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 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) diagnostics.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
$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
(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
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 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.
AUTHOR
Tom Christiansen <tchrist@mox.perl.com>, 25 June 1995.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
