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Config - access Perl configuration information


    use Config;
    if ($Config{'cc'} =~ /gcc/) {
        print "built by gcc\n";

    use Config qw(myconfig config_sh config_vars);

    print myconfig();

    print config_sh();

    config_vars(qw(osname archname));


The Config module contains all the information that was available to the Configure program at Perl build time (over 900 values).

Shell variables from the file (written by Configure) are stored in the readonly-variable %Config, indexed by their names.

Values stored in as 'undef' are returned as undefined values. The perl exists function can be used to check if a named variable exists.


Returns a textual summary of the major perl configuration values. See also -V in "Switches" in perlrun.


Returns the entire perl configuration information in the form of the original shell variable assignment script.


Prints to STDOUT the values of the named configuration variable. Each is printed on a separate line in the form:


Names which are unknown are output as name='UNKNOWN';. See also -V:name in "Switches" in perlrun.


Here's a more sophisticated example of using %Config:

    use Config;
    use strict;

    my %sig_num;
    my @sig_name;
    unless($Config{sig_name} && $Config{sig_num}) {
        die "No sigs?";
    } else {
        my @names = split ' ', $Config{sig_name};
        @sig_num{@names} = split ' ', $Config{sig_num};
        foreach (@names) {
            $sig_name[$sig_num{$_}] ||= $_;

    print "signal #17 = $sig_name[17]\n";
    if ($sig_num{ALRM}) { 
        print "SIGALRM is $sig_num{ALRM}\n";


Because this information is not stored within the perl executable itself it is possible (but unlikely) that the information does not relate to the actual perl binary which is being used to access it.

The Config module is installed into the architecture and version specific library directory ($Config{installarchlib}) and it checks the perl version number when loaded.

The values stored in may be either single-quoted or double-quoted. Double-quoted strings are handy for those cases where you need to include escape sequences in the strings. To avoid runtime variable interpolation, any $ and @ characters are replaced by \$ and \@, respectively. This isn't foolproof, of course, so don't embed \$ or \@ in double-quoted strings unless you're willing to deal with the consequences. (The slashes will end up escaped and the $ or @ will trigger variable interpolation)


Most Config variables are determined by the Configure script on platforms supported by it (which is most UNIX platforms). Some platforms have custom-made Config variables, and may thus not have some of the variables described below, or may have extraneous variables specific to that particular port. See the port specific documentation in such cases.


if ($opts{glossary}) { open(GLOS, "<$glossary") or die "Can't open $glossary: $!"; } %seen = (); $text = 0; $/ = '';

sub process { if (s/\A(\w*)\s+\(([\w.]+)\):\s*\n(\t?)/=item $1\n\nFrom $2:\n\n/m) { my $c = substr $1, 0, 1; unless ($seen{$c}++) { print CONFIG <<EOF if $text; =back

EOF print CONFIG <<EOF; =head2 $c

    EOF $text = 1; } } elsif (!$text || !/\A\t/) { warn "Expected a Configure variable header", ($text ? " or another paragraph of description" : () ); } s/n't/n\00t/g; # leave can't, won't etc untouched s/^\t\s+(.*)/\n$1/gm; # Indented lines ===> new paragraph s/^(?<!\n\n)\t(.*)/$1/gm; # Not indented lines ===> text s{([\'\"])(?=[^\'\"\s]*[./][^\'\"\s]*\1)([^\'\"\s]+)\1}($2)g; # '.o' s{([\'\"])([^\'\"\s]+)\1}($2)g; # "date" command s{\'([A-Za-z_\- *=/]+)\'}($1)g; # 'ln -s' s{ (?<! [\w./<\'\"] ) # Only standalone file names (?! e \. g \. ) # Not e.g. (?! \. \. \. ) # Not ... (?! \d ) # Not 5.004 (?! read/ ) # Not read/write (?! etc\. ) # Not etc. (?! I/O ) # Not I/O ( \$ ? # Allow leading $ [\w./]* [./] [\w./]* # Require . or / inside ) (?<! \. (?= [\s)] ) ) # Do not include trailing dot (?! [\w/] ) # Include all of it } ($1)xg; # /usr/local s/((?<=\s)~\w*)/$1/g; # ~name s/(?<![.<\'\"])\b([A-Z_]{2,})\b(?![\'\"])/$1/g; # UNISTD s/(?<![.<\'\"])\b(?!the\b)(\w+)\s+macro\b/$1 macro/g; # FILE_cnt macro s/n[\0]t/n't/g; # undo can't, won't damage }

    if ($opts{glossary}) { <GLOS>; # Skip the "DO NOT EDIT" <GLOS>; # Skip the preamble while (<GLOS>) { process; print CONFIG; } }

    print CONFIG <<'ENDOFTAIL';


This module contains a good example of how to use tie to implement a cache and an example of how to make a tied variable readonly to those outside of it.