with format C<B> it determines the most-significant bit of
+a character.
If the length of the input string is not exactly divisible by 8, the
-remainder is packed as if the input string were padded by null bytes
+remainder is packed as if the input string were padded by null characters
at the end. Similarly, during unpack()ing the "extra" bits are ignored.
-If the input string of pack() is longer than needed, extra bytes are ignored.
-A C<*> for the repeat count of pack() means to use all the bytes of
-the input field. On unpack()ing the bits are converted to a string
-of C<"0">s and C<"1">s.
+If the input string of pack() is longer than needed, extra characters are
+ignored. A C<*> for the repeat count of pack() means to use all the
+characters of the input field. On unpack()ing the bits are converted to a
+string of C<"0">s and C<"1">s.
=item *
The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, 0-9a-f) long.
-Each byte of the input field of pack() generates 4 bits of the result.
-For non-alphabetical bytes the result is based on the 4 least-significant
-bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
-bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
-C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
+Each character of the input field of pack() generates 4 bits of the result.
+For non-alphabetical characters the result is based on the 4 least-significant
+bits of the input character, i.e., on C<ord($char)%16>. In particular,
+characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
+C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
is compatible with the usual hexadecimal digits, so that C<"a"> and
-C<"A"> both generate the nybble C<0xa==10>. The result for bytes
+C<"A"> both generate the nybble C<0xa==10>. The result for characters
C<"g".."z"> and C<"G".."Z"> is not well-defined.
Starting from the beginning of the input string of pack(), each pair
-of bytes is converted to 1 byte of output. With format C<h> the
-first byte of the pair determines the least-significant nybble of the
-output byte, and with format C<H> it determines the most-significant
+of characters is converted to 1 character of output. With format C<h> the
+first character of the pair determines the least-significant nybble of the
+output character, and with format C<H> it determines the most-significant
nybble.
If the length of the input string is not even, it behaves as if padded
-by a null byte at the end. Similarly, during unpack()ing the "extra"
+by a null character at the end. Similarly, during unpack()ing the "extra"
nybbles are ignored.
-If the input string of pack() is longer than needed, extra bytes are ignored.
-A C<*> for the repeat count of pack() means to use all the bytes of
-the input field. On unpack()ing the bits are converted to a string
+If the input string of pack() is longer than needed, extra characters are
+ignored.
+A C<*> for the repeat count of pack() means to use all the characters of
+the input field. On unpack()ing the nybbles are converted to a string
of hexadecimal digits.
=item *
=item *
-The C</> template character allows packing and unpacking of strings where
-the packed structure contains a byte count followed by the string itself.
-You write I<length-item>C</>I<string-item>.
+The C</> template character allows packing and unpacking of a sequence of
+items where the packed structure contains a packed item count followed by
+the packed items themselves.
+You write I<length-item>C</>I<sequence-item>.
The I<length-item> can be any C<pack> template letter, and describes
how the length value is packed. The ones likely to be of most use are
integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
SNMP) and C<N> (for Sun XDR).
-For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
-C<"Z*">. For C<unpack> the length of the string is obtained from the
-I<length-item>, but if you put in the '*' it will be ignored. For all other
-codes, C<unpack> applies the length value to the next item, which must not
-have a repeat count.
+For C<pack>, the I<sequence-item> may have a repeat count, in which case
+the minimum of that and the number of available items is used as argument
+for the I<length-item>. If it has no repeat count or uses a '*', the number
+of available items is used. For C<unpack> the repeat count is always obtained
+by decoding the packed item count, and the I<sequence-item> must not have a
+repeat count.
+
+If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
+the I<length-item> is a string length, not a number of strings. If there is
+an explicit repeat count for pack, the packed string will be adjusted to that
+given length.
- unpack 'C/a', "\04Gurusamy"; gives 'Guru'
- unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
- pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
+ unpack 'W/a', "\04Gurusamy"; gives ('Guru')
+ unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
+ pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
+ pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
The I<length-item> is not returned explicitly from C<unpack>.
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
- unpack("C*",pack("L",0x12345678))), "\n";
+ unpack("W*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was built is also available
via L<Config>:
If you want portable packed integers you can either use the formats
C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
-modifiers. These modifiers are only available as of perl 5.8.5.
+modifiers. These modifiers are only available as of perl 5.9.2.
See also L<perlport>.
=item *
platforms are using IEEE, there may be subtle differences. Being able
to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
but also very dangerous if you don't know exactly what you're doing.
-It is definetely not a general way to portably store floating point
+It is definitely not a general way to portably store floating point
values.
When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
=item *
-If the pattern begins with a C<U>, the resulting string will be
-treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a
-string with an initial C<U0>, and the bytes that follow will be
-interpreted as Unicode characters. If you don't want this to happen,
-you can begin your pattern with C<C0> (or anything else) to force Perl
-not to UTF-8 encode your string, and then follow this with a C<U*>
-somewhere in your pattern.
+Pack and unpack can operate in two modes, character mode (C<C0> mode) where
+the packed string is processed per character and UTF-8 mode (C<U0> mode)
+where the packed string is processed in its UTF-8-encoded Unicode form on
+a byte by byte basis. Character mode is the default unless the format string
+starts with an C<U>. You can switch mode at any moment with an explicit
+C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
+or until the end of the ()-group in which it was entered.
=item *
You must yourself do any alignment or padding by inserting for example
enough C<'x'>es while packing. There is no way to pack() and unpack()
-could know where the bytes are going to or coming from. Therefore
+could know where the characters are going to or coming from. Therefore
C<pack> (and C<unpack>) handle their output and input as flat
-sequences of bytes.
+sequences of characters.
=item *
is the string "\0a\0\0bc".
-
=item *
C<x> and C<X> accept C<!> modifier. In this case they act as
alignment commands: they jump forward/back to the closest position
-aligned at a multiple of C<count> bytes. For example, to pack() or
+aligned at a multiple of C<count> characters. For example, to pack() or
unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
-use the template C<C x![d] d C[2]>; this assumes that doubles must be
+use the template C<W x![d] d W[2]>; this assumes that doubles must be
aligned on the double's size.
For alignment commands C<count> of 0 is equivalent to C<count> of 1;
=item *
If TEMPLATE requires more arguments to pack() than actually given, pack()
-assumes additional C<""> arguments. If TEMPLATE requires less arguments
+assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
to pack() than actually given, extra arguments are ignored.
=back
Examples:
- $foo = pack("CCCC",65,66,67,68);
+ $foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
- $foo = pack("C4",65,66,67,68);
+ $foo = pack("W4",65,66,67,68);
# same thing
+ $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
- # same thing with Unicode circled letters
+ # same thing with Unicode circled letters. You don't get the UTF-8
+ # bytes because the U at the start of the format caused a switch to
+ # U0-mode, so the UTF-8 bytes get joined into characters
+ $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
+ # This is the UTF-8 encoding of the string in the previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
- # note: the above examples featuring "C" and "c" are true
+ # note: the above examples featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
- # $foo = pack("CCCC",193,194,195,196);
+ # $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
+ $baz = pack('s.l', 12, 4, 34);
+ # short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
=item pos
Returns the offset of where the last C<m//g> search left off for the variable
-in question (C<$_> is used when the variable is not specified). May be
-modified to change that offset. Such modification will also influence
-the C<\G> zero-width assertion in regular expressions. See L<perlre> and
+in question (C<$_> is used when the variable is not specified). Note that
+0 is a valid match offset. C<undef> indicates that the search position
+is reset (usually due to match failure, but can also be because no match has
+yet been performed on the scalar). C<pos> directly accesses the location used
+by the regexp engine to store the offset, so assigning to C<pos> will change
+that offset, and so will also influence the C<\G> zero-width assertion in
+regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
+the return from C<pos> won't change either in this case. See L<perlre> and
L<perlop>.
=item print FILEHANDLE LIST
the print--interpose a C<+> or put parentheses around all the
arguments.
-Note that if you're storing FILEHANDLES in an array or other expression,
-you will have to use a block returning its value instead:
+Note that if you're storing FILEHANDLEs in an array, or if you're using
+any other expression more complex than a scalar variable to retrieve it,
+you will have to use a block returning the filehandle value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
The C<redo> command restarts the loop block without evaluating the
conditional again. The C<continue> block, if any, is not executed. If
the LABEL is omitted, the command refers to the innermost enclosing
-loop. This command is normally used by programs that want to lie to
-themselves about what was just input:
+loop. Programs that want to lie to themselves about what was just input
+normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
unless (ref($r)) {
print "r is not a reference at all.\n";
}
- if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
- print "r is a reference to something that isa hash.\n";
- }
See also L<perlref>.
Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
-versions of Perl which do not support this syntax. The equivalent numeric
+versions of Perl that do not support this syntax. The equivalent numeric
version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
-Otherwise, demands that a library file be included if it hasn't already
+Otherwise, C<ref> demands that a library file be included if it hasn't already
been included. The file is included via the do-FILE mechanism, which is
essentially just a variety of C<eval>. Has semantics similar to the
following subroutine:
behind the scenes. Before C<require> looks for a "F<.pm>" extension,
it will first look for a filename with a "F<.pmc>" extension. A file
with this extension is assumed to be Perl bytecode generated by
-L<B::Bytecode|B::Bytecode>. If this file is found, and it's modification
+L<B::Bytecode|B::Bytecode>. If this file is found, and its modification
time is newer than a coinciding "F<.pm>" non-compiled file, it will be
loaded in place of that non-compiled file ending in a "F<.pm>" extension.
...
}
-If the hook is an object, it must provide an INC method, that will be
+If the hook is an object, it must provide an INC method that will be
called as above, the first parameter being the object itself. (Note that
you must fully qualify the sub's name, as it is always forced into package
C<main>.) Here is a typical code layout:
=item seekdir DIRHANDLE,POS
Sets the current position for the C<readdir> routine on DIRHANDLE. POS
-must be a value returned by C<telldir>. Has the same caveats about
-possible directory compaction as the corresponding system library
+must be a value returned by C<telldir>. C<seekdir> also has the same caveats
+about possible directory compaction as the corresponding system library
routine.
=item select FILEHANDLE
is implementation-dependent. See also L<perlport> for notes on the
portability of C<select>.
+On error, C<select> returns C<undef> and sets C<$!>.
+
+Note: on some Unixes, the select(2) system call may report a socket file
+descriptor as "ready for reading", when actually no data is available,
+thus a subsequent read blocks. It can be avoided using always the
+O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
+details.
+
B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
or <FH>) with C<select>, except as permitted by POSIX, and even
then only on POSIX systems. You have to use C<sysread> instead.
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or
-GETALL, then ARG must be a variable which will hold the returned
+GETALL, then ARG must be a variable that will hold the returned
semid_ds structure or semaphore value array. Returns like C<ioctl>:
the undefined value for error, "C<0 but true>" for zero, or the actual
return value otherwise. The ARG must consist of a vector of native
Calls the System V IPC function semop to perform semaphore operations
such as signalling and waiting. OPSTRING must be a packed array of
semop structures. Each semop structure can be generated with
-C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
-operations is implied by the length of OPSTRING. Returns true if
+C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
+implies the number of semaphore operations. Returns true if
successful, or false if there is an error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
array by 1 and moving everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is omitted, shifts the
C<@_> array within the lexical scope of subroutines and formats, and the
-C<@ARGV> array at file scopes or within the lexical scopes established by
-the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
-constructs.
+C<@ARGV> array outside of a subroutine and also within the lexical scopes
+established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
+and C<END {}> constructs.
See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
same thing to the left end of an array that C<pop> and C<push> do to the
use IPC::SysV;
first to get the correct constant definitions. If CMD is C<IPC_STAT>,
-then ARG must be a variable which will hold the returned C<shmid_ds>
+then ARG must be a variable that will hold the returned C<shmid_ds>
structure. Returns like ioctl: the undefined value for error, "C<0> but
true" for zero, or the actual return value otherwise.
See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
$b as lexicals.
In either case, the subroutine may not be recursive. The values to be
-compared are always passed by reference, so don't modify them.
+compared are always passed by reference and should not be modified.
You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in L<perlsyn> or with C<goto>.
When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
current collation locale. See L<perllocale>.
+sort() 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 sort() (for example, in a C<foreach>, C<map> or C<grep>)
+actually modifies the element in the original list. This is usually
+something to be avoided when writing clear code.
+
Perl 5.6 and earlier used a quicksort algorithm to implement sort.
That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
preserves the input order of elements that compare equal. Although
quicksort's run time is O(NlogN) when averaged over all arrays of
length N, the time can be O(N**2), I<quadratic> behavior, for some
inputs.) In 5.7, the quicksort implementation was replaced with
-a stable mergesort algorithm whose worst case behavior is O(NlogN).
+a stable mergesort algorithm whose worst-case behavior is O(NlogN).
But benchmarks indicated that for some inputs, on some platforms,
the original quicksort was faster. 5.8 has a sort pragma for
limited control of the sort. Its rather blunt control of the
-underlying algorithm may not persist into future perls, but the
+underlying algorithm may not persist into future Perls, but the
ability to characterize the input or output in implementation
independent ways quite probably will. See L<sort>.
Splits the string EXPR into a list of strings and returns that list. By
default, empty leading fields are preserved, and empty trailing ones are
-deleted.
+deleted. (If all fields are empty, they are considered to be trailing.)
In scalar context, returns the number of fields found and splits into
the C<@_> array. Use of split in scalar context is deprecated, however,
produces the output 'h:i:t:h:e:r:e'.
-Using the empty pattern C<//> specifically matches the null string, and is
-not be confused with the use of C<//> to mean "the last successful pattern
-match".
+As a special case for C<split>, using the empty pattern C<//> specifically
+matches only the null string, and is not be confused with the regular use
+of C<//> to mean "the last successful pattern match". So, for C<split>,
+the following:
-Empty leading (or trailing) fields are produced when there are positive width
-matches at the beginning (or end) of the string; a zero-width match at the
-beginning (or end) of the string does not produce an empty field. For
-example:
+ print join(':', split(//, 'hi there'));
+
+produces the output 'h:i: :t:h:e:r:e'.
+
+Empty leading (or trailing) fields are produced when there are positive
+width matches at the beginning (or end) of the string; a zero-width match
+at the beginning (or end) of the string does not produce an empty field.
+For example:
print join(':', split(/(?=\w)/, 'hi there!'));
An explicit format parameter index, such as C<2$>. By default sprintf
will format the next unused argument in the list, but this allows you
-to take the arguments out of order. Eg:
+to take the arguments out of order, e.g.:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for
-the join string using eg C<*2$v>:
+the join string using e.g. C<*2$v>:
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
Arguments are usually formatted to be only as wide as required to
display the given value. You can override the width by putting
a number here, or get the width from the next argument (with C<*>)
-or from a specified argument (with eg C<*2$>):
+or from a specified argument (with e.g. C<*2$>):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
You can specify a precision (for numeric conversions) or a maximum
width (for string conversions) by specifying a C<.> followed by a number.
For floating point formats, with the exception of 'g' and 'G', this specifies
-the number of decimal places to show (the default being 6), eg:
+the number of decimal places to show (the default being 6), e.g.:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For 'g' and 'G', this specifies the maximum number of digits to show,
-including prior to the decimal point as well as after it, eg:
+including prior to the decimal point as well as after it, e.g.:
# these examples are subject to system-specific variation
printf '<%g>', 1; # prints "<1>"
You cannot currently get the precision from a specified number,
but it is intended that this will be possible in the future using
-eg C<.*2$>:
+e.g. C<.*2$>:
printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
programs supply their own seed value (often C<time ^ $$> or C<time ^
($$ + ($$ << 15))>), but that isn't necessary any more.
-Note that you need something much more random than the default seed for
-cryptographic purposes. Checksumming the compressed output of one or more
+For cryptographic purposes, however, you need something much more random
+than the default seed. Checksumming the compressed output of one or more
rapidly changing operating system status programs is the usual method. For
example:
(The epoch was at 00:00 January 1, 1970 GMT.)
-(*) The ctime field is non-portable. In particular, you cannot expect
-it to be a "creation time", see L<perlport/"Files and Filesystems">
-for details.
+(*) Not all fields are supported on all filesystem types. Notably, the
+ctime field is non-portable. In particular, you cannot expect it to be a
+"creation time", see L<perlport/"Files and Filesystems"> for details.
If C<stat> is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
run times with and without it to see which runs faster. Those loops
-which scan for many short constant strings (including the constant
+that scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
one C<study> active at a time--if you study a different scalar the first
is "unstudied". (The way C<study> works is this: a linked list of every
parts of the EXPR and return what was there before in one operation,
just as you can with splice().
-Note that the lvalue returned by by the 3-arg version of substr() acts as
+Note that the lvalue returned by the 3-arg version of substr() acts as
a 'magic bullet'; each time it is assigned to, it remembers which part
of the original string is being modified; for example:
Some of the most common values are C<O_RDONLY> for opening the file in
read-only mode, C<O_WRONLY> for opening the file in write-only mode,
-and C<O_RDWR> for opening the file in read-write mode, and.
+and C<O_RDWR> for opening the file in read-write mode.
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
In many systems the C<O_EXCL> flag is available for opening files in
exclusive mode. This is B<not> locking: exclusiveness means here that
-if the file already exists, sysopen() fails. The C<O_EXCL> wins
-C<O_TRUNC>.
+if the file already exists, sysopen() fails. C<O_EXCL> may not work
+on network filesystems, and has no effect unless the C<O_CREAT> flag
+is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
+being opened if it is a symbolic link. It does not protect against
+symbolic links in the file's path.
-Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
+Sometimes you may want to truncate an already-existing file. This
+can be done using the C<O_TRUNC> flag. The behavior of
+C<O_TRUNC> with C<O_RDONLY> is undefined.
You should seldom if ever use C<0644> as argument to C<sysopen>, because
that takes away the user's option to have a more permissive umask.
that would render sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing this with reads (other
-than C<sysread>, for example
>< or read()) C<print>, C<write>,
+than C<sysread>, for example
C<< <> >> or read()) C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion.
For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
of C<IO::Handle> on any open handles.
The return value is the exit status of the program as returned by the
-C<wait> call. To get the actual exit value shift right by eight (see below).
+C<wait> call. To get the actual exit value, shift right by eight (see below).
See also L</exec>. This is I<not> what you want to use to capture
the output from a command, for that you should use merely backticks or
C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
printf "child exited with value %d\n", $? >> 8;
}
-or more portably by using the W*() calls of the POSIX extension;
-see L<perlport> for more information.
+Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
+with the W*() calls of the POSIX extension.
When the arguments get executed via the system shell, results
and return codes will be subject to its quirks and capabilities.
There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
-Do not use tell() on a filehandle that has been opened using
-sysopen(), use sysseek() for that as described above. Why? Because
-sysopen() creates unbuffered, "raw", filehandles, while open() creates
-buffered filehandles. sysseek() make sense only on the first kind,
-tell() only makes sense on the second kind.
+Do not use tell() (or other buffered I/O operations) on a file handle
+that has been manipulated by sysread(), syswrite() or sysseek().
+Those functions ignore the buffering, while tell() does not.
=item telldir DIRHANDLE
Returns the current position of the C<readdir> routines on DIRHANDLE.
Value may be given to C<seekdir> to access a particular location in a
-directory. Has the same caveats about possible directory compaction as
-the corresponding system library routine.
+directory. C<telldir> has the same caveats about possible directory
+compaction as the corresponding system library routine.
=item tie VARIABLE,CLASSNAME,LIST
=item time
Returns the number of non-leap seconds since whatever time the system
-considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
-and 00:00:00 UTC, January 1, 1970 for most other systems).
-Suitable for feeding to C<gmtime> and C<localtime>.
+considers to be the epoch, suitable for feeding to C<gmtime> and
+C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
+a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
+1904 in the current local time zone for its epoch.
For measuring time in better granularity than one second,
you may use either the Time::HiRes module (from CPAN, and starting from
The string is broken into chunks described by the TEMPLATE. Each chunk
is converted separately to a value. Typically, either the string is a result
-of C<pack>, or the bytes of the string represent a C structure of some
+of C<pack>, or the characters of the string represent a C structure of some
kind.
The TEMPLATE has the same format as in the C<pack> function.
and then there's
- sub ordinal { unpack("c",$_[0]); } # same as ord()
+ sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field with
a %<number> to indicate that
$checksum = do {
local $/; # slurp!
- unpack("%32C*",<>) % 65535;
+ unpack("%32W*",<>) % 65535;
};
The following efficiently counts the number of set bits in a bit vector:
Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
-versions of Perl which do not support this syntax. The equivalent numeric
+versions of Perl that do not support this syntax. The equivalent numeric
version should be used instead.
use v5.6.1; # compile time version check
and modification times, in that order. Returns the number of files
successfully changed. The inode change time of each file is set
to the current time. For example, this code has the same effect as the
-Unix touch(1) command when the files I<already exist>.
+Unix touch(1) command when the files I<already exist> and belong to
+the user running the program:
#!/usr/bin/perl
$atime = $mtime = time;
the utime(2) function in the C library will be called with a null second
argument. On most systems, this will set the file's access and
modification times to the current time (i.e. equivalent to the example
-above.)
+above) and will even work on other users' files where you have write
+permission:
utime undef, undef, @ARGV;
The string should not contain any character with the value > 255 (which
can only happen if you're using UTF-8 encoding). If it does, it will be
-treated as something which is not UTF-8 encoded. When the C<vec> was
+treated as something that is not UTF-8 encoded. When the C<vec> was
assigned to, other parts of your program will also no longer consider the
string to be UTF-8 encoded. In other words, if you do have such characters
in your string, vec() will operate on the actual byte string, and not the
Behaves like the wait(2) system call on your system: it waits for a child
process to terminate and returns the pid of the deceased process, or
-C<-1> if there are no child processes. The status is returned in C<$?>.
+C<-1> if there are no child processes. The status is returned in C<$?>
+and C<{^CHILD_ERROR_NATIVE}>.
Note that a return value of C<-1> could mean that child processes are
being automatically reaped, as described in L<perlipc>.
Waits for a particular child process to terminate and returns the pid of
the deceased process, or C<-1> if there is no such child process. On some
systems, a value of 0 indicates that there are processes still running.
-The status is returned in C<$?>. If you say
+The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
use POSIX ":sys_wait_h";
#...
=item wantarray
Returns true if the context of the currently executing subroutine or
-eval() block is looking for a list value. Returns false if the context is
+C<eval> is looking for a list value. Returns false if the context is
looking for a scalar. Returns the undefined value if the context is
looking for no value (void context).
my @a = complex_calculation();
return wantarray ? @a : "@a";
+C<wantarray()>'s result is unspecified in the top level of a file,
+in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
+method.
+
This function should have been named wantlist() instead.
=item warn LIST
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