Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
SPLAIN(1)	       Perl Programmers	Reference Guide		     SPLAIN(1)

       diagnostics, splain - produce verbose warning diagnostics

       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

   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 compilation 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
       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.

       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 actually nothing
       more than a link	to the (executable) 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
       $diagnostics::PRETTY variable.  Since you're post-processing with
       splain, there's no sense	in being able to enable() or disable()

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

       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 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 >/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 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";

       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

       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 }

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

       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 subserious, if you
       use a program named splain, you should expect a bit of whimsy.

       Tom Christiansen	<>,	25 June	1995.

perl v5.32.0			  2020-08-08			     SPLAIN(1)


Want to link to this manual page? Use this URL:

home | help