diagnostics(3)   Perl Programmers Reference Guide  diagnostics(3)



NAME
       diagnostics, splain - produce verbose warning diagnostics

SYNOPSIS
       Using the "diagnostics" pragma:

           use diagnostics;
           use diagnostics -verbose;

           enable  diagnostics;
           disable diagnostics;

       Using the "splain" standalone filter program:

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

       Using diagnostics to get stack traces from a misbehaving
       script:

           perl -Mdiagnostics=-traceonly my_script.pl

DESCRIPTION
       The "diagnostics" Pragma

       This module extends the terse diagnostics normally emitted
       by both the perl compiler and the perl interpreter (from
       running perl with a -w switch or "use warnings"), augment-
       ing them with the more explicative and endearing descrip-
       tions found in perldiag.  Like the other pragmata, it
       affects the compilation phase of your program rather than
       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 whole compi-
       lation will then be subject(ed :-) to the enhanced diag-
       nostics.  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 their behaviour
       at runtime using the disable() and enable() methods to
       turn them off and on respectively.

       The -verbose flag first prints out the perldiag introduc-
       tion before any other diagnostics.  The $diagnos-
       tics::PRETTY variable can generate nicer escape sequences
       for pagers.

       Warnings dispatched from perl itself (or more accurately,
       those that match descriptions found in perldiag) are only
       displayed once (no duplicate descriptions).  User code
       generated warnings a la warn() are unaffected, allowing
       duplicate user messages to be displayed.

       This module also adds a stack trace to the error message
       when perl dies.  This is useful for pinpointing what
       caused the death. The -traceonly (or just -t) flag turns
       off the explanations of warning messages leaving just the
       stack traces. So if your script is dieing, run it again
       with

         perl -Mdiagnostics=-traceonly my_bad_script

       to see the call stack at the time of death. By supplying
       the -warntrace (or just -w) flag, any warnings emitted
       will also come with a stack trace.

       The splain Program

       While apparently a whole nuther program, splain is actu-
       ally nothing more than a link to the (executable) diagnos-
       tics.pm module, as well as a link to the diagnostics.pod
       documentation.  The -v flag is like the "use diagnostics
       -verbose" directive.  The -p flag is like the $diagnos-
       tics::PRETTY variable.  Since you're post-processing with
       splain, there's no sense in being able to enable() or dis-
       able() processing.

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

EXAMPLES
       The following file is certain to trigger a few errors at
       both runtime 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  here: ";
           my $a, $b = scalar ;
           print "\n";
           print $x/$y;

       If you prefer to run your program first and look at its
       problem afterwards, 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-fly warnings, 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 get at 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 at runtime.  Otherwise, they may be embedded in
       the file itself when the splain package is built.   See
       the Makefile for details.

       If an extant $SIG{__WARN__} handler is discovered, it will
       continue to be honored, but only after the diagnos-
       tics::splainthis() function (the module's $SIG{__WARN__}
       interceptor) has had its way with your warnings.

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

           BEGIN { $diagnostics::DEBUG = 1 }

BUGS
       Not being able to say "no diagnostics" is annoying, but
       may not be insurmountable.

       The "-pretty" directive is called too late to affect mat-
       ters.  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 be needed, but this gets a "panic: top_level" when
       using the pragma form in Perl 5.001e.

       While it's true that this documentation is somewhat subse-
       rious, if you use a program named splain, you should
       expect a bit of whimsy.

AUTHOR
       Tom Christiansen , 25 June 1995.



perl v5.8.8                 2001-09-21             diagnostics(3)