PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)NAMEperlvar - Perl predefined variables
DESCRIPTION
Predefined Names
The following names have special meaning to Perl. Most
punctuation names have reasonable mnemonics, or analogues
in one of the shells. Nevertheless, if you wish to use
long variable names, you just need to 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 even
have medium names, generally borrowed from awk.
Due to an unfortunate accident of Perl's implementation,
"use English" imposes a considerable performance penalty
on all regular expression matches in a program, regardless
of whether they occur in the scope of "use English". For
that reason, saying "use English" in libraries is strongly
discouraged. See the Devel::SawAmpersand module
documentation from CPAN
(http://www.perl.com/CPAN/modules/by-module/Devel/Devel-
SawAmpersand-0.10.readme) for more information.
To go a step further, those variables that depend on the
currently selected filehandle may instead (and preferably)
be set by calling an object method on the FileHandle
object. (Summary lines below for this contain the word
HANDLE.) First you must say
use FileHandle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each of the methods returns the old value of the
FileHandle 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
autoflush(), which will assume a 1 for you, just to be
different.
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
16/Sep/1999 perl 5.005, patch 03 1
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
run-time exception.
The following list is ordered by scalar variables first,
then the arrays, then the hashes (except $^M was added in
the wrong place). This is somewhat obscured by the fact
that %ENV and %SIG are listed as $ENV{expr} and
$SIG{expr}.
$ARG
$_ The default input and pattern-searching space.
The following pairs are equivalent:
while (<>) {...} # equivalent in only while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chop
chop($_)
Here are the places where Perl will assume $_ even
if you don't use it:
o Various unary functions, including functions
like ord() and int(), as well as the all file
tests (-f, -d) except for -t, which defaults to
STDIN.
o Various list functions like print() and
unlink().
o The pattern matching operations m//, s///, and
tr/// when used without an =~ operator.
o The default iterator variable in a foreach loop
if no other variable is supplied.
o The implicit iterator variable in the grep()
and map() functions.
o The default place to put an input record when a
<FH> operation's result is tested by itself as
the sole criterion of a while test. Note that
outside of a while test, this will not happen.
(Mnemonic: underline is understood in certain
operations.)
16/Sep/1999 perl 5.005, patch 03 2
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$<digits>
Contains the subpattern 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
\digits.) These variables are all read-only.
$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.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See the
Devel::SawAmpersand module from CPAN for more
information.
$PREMATCH
$` The string preceding whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval enclosed by
the current BLOCK). (Mnemonic: ` 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 the
Devel::SawAmpersand module from CPAN for more
information.
$POSTMATCH
$' The string following whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval() enclosed
by the current BLOCK). (Mnemonic: ' often follows
a quoted string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
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 the
Devel::SawAmpersand module from CPAN for more
information.
16/Sep/1999 perl 5.005, patch 03 3
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$LAST_PAREN_MATCH
$+ The last bracket matched by the last search
pattern. This is useful if you don't know which
of a set of alternative patterns matched. For
example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This
variable is read-only.
$MULTILINE_MATCHING
$* Set to 1 to do multi-line matching within a
string, 0 to tell Perl that it can assume that
strings contain a single line, for the purpose of
optimizing pattern matches. Pattern matches on
strings containing multiple newlines can produce
confusing results when "$*" is 0. Default is 0.
(Mnemonic: * matches multiple things.) Note that
this variable influences the interpretation of
only "^" and "$". A literal newline can be
searched for even when $* == 0.
Use of "$*" is deprecated in modern Perls,
supplanted by the /s and /m modifiers on pattern
matching.
input_line_number HANDLE EXPR
$INPUT_LINE_NUMBER
$NR
$. The current input line number for the last file
handle from which you read (or performed a seek or
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 the
section on $/ on how to affect that. An explicit
close on a filehandle resets the line number.
Because "<>" never does an explicit close, line
numbers increase across ARGV files (but see
examples under eof()). Localizing $. has the
effect of also localizing Perl's notion of "the
last read filehandle". (Mnemonic: many programs
use "." to mean the current line number.)
input_record_separator HANDLE EXPR
$INPUT_RECORD_SEPARATOR
16/Sep/1999 perl 5.005, patch 03 4
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$RS
$/ The input record separator, newline by default.
This is used to influence Perl's idea of what a
"line" is. Works like 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 multi-character string to match a multi-
character delimiter, or to undef to read to end of
file. Note that setting it to "\n\n" means
something slightly different than setting it to
"", if the file contains consecutive empty lines.
Setting it to "" will treat two or more
consecutive empty lines as a single empty line.
Setting it to "\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 $/; # 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 :-)
Setting $/ to a reference to an integer, scalar
containing an integer, or scalar that's
convertable 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 sysread, so it's best not to mix record and
non-record reads on the same file. (This is likely
not a problem, as any file you'd want to read in
record mode is probably usable in line mode) Non-
VMS systems perform normal I/O, so it's safe to
mix record and non-record reads of a file.
16/Sep/1999 perl 5.005, patch 03 5
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
Also see the section on $..
autoflush HANDLE EXPR
$OUTPUT_AUTOFLUSH
$| 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 actually buffered by the
system or not; $| 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.)
output_field_separator HANDLE EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$, 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 awk, set this variable as you
would set awk's OFS variable to specify what is
printed between fields. (Mnemonic: what is
printed when there is a , in your print
statement.)
output_record_separator HANDLE EXPR
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print
operator. Ordinarily the print operator simply
prints out the comma-separated fields you specify,
with no trailing newline or record separator
assumed. To get behavior more like awk, set this
variable as you would set awk's ORS variable to
specify what is printed at the end of the print.
(Mnemonic: you set "$\" instead of adding \n at
the end of the print. Also, it's just like $/,
but it's what you get "back" from Perl.)
16/Sep/1999 perl 5.005, patch 03 6
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$LIST_SEPARATOR
$"""" This is like "$," 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.)
$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array
emulation. If you refer to a hash element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk.
Note that if your keys contain binary data there
might not be any safe value for "$;". (Mnemonic:
comma (the syntactic subscript separator) is a
semi-semicolon. Yeah, I know, it's pretty lame,
but "$," is already taken for something more
important.)
Consider using "real" multidimensional arrays.
$OFMT
$# The output format for printed numbers. This
variable is a half-hearted attempt to emulate
awk's OFMT variable. There are times, however,
when awk and Perl have differing notions of what
is in fact numeric. The initial value is %.ng,
where n is the value of the macro DBL_DIG from
your system's float.h. This is different from
awk's default OFMT setting of %.6g, so you need to
set "$#" explicitly to get awk's value.
(Mnemonic: # is the number sign.)
Use of "$#" is deprecated.
16/Sep/1999 perl 5.005, patch 03 7
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
format_page_number HANDLE EXPR
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected
output channel. (Mnemonic: % is page number in
nroff.)
format_lines_per_page HANDLE EXPR
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the
currently selected output channel. Default is 60.
(Mnemonic: = has horizontal lines.)
format_lines_left HANDLE EXPR
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the
currently selected output channel. (Mnemonic:
lines_on_page - lines_printed.)
format_name HANDLE EXPR
$FORMAT_NAME
$~ The name of the current report format for the
currently selected output channel. Default is
name of the filehandle. (Mnemonic: brother to
"$^".)
format_top_name HANDLE EXPR
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the
currently selected output channel. Default is
name of the filehandle with _TOP appended.
(Mnemonic: points to top of page.)
format_line_break_characters HANDLE EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string
may be broken to fill continuation fields
(starting with ^) in a format. Default is " \n-",
to break on whitespace or hyphens. (Mnemonic: a
"colon" in poetry is a part of a line.)
format_formfeed HANDLE EXPR
16/Sep/1999 perl 5.005, patch 03 8
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$FORMAT_FORMFEED
$^L What formats output to perform a form feed.
Default is \f.
$ACCUMULATOR
$^A The current value of the write() accumulator for
format() lines. A format contains formline()
commands that put their result into $^A. After
calling its format, write() prints out the
contents of $^A and empties. So you never
actually see the contents of $^A unless you call
formline() yourself and then look at it. See the
perlform manpage and the formline() entry in the
perlfunc manpage.
$CHILD_ERROR
$? The status returned by the last pipe close,
backtick (``) command, or system() operator. Note
that this is the status word returned by the
wait() system call (or else is made up to look
like it). Thus, the exit value of the subprocess
is actually ($? >> 8), and $? & 127 gives which
signal, if any, the process died from, and $? &
128 reports whether there was a core dump.
(Mnemonic: similar to sh and ksh.)
Additionally, if the h_errno variable is supported
in C, its value is returned via $? if any of the
gethost*() functions fail.
Note that if you have installed a signal handler
for SIGCHLD, the value of $? will usually be wrong
outside that handler.
Inside an END subroutine $? contains the value
that is going to be given to exit(). You can
modify $? in an END subroutine to change the exit
status of the script.
Under VMS, the pragma use vmsish 'status' makes $?
reflect the actual VMS exit status, instead of the
default emulation of POSIX status.
Also see the section on Error Indicators.
$OS_ERROR
$ERRNO
$! If used in a numeric context, yields the current
value of errno, with all the usual caveats. (This
16/Sep/1999 perl 5.005, patch 03 9
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
means that you shouldn't depend on the value of $!
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 $! to set errno if, for instance, you want "$!"
to return the string for error n, or you want to
set the exit value for the die() operator.
(Mnemonic: What just went bang?)
Also see the section on Error Indicators.
$EXTENDED_OS_ERROR
$^E Error information specific to the current
operating system. At the moment, this differs
from $! under only VMS, OS/2, and Win32 (and for
MacPerl). On all other platforms, $^E is always
just the same as $!.
Under VMS, $^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 $!. This is particularly important
when $! is set to EVMSERR.
Under OS/2, $^E is set to the error code of the
last call to OS/2 API either via CRT, or directly
from perl.
Under Win32, $^E always returns the last error
information reported by the Win32 call
GetLastError() which describes the last error from
within the Win32 API. Most Win32-specific code
will report errors via $^E. ANSI C and UNIX-like
calls set errno and so most portable Perl code
will report errors via $!.
Caveats mentioned in the description of $!
generally apply to $^E, also. (Mnemonic: Extra
error explanation.)
Also see the section on Error Indicators.
$EVAL_ERROR
$@ The Perl syntax error message from the last eval()
command. 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, however, set up a routine
16/Sep/1999 perl 5.005, patch 03 10
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
to process warnings by setting $SIG{__WARN__} as
described below.
Also see the section on Error Indicators.
$PROCESS_ID
$PID
$$ The process number of the Perl running this
script. (Mnemonic: same as shells.)
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it's the
uid you came FROM, if you're running setuid.)
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're
running setuid.) Note: "$<" and "$>" can be
swapped only on machines supporting setreuid().
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a
machine that supports membership in multiple
groups simultaneously, gives a space separated
list of groups you are in. The first number is
the one returned by getgid(), and the subsequent
ones by getgroups(), one of which may be the same
as the first number.
However, a value assigned to "$(" must be a single
number used to set the real gid. So the value
given by "$(" should not be assigned back to "$("
without being forced numeric, such as by adding
zero.
(Mnemonic: parentheses are used to GROUP things.
The real gid is the group you LEFT, if you're
running setgid.)
16/Sep/1999 perl 5.005, patch 03 11
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on
a machine that supports membership in multiple
groups simultaneously, gives a space separated
list of groups you are in. The first number is
the one returned by getegid(), and the subsequent
ones by getgroups(), one of which may be the same
as the first number.
Similarly, a value assigned to "$)" must also be a
space-separated list of numbers. The first number
is used to set 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 $) = "5 5" .
(Mnemonic: parentheses are used to GROUP things.
The effective gid is the group that's RIGHT for
you, if you're running setgid.)
Note: "$<", "$>", "$(" and "$)" can be set only on
machines that support the corresponding
set[re][ug]id() routine. "$(" and "$)" can be
swapped only on machines supporting setregid().
$PROGRAM_NAME
$0 Contains the name of the file containing the Perl
script being executed. On some operating systems
assigning to "$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.
(Mnemonic: same as sh and ksh.)
$[ The index of the first element in an array, and of
the first character in a substring. Default is 0,
but you could set it to 1 to make Perl behave more
like awk (or Fortran) when subscripting and when
evaluating the index() and substr() functions.
(Mnemonic: [ begins subscripts.)
As of Perl 5, assignment to "$[" is treated as a
compiler directive, and cannot influence the
behavior of any other file. Its use is
discouraged.
$PERL_VERSION
16/Sep/1999 perl 5.005, patch 03 12
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$] 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:
warn "No checksumming!\n" if $] < 3.019;
See also the documentation of use VERSION and
require VERSION for a convenient way to fail if
the Perl interpreter is too old.
$COMPILING
$^C The current value of the flag associated with the
-c switch. Mainly of use with -MO=... to allow
code to alter its behaviour when being compiled.
(For example to automatically AUTOLOADing at
compile time rather than normal deferred loading.)
Setting $^C = 1 is similar to calling B::minus_c.
$DEBUGGING
$^D The current value of the debugging flags.
(Mnemonic: value of -D switch.)
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2.
System file 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 status of
a file descriptor will be decided according to the
value of $^F when the open() or pipe() was called,
not the time of the exec().
$^H The current set of syntax checks enabled by use
strict and other block scoped compiler hints. See
the documentation of strict for more details.
$INPLACE_EDIT
$^I The current value of the inplace-edit extension.
Use undef to disable inplace editing. (Mnemonic:
value of -i switch.)
$^M By default, running out of memory it is not
trappable. However, if compiled for this, Perl
may use the contents of $^M as an emergency pool
after die()ing with this message. Suppose that
16/Sep/1999 perl 5.005, patch 03 13
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
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 when in
emergency. See the INSTALL file for information
on how to enable this option. As a disincentive
to casual use of this advanced feature, there is
no the English manpage long name for this
variable.
$OSNAME
$^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
$Config{'osname'}.
$PERLDB
$^P The internal variable for debugging support.
Different bits mean the following (subject to
change):
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging.
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive
inspections.
0x10 Keep info about source lines on which a
subroutine is defined.
0x20 Start with single-step on.
Note that some bits may be relevant at
compile-time only, some at run-time only.
This is a new mechanism and the details may
change.
$^R The result of evaluation of the last successful
the section on (?{ code }) in the perlre manpage
regular expression assertion. (Excluding those
used as switches.) May be written to.
$^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.
16/Sep/1999 perl 5.005, patch 03 14
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
$BASETIME
$^T The time at which the script began running, in
seconds since the epoch (beginning of 1970). The
values returned by the -M, -A, and -C filetests
are based on this value.
$WARNING
$^W The current value of the warning switch, either
TRUE or FALSE. (Mnemonic: related to the -w
switch.)
$EXECUTABLE_NAME
$^X The name that the Perl binary itself was executed
as, from C's argv[0].
$ARGV contains the name of the current file when reading
from <>.
@ARGV The array @ARGV contains the command line
arguments intended for the script. Note that
$#ARGV is the generally number of arguments minus
one, because $ARGV[0] is the first argument, NOT
the command name. See "$0" for the command name.
@INC The array @INC contains the list of places to look
for Perl scripts to be evaluated by the do EXPR,
require, or use constructs. It initially consists
of the arguments to any -I command line switches,
followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent
the current directory. If you need to modify this
at runtime, you should use the use lib pragma to
get the machine-dependent library properly loaded
also:
use lib '/mypath/libdir/';
use SomeMod;
@_ Within a subroutine the array @_ contains the
parameters passed to that subroutine. See the
perlsub manpage.
%INC The hash %INC contains entries for each filename
that has been included via do or require. The key
is the filename you specified, and the value is
the location of the file actually found. The
require command uses this array to determine
whether a given file has already been included.
16/Sep/1999 perl 5.005, patch 03 15
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
%ENV
$ENV{expr}
The hash %ENV contains your current environment.
Setting a value in ENV changes the environment for
child processes.
%SIG
$SIG{expr}
The hash %SIG is used to set signal handlers for
various signals. Example:
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of 'IGNORE' usually has the effect
of ignoring the signal, except for the CHLD
signal. See the perlipc manpage for more about
this special case.
The %SIG array contains values for only the
signals actually set within the Perl script. Here
are some other examples:
$SIG{"PIPE"} = Plumber; # SCARY!!
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
$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 the
perlsub manpage.
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 continue
16/Sep/1999 perl 5.005, patch 03 16
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
rather than returning when a signal arrives. If
you want your system calls to be interrupted by
signal delivery then do something like this:
use POSIX ':signal_h';
my $alarm = 0;
sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
or die "Error setting SIGALRM handler: $!\n";
See the POSIX manpage.
Certain internal hooks can be also set using the
%SIG hash. The routine indicated by
$SIG{__WARN__} is called when a warning message is
about to be printed. The warning message is
passed as the first argument. The presence of a
__WARN__ hook causes the ordinary printing of
warnings to STDERR to be suppressed. You can use
this to save warnings in a variable, or turn
warnings into fatal errors, like this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
The routine indicated by $SIG{__DIE__} is called
when a fatal exception is about to be thrown. The
error message is passed as the first argument.
When a __DIE__ hook routine returns, the exception
processing continues as it would have in the
absence of the hook, unless the hook routine
itself exits via a goto, a loop exit, or a die().
The __DIE__ handler is explicitly disabled during
the call, so that you can die from a __DIE__
handler. Similarly for __WARN__.
Note that the $SIG{__DIE__} hook is called even
inside eval()ed blocks/strings. See the die entry
in the perlfunc manpage and the section on $^S in
the perlvar manpage for how to circumvent this.
Note that __DIE__/__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 calls which result/may-result in
parsing Perl should be used with extreme caution,
like this:
16/Sep/1999 perl 5.005, patch 03 17
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
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 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 the die entry in the perlfunc manpage, the
warn entry in the perlfunc manpage and the eval
entry in the perlfunc manpage for additional info.
Error Indicators
The variables the section on $@, the section on $!, the
section on $^E, and the section on $? contain information
about different types of error conditions that may appear
during execution of Perl script. The variables are shown
ordered by the "distance" between the subsystem which
reported the error and the Perl process, and 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:
eval '
open PIPE, "/cdrom/install |";
@res = <PIPE>;
close PIPE or die "bad pipe: $?, $!";
';
After execution of this statement all 4 variables may have
been set.
$@ is set if the string to be eval-ed did not compile
(this may happen if open or close were imported with bad
prototypes), or if Perl code executed during evaluation
die()d (either implicitly, say, if open was imported from
module the Fatal manpage, or the die after close was
triggered). In these cases the value of $@ is the compile
error, or Fatal error (which will interpolate $!!), or the
argument to die (which will interpolate $! and $?!).
When the above expression is executed, open(), <PIPE>, and
close are translated to C run-time library calls. $! is
set if one of these calls fails. The value is a symbolic
indicator chosen by the C run-time library, say No such
file or directory.
On some systems the above C library calls are further
16/Sep/1999 perl 5.005, patch 03 18
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
translated to calls to the kernel. The kernel may have
set more verbose error indicator that one of the handful
of standard C errors. In such cases $^E contains this
verbose error indicator, which may be, say, CDROM tray not
closed. On systems where C library calls are identical to
system calls $^E is a duplicate of $!.
Finally, $? may be set to non-0 value if the external
program /cdrom/install fails. Upper bits of the
particular value may reflect specific error conditions
encountered by this program (this is program-dependent),
lower-bits reflect mode of failure (segfault, completion,
etc.). Note that in contrast to $@, $!, and $^E, which
are set only if error condition is detected, the variable
$? is set on each wait or pipe close, overwriting the old
value.
For more details, see the individual descriptions at the
section on $@, the section on $!, the section on $^E, and
the section on $?.
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
256 characters) and may contain letters, digits,
underscores, or the special sequence ::. In this case the
part before the last :: is taken to be a package
qualifier; see the perlmod manpage.
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 backreferences after a
regular expression match. Perl has a special syntax for
the single-control-character names: It understands ^X
(caret X) to mean the control-X character. For example,
the notation $^W (dollar-sign caret W) is the scalar
variable whose name is the single character control-W.
This is better than typing a literal control-W into your
program.
All Perl variables that begin with digits, control
characters, or punctuation characters are exempt from the
effects of the package declaration and are always forced
to be in package main. A few other names are also exempt:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
16/Sep/1999 perl 5.005, patch 03 19
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)16/Sep/1999 perl 5.005, patch 03 20
