SPLAIN(1) Perl Programmers Reference Guide SPLAIN(1)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"), augmenting
them with the more explicative and endearing descriptions
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 compila-
tion will then be subject(ed :-) to the enhanced diagnos-
tics. These still go out STDERR.
Due to the interaction between runtime and compiletime
issues, and because it's probably not a very good idea any-
way, 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 introduction
before any other diagnostics. The $diagnostics::PRETTY
variable can generate nicer escape sequences for pagers.
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Warnings dispatched from perl itself (or more accurately,
those that match descriptions found in perldiag) are only
displayed once (no duplicate descriptions). User code gen-
erated warnings a la warn() are unaffected, allowing dupli-
cate 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 actually
nothing more than a link to the (executable) diagnostics.pm
module, as well as a link to the diagnostics.pod documenta-
tion. The -v flag is like the "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
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 <CR> here: ";
my $a, $b = scalar <STDIN>;
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
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(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
diagnostics::splainthis() function (the module's
$SIG{__WARN__} interceptor) has had its way with your warn-
ings.
There is a $diagnostics::DEBUG variable you may set if
you're desperately curious what sorts of things are being
intercepted.
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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
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 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 subseri-
ous, if you use a program named splain, you should expect a
bit of whimsy.
AUTHOR
Tom Christiansen <tchrist@mox.perl.com>, 25 June 1995.
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