08-26-2008
Perl Exception - $!,$?,$@
Hi,
I am trying to understand the significance of the special variables $!,$@ and $? in perl. I have a code block as follows:
eval {
Code Segment 1:
#authenticating to the remote server
$ftpobj -> login($username,$password) or die "Can't login to $remote_host";
Code Segment 2:
#setting up the current working directory
$ftpobj -> cwd ($cwd) or die "Can't change directory";
Code Segment 3:
#putting the file in remote server
$ftpobj -> put($filename) or die "Can't ftp the file:",$!;
}
[Where $ftpobj is an Net::FTP object and $filename is an invalid filename]
In Code Segment 1 and 2,if there were any errors,i can able to catch it using if($@) since the above code is enclosed by eval.
But the trapping is not successful in the case of Code Segment 3.Although the control comes inside the conditional loop if($@) after eval,error message
is written to the Standard output with this.The error message is:
"Cannot open Local file test2.log: No such file or directory
at ftp_file.pl line 44"
Why is this happening?Exception handling in perl is implemented using eval{} and if($@), but in this case the error is going out-of control and got printed to STDOUT or STDERR
Can anyone suggest what is going on and what is the reason for this behaviour?
Can anyone explain me what makes $!,$@,$? different in various situations?
Your help and support is appreciated!
With Regards
Dileep
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LEARN ABOUT OSX
universal::require
UNIVERSAL::require(3) User Contributed Perl Documentation UNIVERSAL::require(3)
NAME
UNIVERSAL::require - require() modules from a variable
SYNOPSIS
# This only needs to be said once in your program.
require UNIVERSAL::require;
# Same as "require Some::Module"
my $module = 'Some::Module';
$module->require or die $@;
# Same as "use Some::Module"
BEGIN { $module->use or die $@ }
DESCRIPTION
If you've ever had to do this...
eval "require $module";
to get around the bareword caveats on require(), this module is for you. It creates a universal require() class method that will work with
every Perl module and its secure. So instead of doing some arcane eval() work, you can do this:
$module->require;
It doesn't save you much typing, but it'll make alot more sense to someone who's not a ninth level Perl acolyte.
Methods
require
my $return_val = $module->require or die $@;
my $return_val = $module->require($version) or die $@;
This works exactly like Perl's require, except without the bareword restriction, and it doesn't die. Since require() is placed in the
UNIVERSAL namespace, it will work on any module. You just have to use UNIVERSAL::require somewhere in your code.
Should the module require fail, or not be a high enough $version, it will simply return false and not die. The error will be in $@ as well
as $UNIVERSAL::require::ERROR.
$module->require or die $@;
use
my $require_return = $module->use or die $@;
my $require_return = $module->use(@imports) or die $@;
Like "UNIVERSAL::require", this allows you to "use" a $module without having to eval to work around the bareword requirement. It returns
the same as require.
Should either the require or the import fail it will return false. The error will be in $@.
If possible, call this inside a BEGIN block to emulate a normal "use" as closely as possible.
BEGIN { $module->use }
SECURITY NOTES
UNIVERSAL::require makes use of "eval STRING". In previous versions of UNIVERSAL::require it was discovered that one could craft a class
name which would result in code being executed. This hole has been closed. The only variables now exposed to "eval STRING" are the
caller's package, filename and line which are not tainted.
UNIVERSAL::require is taint clean.
COPYRIGHT
Copyright 2001, 2005 by Michael G Schwern <schwern@pobox.com>.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
See http://www.perl.com/perl/misc/Artistic.html
AUTHOR
Michael G Schwern <schwern@pobox.com>
SEE ALSO
Module::Load, "require" in perlfunc, <http://dev.perl.org/rfc/253.pod>
perl v5.16.2 2009-03-30 UNIVERSAL::require(3)