diagnostics(3p) Perl Programmers Reference Guide diagnostics(3p) #
diagnostics(3p) Perl Programmers Reference Guide diagnostics(3p)
NNAAMMEE #
diagnostics, splain - produce verbose warning diagnostics
SSYYNNOOPPSSIISS #
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
DDEESSCCRRIIPPTTIIOONN #
TThhee “"ddiiaaggnnoossttiiccss"” PPrraaggmmaa 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 _d_o_e_s
enable perl's --ww flag.) Your whole compilation will then be subject(ed
:-) to the enhanced diagnostics. These still go out SSTTDDEERRRR.
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 ddiissaabbllee(()) and eennaabbllee(()) methods to
turn them off and on respectively.
The --vveerrbboossee 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 wwaarrnn(()) 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 --ttrraacceeoonnllyy (or
just --tt) 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 --wwaarrnnttrraaccee
(or just --ww) flag, any warnings emitted will also come with a stack
trace.
TThhee _s_p_l_a_i_n PPrrooggrraamm Another program, _s_p_l_a_i_n is actually nothing more than a link to the (executable) _d_i_a_g_n_o_s_t_i_c_s_._p_m module, as well as a link to the _d_i_a_g_n_o_s_t_i_c_s_._p_o_d documentation. The --vv flag is like the “use diagnostics -verbose” directive. The --pp flag is like the $diagnostics::PRETTY variable. Since you’re post-processing with _s_p_l_a_i_n, there’s no sense in being able to eennaabbllee(()) or ddiissaabbllee(()) processing.
Output from _s_p_l_a_i_n is directed to SSTTDDOOUUTT, unlike the pragma.
EEXXAAMMPPLLEESS #
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 ssttddoouutt 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 eennaabbllee(())
or ddiissaabbllee(()) 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";
IINNTTEERRNNAALLSS #
Diagnostic messages derive from the _p_e_r_l_d_i_a_g_._p_o_d file when available at
runtime. Otherwise, they may be embedded in the file itself when the
splain package is built. See the _M_a_k_e_f_i_l_e for details.
If an extant $SIG{__WARN__} handler is discovered, it will continue to be
honored, but only after the ddiiaaggnnoossttiiccss::::ssppllaaiinntthhiiss(()) 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 }
BBUUGGSS #
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 _b_e_f_o_r_e 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 _s_p_l_a_i_n, you should expect a bit of whimsy.
AAUUTTHHOORR #
Tom Christiansen <_t_c_h_r_i_s_t_@_m_o_x_._p_e_r_l_._c_o_m>, 25 June 1995.
perl v5.36.3 2023-02-15 diagnostics(3p)