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diagnostics

Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index

NAME

diagnostics - Perl compiler pragma to force verbose warning diagnostics

splain - standalone program to do the same thing

SYNOPSIS

As a pragma:

    use diagnostics;    use diagnostics -verbose;

    enable  diagnostics;    disable diagnostics;

Aa a program:

    perl program 2>diag.out    splain [-v] [-p] diag.out

DESCRIPTION

The `diagnostics` Pragma

This module extends the terse diagnostics normally emitted by both theperl compiler and the perl interpeter, augmenting them with the moreexplicative and endearing descriptions found in the perldiag manpage. Like theother pragmata, it affects the compilation phase of your program ratherthan merely the execution phase.

To use in your program as a pragma, merely invoke

    use diagnostics;

at the start (or near the start) of your program. (Note that this does enable perl's -w flag.) Your wholecompilation will then be subject(ed :-) to the enhanced diagnostics.These still go out STDERR.

Due to the interaction between runtime and compiletime issues,and because it's probably not a very good idea anyway,you may not use no diagnostics to turn them off at compiletime.However, you may control there behaviour at runtime using the disable() and enable() methods to turn them off and on respectively.

The -verbose flag first prints out the the perldiag manpage introduction beforeany other diagnostics. The $diagnostics::PRETTY variable can generate nicerescape sequences for pagers.

The splain Program

While apparently a whole nuther program, splain is actually nothingmore than a link to the (executable) diagnostics.pm module, as well asa link to the diagnostics.pod documentation. The -v flag is likethe use diagnostics -verbose directive.The -p flag is like the$diagnostics::PRETTY variable. Since you're post-processing with splain, there's no sense in being able to enable() or disable() processing.

Output from splain is directed to STDOUT, unlike the pragma.

EXAMPLES

The following file is certain to trigger a few errors at bothruntime and compiletime:

    use diagnostics;    print NOWHERE "nothing\n";    print STDERR "\n\tThis message should be unadorned.\n";    warn "\tThis is a user warning";    print "\nDIAGNOSTIC TESTER: Please enter a <CR> here: ";    my $a, $b = scalar <STDIN>;    print "\n";    print $x/$y;

If you prefer to run your program first and look at its problemafterwards, do this:

    perl -w test.pl 2>test.out    ./splain < test.out

Note that this is not in general possible in shells of more dubious heritage, as the theoretical

    (perl -w test.pl >/dev/tty) >& test.out    ./splain < test.out

Because you just moved the existing stdout to somewhere else.

If you don't want to modify your source code, but still have on-the-flywarnings, do this:

    exec 3>&1; perl -w test.pl 2>&1 1>&3 3>&- | splain 1>&2 3>&-

Nifty, eh?

If you want to control warnings on the fly, do something like this.Make sure you do the use first, or you won't be able to getat the enable() or disable() methods.

    use diagnostics; # checks entire compilation phase         print "\ntime for 1st bogus diags: SQUAWKINGS\n";        print BOGUS1 'nada';        print "done with 1st bogus\n";

    disable diagnostics; # only turns off runtime warnings        print "\ntime for 2nd bogus: (squelched)\n";        print BOGUS2 'nada';        print "done with 2nd bogus\n";

    enable diagnostics; # turns back on runtime warnings        print "\ntime for 3rd bogus: SQUAWKINGS\n";        print BOGUS3 'nada';        print "done with 3rd bogus\n";

    disable diagnostics;        print "\ntime for 4th bogus: (squelched)\n";        print BOGUS4 'nada';        print "done with 4th bogus\n";

INTERNALS

Diagnostic messages derive from the perldiag.pod file when available atruntime. Otherwise, they may be embedded in the file itself when thesplain package is built. See the Makefile for details.

If an extant $SIG{__WARN__} handler is discovered, it will continueto be honored, but only after the diagnostics::splainthis() function (the module's $SIG{__WARN__} interceptor) has had its way with yourwarnings.

There is a $diagnostics::DEBUG variable you may set if you're desperatelycurious what sorts of things are being intercepted.

    BEGIN { $diagnostics::DEBUG = 1 }

BUGS

Not being able to say ``no diagnostics'' is annoying, but may not beinsurmountable.

The -pretty directive is called too late to affect matters.You have to do this instead, and before you load the module.

    BEGIN { $diagnostics::PRETTY = 1 }

I could start up faster by delaying compilation until it should beneeded, but this gets a ``panic: top_level'' when using the pragma formin Perl 5.001e.

While it's true that this documentation is somewhat subserious, if you usea program named splain, you should expect a bit of whimsy.

AUTHOR

Tom Christiansen <tchristAATTmox.perl.com>, 25 June 1995.

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