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Errno(3) Perl Programmers Reference Guide Errno(3)

Errno - System errno constants

    use Errno qw(EINTR EIO :POSIX);

"Errno" defines and conditionally exports all the error constants defined in your system errno.h include file. It has a single export tag, ":POSIX", which will export all POSIX defined error numbers.
On Windows, "Errno" also defines and conditionally exports all the Winsock error constants defined in your system WinError.h include file. These are included in a second export tag, ":WINSOCK".
"Errno" also makes "%!" magic such that each element of "%!" has a non-zero value only if $! is set to that value. For example:
    my $fh;
    unless (open($fh, "<", "/fangorn/spouse")) {
        if ($!{ENOENT}) {
            warn "Get a wife!\n";
        } else {
            warn "This path is barred: $!";
If a specified constant "EFOO" does not exist on the system, $!{EFOO} returns "". You may use "exists $!{EFOO}" to check whether the constant is available on the system.
Perl automatically loads "Errno" the first time you use "%!", so you don't need an explicit "use".

Importing a particular constant may not be very portable, because the import will fail on platforms that do not have that constant. A more portable way to set $! to a valid value is to use:
    if (exists &Errno::EFOO) {
        $! = &Errno::EFOO;

Graham Barr <>

Copyright (c) 1997-8 Graham Barr. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
2019-01-01 perl v5.28.1

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