=head2 Predefined Names
-The following names have special meaning to Perl. Most of the
-punctuation names have reasonable mnemonics, or analogues in one of
-the shells. Nevertheless, if you wish to use the long variable names,
-you just need to say
+The following names have special meaning to Perl. Most
+punctuation names have reasonable mnemonics, or analogs in the
+shells. Nevertheless, if you wish to use long variable names,
+you need only say
use English;
at the top of your program. This will alias all the short names to the
-long names in the current package. Some of them even have medium names,
+long names in the current package. Some even have medium names,
generally borrowed from B<awk>.
-To go a step further, those variables that depend on the currently
-selected filehandle may instead be set by calling an object method on
-the FileHandle object. (Summary lines below for this contain the word
-HANDLE.) First you must say
+If you don't mind the performance hit, variables that depend on the
+currently selected filehandle may instead be set by calling an
+appropriate object method on the IO::Handle object. (Summary lines
+below for this contain the word HANDLE.) First you must say
- use FileHandle;
+ use IO::Handle;
after which you may use either
method HANDLE EXPR
-or
+or more safely,
HANDLE->method(EXPR)
-Each of the methods returns the old value of the FileHandle attribute.
+Each method returns the old value of the IO::Handle attribute.
The methods each take an optional EXPR, which if supplied specifies the
-new value for the FileHandle attribute in question. If not supplied,
-most of the methods do nothing to the current value, except for
+new value for the IO::Handle attribute in question. If not supplied,
+most methods do nothing to the current value--except for
autoflush(), which will assume a 1 for you, just to be different.
+Because loading in the IO::Handle class is an expensive operation, you should
+learn how to use the regular built-in variables.
A few of these variables are considered "read-only". This means that if
you try to assign to this variable, either directly or indirectly through
a reference, you'll raise a run-time exception.
+The following list is ordered by scalar variables first, then the
+arrays, then the hashes.
+
=over 8
=item $ARG
The default input and pattern-searching space. The following pairs are
equivalent:
- while (<>) {...} # equivalent in only while!
- while ($_ = <>) {...}
+ while (<>) {...} # equivalent only in while!
+ while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
- chop
- chop($_)
+ chomp
+ chomp($_)
-Here are the places where Perl will assume $_ even if you
+Here are the places where Perl will assume $_ even if you
don't use it:
=over 3
The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
without an C<=~> operator.
-=item *
+=item *
The default iterator variable in a C<foreach> loop if no other
variable is supplied.
-=item *
+=item *
The implicit iterator variable in the grep() and map() functions.
-=item *
+=item *
-The default place to put an input record when a C<E<lt>FHE<gt>>
+The default place to put an input record when a C<< <FH> >>
operation's result is tested by itself as the sole criterion of a C<while>
-test. Note that outside of a C<while> test, this will not happen.
+test. Outside a C<while> test, this will not happen.
=back
=over 8
-=item $E<lt>I<digit>E<gt>
+=item $<I<digits>>
-Contains the sub-pattern from the corresponding set of parentheses in
-the last pattern matched, not counting patterns matched in nested
-blocks that have been exited already. (Mnemonic: like \digit.)
-These variables are all read-only.
+Contains the subpattern from the corresponding set of capturing
+parentheses from the last pattern match, not counting patterns
+matched in nested blocks that have been exited already. (Mnemonic:
+like \digits.) These variables are all read-only and dynamically
+scoped to the current BLOCK.
=item $MATCH
The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
-BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
+BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
+and dynamically scoped to the current BLOCK.
+
+The use of this variable anywhere in a program imposes a considerable
+performance penalty on all regular expression matches. See L<BUGS>.
=item $PREMATCH
enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
string.) This variable is read-only.
+The use of this variable anywhere in a program imposes a considerable
+performance penalty on all regular expression matches. See L<BUGS>.
+
=item $POSTMATCH
=item $'
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
-This variable is read-only.
+This variable is read-only and dynamically scoped to the current BLOCK.
+
+The use of this variable anywhere in a program imposes a considerable
+performance penalty on all regular expression matches. See L<BUGS>.
=item $LAST_PAREN_MATCH
=item $+
The last bracket matched by the last search pattern. This is useful if
-you don't know which of a set of alternative patterns matched. For
+you don't know which one of a set of alternative patterns matched. For
example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
-This variable is read-only.
+This variable is read-only and dynamically scoped to the current BLOCK.
+
+=item @LAST_MATCH_END
+
+=item @+
+
+This array holds the offsets of the ends of the last successful
+submatches in the currently active dynamic scope. C<$+[0]> is
+the offset into the string of the end of the entire match. This
+is the same value as what the C<pos> function returns when called
+on the variable that was matched against. The I<n>th element
+of this array holds the offset of the I<n>th submatch, so
+C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
+past where $2 ends, and so on. You can use C<$#+> to determine
+how many subgroups were in the last successful match. See the
+examples given for the C<@-> variable.
=item $MULTILINE_MATCHING
=item $*
-Set to 1 to do multi-line matching within a string, 0 to tell Perl
-that it can assume that strings contain a single line, for the purpose
-of optimizing pattern matches. Pattern matches on strings containing
-multiple newlines can produce confusing results when "C<$*>" is 0. Default
-is 0. (Mnemonic: * matches multiple things.) Note that this variable
-influences the interpretation of only "C<^>" and "C<$>". A literal newline can
-be searched for even when C<$* == 0>.
+Set to a non-zero integer value to do multi-line matching within a
+string, 0 (or undefined) to tell Perl that it can assume that strings
+contain a single line, for the purpose of optimizing pattern matches.
+Pattern matches on strings containing multiple newlines can produce
+confusing results when C<$*> is 0 or undefined. Default is undefined.
+(Mnemonic: * matches multiple things.) This variable influences the
+interpretation of only C<^> and C<$>. A literal newline can be searched
+for even when C<$* == 0>.
-Use of "C<$*>" is deprecated in modern perls.
+Use of C<$*> is deprecated in modern Perl, supplanted by
+the C</s> and C</m> modifiers on pattern matching.
+
+Assigning a non-numerical value to C<$*> triggers a warning (and makes
+C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
+makes that an implicit C<int> is applied on the value.
=item input_line_number HANDLE EXPR
=item $.
-The current input line number for the last file handle from
-which you read (or performed a C<seek> or C<tell> on). An
-explicit close on a filehandle resets the line number. Because
-"C<E<lt>E<gt>>" never does an explicit close, line numbers increase
-across ARGV files (but see examples under eof()). Localizing C<$.> has
-the effect of also localizing Perl's notion of "the last read
+The current input record number for the last file handle from which
+you just read() (or called a C<seek> or C<tell> on). The value
+may be different from the actual physical line number in the file,
+depending on what notion of "line" is in effect--see C<$/> on how
+to change that. An explicit close on a filehandle resets the line
+number. Because C<< <> >> never does an explicit close, line
+numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
+Consider this variable read-only: setting it does not reposition
+the seek pointer; you'll have to do that on your own. Localizing C<$.>
+has the effect of also localizing Perl's notion of "the last read
filehandle". (Mnemonic: many programs use "." to mean the current line
number.)
=item $/
-The input record separator, newline by default. Works like B<awk>'s RS
-variable, including treating empty lines as delimiters if set to the
-null string. (Note: An empty line cannot contain any spaces or
-tabs.) You may set it to a multicharacter string to match a
-multi-character delimiter. Note that setting it to C<"\n\n"> means
-something slightly different than setting it to C<"">, if the file
-contains consecutive empty lines. Setting it to C<""> will treat two
-or more consecutive empty lines as a single empty line. Setting it to
-C<"\n\n"> will blindly assume that the next input character belongs to
-the next paragraph, even if it's a newline. (Mnemonic: / is used to
-delimit line boundaries when quoting poetry.)
-
- undef $/;
- $_ = <FH>; # whole file now here
+The input record separator, newline by default. This
+influences Perl's idea of what a "line" is. Works like B<awk>'s RS
+variable, including treating empty lines as a terminator if set to
+the null string. (An empty line cannot contain any spaces
+or tabs.) You may set it to a multi-character string to match a
+multi-character terminator, or to C<undef> to read through the end
+of file. Setting it to C<"\n\n"> means something slightly
+different than setting to C<"">, if the file contains consecutive
+empty lines. Setting to C<""> will treat two or more consecutive
+empty lines as a single empty line. Setting to C<"\n\n"> will
+blindly assume that the next input character belongs to the next
+paragraph, even if it's a newline. (Mnemonic: / delimits
+line boundaries when quoting poetry.)
+
+ undef $/; # enable "slurp" mode
+ $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
-Remember: the value of $/ is a string, not a regexp. AWK has to be
-better for something :-)
+Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
+better for something. :-)
+
+Setting C<$/> to a reference to an integer, scalar containing an integer, or
+scalar that's convertible to an integer will attempt to read records
+instead of lines, with the maximum record size being the referenced
+integer. So this:
+
+ $/ = \32768; # or \"32768", or \$var_containing_32768
+ open(FILE, $myfile);
+ $_ = <FILE>;
+
+will read a record of no more than 32768 bytes from FILE. If you're
+not reading from a record-oriented file (or your OS doesn't have
+record-oriented files), then you'll likely get a full chunk of data
+with every read. If a record is larger than the record size you've
+set, you'll get the record back in pieces.
+
+On VMS, record reads are done with the equivalent of C<sysread>,
+so it's best not to mix record and non-record reads on the same
+file. (This is unlikely to be a problem, because any file you'd
+want to read in record mode is probably unusable in line mode.)
+Non-VMS systems do normal I/O, so it's safe to mix record and
+non-record reads of a file.
+
+See also L<perlport/"Newlines">. Also see C<$.>.
=item autoflush HANDLE EXPR
=item $|
-If set to nonzero, forces a flush after every write or print on the
-currently selected output channel. Default is 0 (regardless of whether
-the channel is actually buffered by the system or not; C<$|> tells you
-only whether you've asked Perl explicitly to flush after each write).
-Note that STDOUT will typically be line buffered if output is to the
-terminal and block buffered otherwise. Setting this variable is useful
-primarily when you are outputting to a pipe, such as when you are running
-a Perl script under rsh and want to see the output as it's happening. This
-has no effect on input buffering.
-(Mnemonic: when you want your pipes to be piping hot.)
+If set to nonzero, forces a flush right away and after every write
+or print on the currently selected output channel. Default is 0
+(regardless of whether the channel is really buffered by the
+system or not; C<$|> tells you only whether you've asked Perl
+explicitly to flush after each write). STDOUT will
+typically be line buffered if output is to the terminal and block
+buffered otherwise. Setting this variable is useful primarily when
+you are outputting to a pipe or socket, such as when you are running
+a Perl program under B<rsh> and want to see the output as it's
+happening. This has no effect on input buffering. See L<perlfunc/getc>
+for that. (Mnemonic: when you want your pipes to be piping hot.)
=item output_field_separator HANDLE EXPR
=item $,
The output field separator for the print operator. Ordinarily the
-print operator simply prints out the comma-separated fields you
-specify. To get behavior more like B<awk>, set this variable
-as you would set B<awk>'s OFS variable to specify what is printed
-between fields. (Mnemonic: what is printed when there is a , in your
-print statement.)
+print operator simply prints out its arguments without further
+adornment. To get behavior more like B<awk>, set this variable as
+you would set B<awk>'s OFS variable to specify what is printed
+between fields. (Mnemonic: what is printed when there is a "," in
+your print statement.)
=item output_record_separator HANDLE EXPR
=item $\
The output record separator for the print operator. Ordinarily the
-print operator simply prints out the comma-separated fields you
-specify, with no trailing newline or record separator assumed.
-To get behavior more like B<awk>, set this variable as you would
-set B<awk>'s ORS variable to specify what is printed at the end of the
-print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the
-print. Also, it's just like C<$/>, but it's what you get "back" from
-Perl.)
+print operator simply prints out its arguments as is, with no
+trailing newline or other end-of-record string added. To get
+behavior more like B<awk>, set this variable as you would set
+B<awk>'s ORS variable to specify what is printed at the end of the
+print. (Mnemonic: you set C<$\> instead of adding "\n" at the
+end of the print. Also, it's just like C<$/>, but it's what you
+get "back" from Perl.)
=item $LIST_SEPARATOR
=item $"
-This is like "C<$,>" except that it applies to array values interpolated
-into a double-quoted string (or similar interpreted string). Default
-is a space. (Mnemonic: obvious, I think.)
+This is like C<$,> except that it applies to array and slice values
+interpolated into a double-quoted string (or similar interpreted
+string). Default is a space. (Mnemonic: obvious, I think.)
=item $SUBSCRIPT_SEPARATOR
=item $;
-The subscript separator for multi-dimensional array emulation. If you
+The subscript separator for multidimensional array emulation. If you
refer to a hash element as
$foo{$a,$b,$c}
($foo{$a},$foo{$b},$foo{$c})
-Default is "\034", the same as SUBSEP in B<awk>. Note that if your
-keys contain binary data there might not be any safe value for "C<$;>".
+Default is "\034", the same as SUBSEP in B<awk>. If your
+keys contain binary data there might not be any safe value for C<$;>.
(Mnemonic: comma (the syntactic subscript separator) is a
-semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already
+semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
taken for something more important.)
-Consider using "real" multi-dimensional arrays.
+Consider using "real" multidimensional arrays as described
+in L<perllol>.
=item $OFMT
The output format for printed numbers. This variable is a half-hearted
attempt to emulate B<awk>'s OFMT variable. There are times, however,
-when B<awk> and Perl have differing notions of what is in fact
-numeric. The initial value is %.I<n>g, where I<n> is the value
+when B<awk> and Perl have differing notions of what counts as
+numeric. The initial value is "%.I<n>g", where I<n> is the value
of the macro DBL_DIG from your system's F<float.h>. This is different from
-B<awk>'s default OFMT setting of %.6g, so you need to set "C<$#>"
+B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
-Use of "C<$#>" is deprecated.
+Use of C<$#> is deprecated.
=item format_page_number HANDLE EXPR
=item $%
The current page number of the currently selected output channel.
+Used with formats.
(Mnemonic: % is page number in B<nroff>.)
=item format_lines_per_page HANDLE EXPR
=item $=
The current page length (printable lines) of the currently selected
-output channel. Default is 60. (Mnemonic: = has horizontal lines.)
+output channel. Default is 60.
+Used with formats.
+(Mnemonic: = has horizontal lines.)
=item format_lines_left HANDLE EXPR
=item $-
The number of lines left on the page of the currently selected output
-channel. (Mnemonic: lines_on_page - lines_printed.)
+channel.
+Used with formats.
+(Mnemonic: lines_on_page - lines_printed.)
+
+=item @LAST_MATCH_START
+
+=item @-
+
+$-[0] is the offset of the start of the last successful match.
+C<$-[>I<n>C<]> is the offset of the start of the substring matched by
+I<n>-th subpattern, or undef if the subpattern did not match.
+
+Thus after a match against $_, $& coincides with C<substr $_, $-[0],
+$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
+$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
+C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
+matched subgroup in the last successful match. Contrast with
+C<$#+>, the number of subgroups in the regular expression. Compare
+with C<@+>.
+
+This array holds the offsets of the beginnings of the last
+successful submatches in the currently active dynamic scope.
+C<$-[0]> is the offset into the string of the beginning of the
+entire match. The I<n>th element of this array holds the offset
+of the I<n>th submatch, so C<$+[1]> is the offset where $1
+begins, C<$+[2]> the offset where $2 begins, and so on.
+You can use C<$#-> to determine how many subgroups were in the
+last successful match. Compare with the C<@+> variable.
+
+After a match against some variable $var:
+
+=over 5
+
+=item C<$`> is the same as C<substr($var, 0, $-[0])>
+
+=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
+
+=item C<$'> is the same as C<substr($var, $+[0])>
+
+=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
+
+=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
+
+=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
+
+=back
=item format_name HANDLE EXPR
=item $~
The name of the current report format for the currently selected output
-channel. Default is name of the filehandle. (Mnemonic: brother to
-"C<$^>".)
+channel. Default is the name of the filehandle. (Mnemonic: brother to
+C<$^>.)
=item format_top_name HANDLE EXPR
=item $^
The name of the current top-of-page format for the currently selected
-output channel. Default is name of the filehandle with _TOP
+output channel. Default is the name of the filehandle with _TOP
appended. (Mnemonic: points to top of page.)
=item format_line_break_characters HANDLE EXPR
=item $:
The current set of characters after which a string may be broken to
-fill continuation fields (starting with ^) in a format. Default is
+fill continuation fields (starting with ^) in a format. Default is
S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
poetry is a part of a line.)
=item $^L
-What formats output to perform a form feed. Default is \f.
+What formats output as a form feed. Default is \f.
=item $ACCUMULATOR
=item $^A
The current value of the write() accumulator for format() lines. A format
-contains formline() commands that put their result into C<$^A>. After
+contains formline() calls that put their result into C<$^A>. After
calling its format, write() prints out the contents of C<$^A> and empties.
-So you never actually see the contents of C<$^A> unless you call
+So you never really see the contents of C<$^A> unless you call
formline() yourself and then look at it. See L<perlform> and
L<perlfunc/formline()>.
=item $?
-The status returned by the last pipe close, back-tick (C<``>) command,
-or system() operator. Note that this is the status word returned by
-the wait() system call (or else is made up to look like it). Thus,
-the exit value of the subprocess is actually (C<$? E<gt>E<gt> 8>), and
-C<$? & 255> gives which signal, if any, the process died from, and
-whether there was a core dump. (Mnemonic: similar to B<sh> and
-B<ksh>.)
+The status returned by the last pipe close, backtick (C<``>) command,
+successful call to wait() or waitpid(), or from the system()
+operator. This is just the 16-bit status word returned by the
+wait() system call (or else is made up to look like it). Thus, the
+exit value of the subprocess is really (C<<< $? >> 8 >>>), and
+C<$? & 127> gives which signal, if any, the process died from, and
+C<$? & 128> reports whether there was a core dump. (Mnemonic:
+similar to B<sh> and B<ksh>.)
+
+Additionally, if the C<h_errno> variable is supported in C, its value
+is returned via $? if any C<gethost*()> function fails.
-Note that if you have installed a signal handler for C<SIGCHLD>, the
+If you have installed a signal handler for C<SIGCHLD>, the
value of C<$?> will usually be wrong outside that handler.
Inside an C<END> subroutine C<$?> contains the value that is going to be
given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
-change the exit status of the script.
+change the exit status of your program. For example:
+
+ END {
+ $? = 1 if $? == 255; # die would make it 255
+ }
Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
actual VMS exit status, instead of the default emulation of POSIX
status.
+Also see L<Error Indicators>.
+
=item $OS_ERROR
=item $ERRNO
=item $!
-If used in a numeric context, yields the current value of errno, with
-all the usual caveats. (This means that you shouldn't depend on the
-value of "C<$!>" to be anything in particular unless you've gotten a
-specific error return indicating a system error.) If used in a string
-context, yields the corresponding system error string. You can assign
-to "C<$!>" to set I<errno> if, for instance, you want "C<$!>" to return the
-string for error I<n>, or you want to set the exit value for the die()
-operator. (Mnemonic: What just went bang?)
+If used numerically, yields the current value of the C C<errno>
+variable, with all the usual caveats. (This means that you shouldn't
+depend on the value of C<$!> to be anything in particular unless
+you've gotten a specific error return indicating a system error.)
+If used an a string, yields the corresponding system error string.
+You can assign a number to C<$!> to set I<errno> if, for instance,
+you want C<"$!"> to return the string for error I<n>, or you want
+to set the exit value for the die() operator. (Mnemonic: What just
+went bang?)
+
+Also see L<Error Indicators>.
=item $EXTENDED_OS_ERROR
=item $^E
-More specific information about the last system error than that provided by
-C<$!>, if available. (If not, it's just C<$!> again, except under OS/2.)
-At the moment, this differs from C<$!> under only VMS and OS/2, where it
-provides the VMS status value from the last system error, and OS/2 error
-code of the last call to OS/2 API which was not directed via CRT. The
-caveats mentioned in the description of C<$!> apply here, too.
-(Mnemonic: Extra error explanation.)
+Error information specific to the current operating system. At
+the moment, this differs from C<$!> under only VMS, OS/2, and Win32
+(and for MacPerl). On all other platforms, C<$^E> is always just
+the same as C<$!>.
+
+Under VMS, C<$^E> provides the VMS status value from the last
+system error. This is more specific information about the last
+system error than that provided by C<$!>. This is particularly
+important when C<$!> is set to B<EVMSERR>.
-Note that under OS/2 C<$!> and C<$^E> do not track each other, so if an
-OS/2-specific call is performed, you may need to check both.
+Under OS/2, C<$^E> is set to the error code of the last call to
+OS/2 API either via CRT, or directly from perl.
+
+Under Win32, C<$^E> always returns the last error information
+reported by the Win32 call C<GetLastError()> which describes
+the last error from within the Win32 API. Most Win32-specific
+code will report errors via C<$^E>. ANSI C and Unix-like calls
+set C<errno> and so most portable Perl code will report errors
+via C<$!>.
+
+Caveats mentioned in the description of C<$!> generally apply to
+C<$^E>, also. (Mnemonic: Extra error explanation.)
+
+Also see L<Error Indicators>.
=item $EVAL_ERROR
=item $@
-The Perl syntax error message from the last eval() command. If null, the
+The Perl syntax error message from the last eval() operator. If null, the
last eval() parsed and executed correctly (although the operations you
invoked may have failed in the normal fashion). (Mnemonic: Where was
the syntax error "at"?)
-Note that warning messages are not collected in this variable. You can,
+Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
-below.
+as described below.
+
+Also see L<Error Indicators>.
=item $PROCESS_ID
=item $$
-The process number of the Perl running this script. (Mnemonic: same
-as shells.)
+The process number of the Perl running this script. You should
+consider this variable read-only, although it will be altered
+across fork() calls. (Mnemonic: same as shells.)
=item $REAL_USER_ID
=item $<
-The real uid of this process. (Mnemonic: it's the uid you came I<FROM>,
-if you're running setuid.)
+The real uid of this process. (Mnemonic: it's the uid you came I<from>,
+if you're running setuid.) You can change both the real uid and
+the effective uid at the same time by using POSIX::setuid().
=item $EFFECTIVE_USER_ID
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
-(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note:
-"C<$E<lt>>" and "C<$E<gt>>" can be swapped on only machines supporting setreuid().
+You can change both the effective uid and the real uid at the same
+time by using POSIX::setuid().
+
+(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
+C<< $< >> and C<< $> >> can be swapped only on machines
+supporting setreuid().
=item $REAL_GROUP_ID
membership in multiple groups simultaneously, gives a space separated
list of groups you are in. The first number is the one returned by
getgid(), and the subsequent ones by getgroups(), one of which may be
-the same as the first number. (Mnemonic: parentheses are used to I<GROUP>
-things. The real gid is the group you I<LEFT>, if you're running setgid.)
+the same as the first number.
+
+However, a value assigned to C<$(> must be a single number used to
+set the real gid. So the value given by C<$(> should I<not> be assigned
+back to C<$(> without being forced numeric, such as by adding zero.
+
+You can change both the real gid and the effective gid at the same
+time by using POSIX::setgid().
+
+(Mnemonic: parentheses are used to I<group> things. The real gid is the
+group you I<left>, if you're running setgid.)
=item $EFFECTIVE_GROUP_ID
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in. The first number is the one
returned by getegid(), and the subsequent ones by getgroups(), one of
-which may be the same as the first number. (Mnemonic: parentheses are
-used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for
-you, if you're running setgid.)
+which may be the same as the first number.
-Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can be set only on
-machines that support the corresponding I<set[re][ug]id()> routine. "C<$(>"
-and "C<$)>" can be swapped on only machines supporting setregid(). Because
-Perl doesn't currently use initgroups(), you can't set your group vector to
-multiple groups.
+Similarly, a value assigned to C<$)> must also be a space-separated
+list of numbers. The first number sets the effective gid, and
+the rest (if any) are passed to setgroups(). To get the effect of an
+empty list for setgroups(), just repeat the new effective gid; that is,
+to force an effective gid of 5 and an effectively empty setgroups()
+list, say C< $) = "5 5" >.
+
+You can change both the effective gid and the real gid at the same
+time by using POSIX::setgid() (use only a single numeric argument).
+
+(Mnemonic: parentheses are used to I<group> things. The effective gid
+is the group that's I<right> for you, if you're running setgid.)
+
+C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
+machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
+and C<$)> can be swapped only on machines supporting setregid().
=item $PROGRAM_NAME
=item $0
-Contains the name of the file containing the Perl script being
-executed. Assigning to "C<$0>" modifies the argument area that the ps(1)
-program sees. This is more useful as a way of indicating the
-current program state than it is for hiding the program you're running.
+Contains the name of the program being executed. On some operating
+systems assigning to C<$0> modifies the argument area that the B<ps>
+program sees. This is more useful as a way of indicating the current
+program state than it is for hiding the program you're running.
(Mnemonic: same as B<sh> and B<ksh>.)
+Note for BSD users: setting C<$0> does not completely remove "perl"
+from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
+result in C<"perl: foobar (perl)">. This is an operating system
+feature.
+
=item $[
The index of the first element in an array, and of the first character
-in a substring. Default is 0, but you could set it to 1 to make
-Perl behave more like B<awk> (or Fortran) when subscripting and when
-evaluating the index() and substr() functions. (Mnemonic: [ begins
-subscripts.)
+in a substring. Default is 0, but you could theoretically set it
+to 1 to make Perl behave more like B<awk> (or Fortran) when
+subscripting and when evaluating the index() and substr() functions.
+(Mnemonic: [ begins subscripts.)
-As of Perl 5, assignment to "C<$[>" is treated as a compiler directive,
-and cannot influence the behavior of any other file. Its use is
-discouraged.
-
-=item $PERL_VERSION
+As of release 5 of Perl, assignment to C<$[> is treated as a compiler
+directive, and cannot influence the behavior of any other file.
+Its use is highly discouraged.
=item $]
-The string printed out when you say C<perl -v>.
-(This is currently I<BROKEN>).
-It can be used to
-determine at the beginning of a script whether the perl interpreter
-executing the script is in the right range of versions. If used in a
-numeric context, returns the version + patchlevel / 1000. Example:
+The version + patchlevel / 1000 of the Perl interpreter. This variable
+can be used to determine whether the Perl interpreter executing a
+script is in the right range of versions. (Mnemonic: Is this version
+of perl in the right bracket?) Example:
- # see if getc is available
- ($version,$patchlevel) =
- $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
- print STDERR "(No filename completion available.)\n"
- if $version * 1000 + $patchlevel < 2016;
+ warn "No checksumming!\n" if $] < 3.019;
-or, used numerically,
+See also the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
- warn "No checksumming!\n" if $] < 3.019;
+The use of this variable is deprecated. The floating point representation
+can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
+more modern representation of the Perl version that allows accurate string
+comparisons.
-(Mnemonic: Is this version of perl in the right bracket?)
+=item $COMPILING
+
+=item $^C
+
+The current value of the flag associated with the B<-c> switch.
+Mainly of use with B<-MO=...> to allow code to alter its behavior
+when being compiled, such as for example to AUTOLOAD at compile
+time rather than normal, deferred loading. See L<perlcc>. Setting
+C<$^C = 1> is similar to calling C<B::minus_c>.
=item $DEBUGGING
descriptors are passed to exec()ed processes, while higher file
descriptors are not. Also, during an open(), system file descriptors are
preserved even if the open() fails. (Ordinary file descriptors are
-closed before the open() is attempted.) Note that the close-on-exec
+closed before the open() is attempted.) The close-on-exec
status of a file descriptor will be decided according to the value of
-C<$^F> at the time of the open, not the time of the exec.
+C<$^F> when the corresponding file, pipe, or socket was opened, not the
+time of the exec().
=item $^H
-The current set of syntax checks enabled by C<use strict>. See the
-documentation of C<strict> for more details.
+WARNING: This variable is strictly for internal use only. Its availability,
+behavior, and contents are subject to change without notice.
+
+This variable contains compile-time hints for the Perl interpreter. At the
+end of compilation of a BLOCK the value of this variable is restored to the
+value when the interpreter started to compile the BLOCK.
+
+When perl begins to parse any block construct that provides a lexical scope
+(e.g., eval body, required file, subroutine body, loop body, or conditional
+block), the existing value of $^H is saved, but its value is left unchanged.
+When the compilation of the block is completed, it regains the saved value.
+Between the points where its value is saved and restored, code that
+executes within BEGIN blocks is free to change the value of $^H.
+
+This behavior provides the semantic of lexical scoping, and is used in,
+for instance, the C<use strict> pragma.
+
+The contents should be an integer; different bits of it are used for
+different pragmatic flags. Here's an example:
+
+ sub add_100 { $^H |= 0x100 }
+
+ sub foo {
+ BEGIN { add_100() }
+ bar->baz($boon);
+ }
+
+Consider what happens during execution of the BEGIN block. At this point
+the BEGIN block has already been compiled, but the body of foo() is still
+being compiled. The new value of $^H will therefore be visible only while
+the body of foo() is being compiled.
+
+Substitution of the above BEGIN block with:
+
+ BEGIN { require strict; strict->import('vars') }
+
+demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
+version of the same lexical pragma:
+
+ BEGIN { require strict; strict->import('vars') if $condition }
+
+=item %^H
+
+WARNING: This variable is strictly for internal use only. Its availability,
+behavior, and contents are subject to change without notice.
+
+The %^H hash provides the same scoping semantic as $^H. This makes it
+useful for implementation of lexically scoped pragmas.
=item $INPLACE_EDIT
The current value of the inplace-edit extension. Use C<undef> to disable
inplace editing. (Mnemonic: value of B<-i> switch.)
+=item $^M
+
+By default, running out of memory is an untrappable, fatal error.
+However, if suitably built, Perl can use the contents of C<$^M>
+as an emergency memory pool after die()ing. Suppose that your Perl
+were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
+Then
+
+ $^M = 'a' x (1 << 16);
+
+would allocate a 64K buffer for use in an emergency. See the
+F<INSTALL> file in the Perl distribution for information on how to
+enable this option. To discourage casual use of this advanced
+feature, there is no L<English|English> long name for this variable.
+
=item $OSNAME
=item $^O
The name of the operating system under which this copy of Perl was
built, as determined during the configuration process. The value
-is identical to C<$Config{'osname'}>.
+is identical to C<$Config{'osname'}>. See also L<Config> and the
+B<-V> command-line switch documented in L<perlrun>.
=item $PERLDB
=item $^P
-The internal flag that the debugger clears so that it doesn't debug
-itself. You could conceivably disable debugging yourself by clearing
-it.
+The internal variable for debugging support. The meanings of the
+various bits are subject to change, but currently indicate:
+
+=over 6
+
+=item 0x01
+
+Debug subroutine enter/exit.
+
+=item 0x02
+
+Line-by-line debugging.
+
+=item 0x04
+
+Switch off optimizations.
+
+=item 0x08
+
+Preserve more data for future interactive inspections.
+
+=item 0x10
+
+Keep info about source lines on which a subroutine is defined.
+
+=item 0x20
+
+Start with single-step on.
+
+=item 0x40
+
+Use subroutine address instead of name when reporting.
+
+=item 0x80
+
+Report C<goto &subroutine> as well.
+
+=item 0x100
+
+Provide informative "file" names for evals based on the place they were compiled.
+
+=item 0x200
+
+Provide informative names to anonymous subroutines based on the place they
+were compiled.
+
+=back
+
+Some bits may be relevant at compile-time only, some at
+run-time only. This is a new mechanism and the details may change.
+
+=item $LAST_REGEXP_CODE_RESULT
+
+=item $^R
+
+The result of evaluation of the last successful C<(?{ code })>
+regular expression assertion (see L<perlre>). May be written to.
+
+=item $EXCEPTIONS_BEING_CAUGHT
+
+=item $^S
+
+Current state of the interpreter. Undefined if parsing of the current
+module/eval is not finished (may happen in $SIG{__DIE__} and
+$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
=item $BASETIME
=item $^T
-The time at which the script began running, in seconds since the
+The time at which the program began running, in seconds since the
epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
-and B<-C> filetests are
-based on this value.
+and B<-C> filetests are based on this value.
+
+=item $PERL_VERSION
+
+=item $^V
+
+The revision, version, and subversion of the Perl interpreter, represented
+as a string composed of characters with those ordinals. Thus in Perl v5.6.0
+it equals C<chr(5) . chr(6) . chr(0)> and will return true for
+C<$^V eq v5.6.0>. Note that the characters in this string value can
+potentially be in Unicode range.
+
+This can be used to determine whether the Perl interpreter executing a
+script is in the right range of versions. (Mnemonic: use ^V for Version
+Control.) Example:
+
+ warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
+
+See the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
+
+See also C<$]> for an older representation of the Perl version.
=item $WARNING
=item $^W
-The current value of the warning switch, either TRUE or FALSE.
-(Mnemonic: related to the B<-w> switch.)
+The current value of the warning switch, initially true if B<-w>
+was used, false otherwise, but directly modifiable. (Mnemonic:
+related to the B<-w> switch.) See also L<warnings>.
+
+=item ${^WARNING_BITS}
+
+The current set of warning checks enabled by the C<use warnings> pragma.
+See the documentation of C<warnings> for more details.
+
+=item ${^WIDE_SYSTEM_CALLS}
+
+Global flag that enables system calls made by Perl to use wide character
+APIs native to the system, if available. This is currently only implemented
+on the Windows platform.
+
+This can also be enabled from the command line using the C<-C> switch.
+
+The initial value is typically C<0> for compatibility with Perl versions
+earlier than 5.6, but may be automatically set to C<1> by Perl if the system
+provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
+
+The C<bytes> pragma always overrides the effect of this flag in the current
+lexical scope. See L<bytes>.
=item $EXECUTABLE_NAME
=item $^X
The name that the Perl binary itself was executed as, from C's C<argv[0]>.
+This may not be a full pathname, nor even necessarily in your path.
=item $ARGV
-contains the name of the current file when reading from E<lt>E<gt>.
+contains the name of the current file when reading from <>.
=item @ARGV
-The array @ARGV contains the command line arguments intended for the
-script. Note that C<$#ARGV> is the generally number of arguments minus
-one, because C<$ARGV[0]> is the first argument, I<NOT> the command name. See
-"C<$0>" for the command name.
+The array @ARGV contains the command-line arguments intended for
+the script. C<$#ARGV> is generally the number of arguments minus
+one, because C<$ARGV[0]> is the first argument, I<not> the program's
+command name itself. See C<$0> for the command name.
=item @INC
-The array @INC contains the list of places to look for Perl scripts to
-be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It
-initially consists of the arguments to any B<-I> command line switches,
-followed by the default Perl library, probably F</usr/local/lib/perl>,
-followed by ".", to represent the current directory. If you need to
-modify this at runtime, you should use the C<use lib> pragma
-to get the machine-dependent library properly loaded also:
+The array @INC contains the list of places that the C<do EXPR>,
+C<require>, or C<use> constructs look for their library files. It
+initially consists of the arguments to any B<-I> command-line
+switches, followed by the default Perl library, probably
+F</usr/local/lib/perl>, followed by ".", to represent the current
+directory. If you need to modify this at runtime, you should use
+the C<use lib> pragma to get the machine-dependent library properly
+loaded also:
use lib '/mypath/libdir/';
use SomeMod;
+=item @_
+
+Within a subroutine the array @_ contains the parameters passed to that
+subroutine. See L<perlsub>.
+
=item %INC
-The hash %INC contains entries for each filename that has
-been included via C<do> or C<require>. The key is the filename you
-specified, and the value is the location of the file actually found.
-The C<require> command uses this array to determine whether a given file
-has already been included.
+The hash %INC contains entries for each filename included via the
+C<do>, C<require>, or C<use> operators. The key is the filename
+you specified (with module names converted to pathnames), and the
+value is the location of the file found. The C<require>
+operator uses this hash to determine whether a particular file has
+already been included.
+
+=item %ENV
=item $ENV{expr}
The hash %ENV contains your current environment. Setting a
-value in C<ENV> changes the environment for child processes.
+value in C<ENV> changes the environment for any child processes
+you subsequently fork() off.
+
+=item %SIG
=item $SIG{expr}
-The hash %SIG is used to set signal handlers for various
-signals. Example:
+The hash %SIG contains signal handlers for signals. For example:
sub handler { # 1st argument is signal name
- local($sig) = @_;
+ my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
- $SIG{'INT'} = 'handler';
- $SIG{'QUIT'} = 'handler';
+ $SIG{'INT'} = \&handler;
+ $SIG{'QUIT'} = \&handler;
...
- $SIG{'INT'} = 'DEFAULT'; # restore default action
+ $SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
-The %SIG array contains values for only the signals actually set within
-the Perl script. Here are some other examples:
+Using a value of C<'IGNORE'> usually has the effect of ignoring the
+signal, except for the C<CHLD> signal. See L<perlipc> for more about
+this special case.
- $SIG{PIPE} = Plumber; # SCARY!!
- $SIG{"PIPE"} = "Plumber"; # just fine, assumes main::Plumber
+Here are some other examples:
+
+ $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
+ $SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
-The one marked scary is problematic because it's a bareword, which means
-sometimes it's a string representing the function, and sometimes it's
-going to call the subroutine call right then and there! Best to be sure
-and quote it or take a reference to it. *Plumber works too. See L<perlsub>.
+Be sure not to use a bareword as the name of a signal handler,
+lest you inadvertently call it.
If your system has the sigaction() function then signal handlers are
installed using it. This means you get reliable signal handling. If
your system has the SA_RESTART flag it is used when signals handlers are
-installed. This means that system calls for which it is supported
+installed. This means that system calls for which restarting is supported
continue rather than returning when a signal arrives. If you want your
system calls to be interrupted by signal delivery then do something like
this:
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
The C<__DIE__> handler is explicitly disabled during the call, so that you
-can die from a C<__DIE__> handler. Similarly for C<__WARN__>. See
-L<perlfunc/die>, L<perlfunc/warn> and L<perlfunc/eval>.
+can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
+
+Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
+even inside an eval(). Do not use this to rewrite a pending exception
+in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
+This strange action at a distance may be fixed in a future release
+so that C<$SIG{__DIE__}> is only called if your program is about
+to exit, as was the original intent. Any other use is deprecated.
+
+C<__DIE__>/C<__WARN__> handlers are very special in one respect:
+they may be called to report (probable) errors found by the parser.
+In such a case the parser may be in inconsistent state, so any
+attempt to evaluate Perl code from such a handler will probably
+result in a segfault. This means that warnings or errors that
+result from parsing Perl should be used with extreme caution, like
+this:
+
+ require Carp if defined $^S;
+ Carp::confess("Something wrong") if defined &Carp::confess;
+ die "Something wrong, but could not load Carp to give backtrace...
+ To see backtrace try starting Perl with -MCarp switch";
+
+Here the first line will load Carp I<unless> it is the parser who
+called the handler. The second line will print backtrace and die if
+Carp was available. The third line will be executed only if Carp was
+not available.
+
+See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
+L<warnings> for additional information.
=back
+
+=head2 Error Indicators
+
+The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
+about different types of error conditions that may appear during
+execution of a Perl program. The variables are shown ordered by
+the "distance" between the subsystem which reported the error and
+the Perl process. They correspond to errors detected by the Perl
+interpreter, C library, operating system, or an external program,
+respectively.
+
+To illustrate the differences between these variables, consider the
+following Perl expression, which uses a single-quoted string:
+
+ eval q{
+ open PIPE, "/cdrom/install |";
+ @res = <PIPE>;
+ close PIPE or die "bad pipe: $?, $!";
+ };
+
+After execution of this statement all 4 variables may have been set.
+
+C<$@> is set if the string to be C<eval>-ed did not compile (this
+may happen if C<open> or C<close> were imported with bad prototypes),
+or if Perl code executed during evaluation die()d . In these cases
+the value of $@ is the compile error, or the argument to C<die>
+(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
+though.)
+
+When the eval() expression above is executed, open(), C<< <PIPE> >>,
+and C<close> are translated to calls in the C run-time library and
+thence to the operating system kernel. C<$!> is set to the C library's
+C<errno> if one of these calls fails.
+
+Under a few operating systems, C<$^E> may contain a more verbose
+error indicator, such as in this case, "CDROM tray not closed."
+Systems that do not support extended error messages leave C<$^E>
+the same as C<$!>.
+
+Finally, C<$?> may be set to non-0 value if the external program
+F</cdrom/install> fails. The upper eight bits reflect specific
+error conditions encountered by the program (the program's exit()
+value). The lower eight bits reflect mode of failure, like signal
+death and core dump information See wait(2) for details. In
+contrast to C<$!> and C<$^E>, which are set only if error condition
+is detected, the variable C<$?> is set on each C<wait> or pipe
+C<close>, overwriting the old value. This is more like C<$@>, which
+on every eval() is always set on failure and cleared on success.
+
+For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
+and C<$?>.
+
+=head2 Technical Note on the Syntax of Variable Names
+
+Variable names in Perl can have several formats. Usually, they
+must begin with a letter or underscore, in which case they can be
+arbitrarily long (up to an internal limit of 251 characters) and
+may contain letters, digits, underscores, or the special sequence
+C<::> or C<'>. In this case, the part before the last C<::> or
+C<'> is taken to be a I<package qualifier>; see L<perlmod>.
+
+Perl variable names may also be a sequence of digits or a single
+punctuation or control character. These names are all reserved for
+special uses by Perl; for example, the all-digits names are used
+to hold data captured by backreferences after a regular expression
+match. Perl has a special syntax for the single-control-character
+names: It understands C<^X> (caret C<X>) to mean the control-C<X>
+character. For example, the notation C<$^W> (dollar-sign caret
+C<W>) is the scalar variable whose name is the single character
+control-C<W>. This is better than typing a literal control-C<W>
+into your program.
+
+Finally, new in Perl 5.6, Perl variable names may be alphanumeric
+strings that begin with control characters (or better yet, a caret).
+These variables must be written in the form C<${^Foo}>; the braces
+are not optional. C<${^Foo}> denotes the scalar variable whose
+name is a control-C<F> followed by two C<o>'s. These variables are
+reserved for future special uses by Perl, except for the ones that
+begin with C<^_> (control-underscore or caret-underscore). No
+control-character name that begins with C<^_> will acquire a special
+meaning in any future version of Perl; such names may therefore be
+used safely in programs. C<$^_> itself, however, I<is> reserved.
+
+Perl identifiers that begin with digits, control characters, or
+punctuation characters are exempt from the effects of the C<package>
+declaration and are always forced to be in package C<main>. A few
+other names are also exempt:
+
+ ENV STDIN
+ INC STDOUT
+ ARGV STDERR
+ ARGVOUT
+ SIG
+
+In particular, the new special C<${^_XYZ}> variables are always taken
+to be in package C<main>, regardless of any C<package> declarations
+presently in scope.
+
+=head1 BUGS
+
+Due to an unfortunate accident of Perl's implementation, C<use
+English> imposes a considerable performance penalty on all regular
+expression matches in a program, regardless of whether they occur
+in the scope of C<use English>. For that reason, saying C<use
+English> in libraries is strongly discouraged. See the
+Devel::SawAmpersand module documentation from CPAN
+(http://www.perl.com/CPAN/modules/by-module/Devel/)
+for more information.
+
+Having to even think about the C<$^S> variable in your exception
+handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
+invites grievous and difficult to track down errors. Avoid it
+and use an C<END{}> or CORE::GLOBAL::die override instead.
projects / p5sagit/p5-mst-13.2.git / blobdiff