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Diffstat (limited to 'gnu/usr.bin/perl/pod/perlfaq8.pod')
| -rw-r--r-- | gnu/usr.bin/perl/pod/perlfaq8.pod | 117 |
1 files changed, 92 insertions, 25 deletions
diff --git a/gnu/usr.bin/perl/pod/perlfaq8.pod b/gnu/usr.bin/perl/pod/perlfaq8.pod index 22f6a0727c7..052ce7a001a 100644 --- a/gnu/usr.bin/perl/pod/perlfaq8.pod +++ b/gnu/usr.bin/perl/pod/perlfaq8.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq8 - System Interaction ($Revision: 1.6 $, $Date: 2003/12/03 03:02:45 $) +perlfaq8 - System Interaction ($Revision: 1.7 $, $Date: 2004/08/09 18:10:15 $) =head1 DESCRIPTION @@ -413,31 +413,30 @@ foregrounded process group, which you then trap in your process. Signals are documented in L<perlipc/"Signals"> and the section on ``Signals'' in the Camel. -Be warned that very few C libraries are re-entrant. Therefore, if you -attempt to print() in a handler that got invoked during another stdio -operation your internal structures will likely be in an -inconsistent state, and your program will dump core. You can -sometimes avoid this by using syswrite() instead of print(). +You can set the values of the %SIG hash to be the functions you want +to handle the signal. After perl catches the signal, it looks in %SIG +for a key with the same name as the signal, then calls the subroutine +value for that key. + + # as an anonymous subroutine + + $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) }; + + # or a reference to a function + + $SIG{INT} = \&ouch; + + # or the name of the function as a string + + $SIG{INT} = "ouch"; + +Perl versions before 5.8 had in its C source code signal handlers which +would catch the signal and possibly run a Perl function that you had set +in %SIG. This violated the rules of signal handling at that level +causing perl to dump core. Since version 5.8.0, perl looks at %SIG +*after* the signal has been caught, rather than while it is being caught. +Previous versions of this answer were incorrect. -Unless you're exceedingly careful, the only safe things to do inside a -signal handler are (1) set a variable and (2) exit. In the first case, -you should only set a variable in such a way that malloc() is not -called (eg, by setting a variable that already has a value). - -For example: - - $Interrupted = 0; # to ensure it has a value - $SIG{INT} = sub { - $Interrupted++; - syswrite(STDERR, "ouch\n", 5); - } - -However, because syscalls restart by default, you'll find that if -you're in a "slow" call, such as <FH>, read(), connect(), or -wait(), that the only way to terminate them is by "longjumping" out; -that is, by raising an exception. See the time-out handler for a -blocking flock() in L<perlipc/"Signals"> or the section on ``Signals'' -in the Camel book. =head2 How do I modify the shadow password file on a Unix system? @@ -1066,6 +1065,74 @@ sysopen(): sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) or die "can't open /foo/somefile: $!": +=head2 How do I tell the difference between errors from the shell and perl? + +(answer contributed by brian d foy, C<< <bdfoy@cpan.org> >> + +When you run a Perl script, something else is running the script for you, +and that something else may output error messages. The script might +emit its own warnings and error messages. Most of the time you cannot +tell who said what. + +You probably cannot fix the thing that runs perl, but you can change how +perl outputs its warnings by defining a custom warning and die functions. + +Consider this script, which has an error you may not notice immediately. + + #!/usr/locl/bin/perl + + print "Hello World\n"; + +I get an error when I run this from my shell (which happens to be +bash). That may look like perl forgot it has a print() function, +but my shebang line is not the path to perl, so the shell runs the +script, and I get the error. + + $ ./test + ./test: line 3: print: command not found + +A quick and dirty fix involves a little bit of code, but this may be all +you need to figure out the problem. + + #!/usr/bin/perl -w + + BEGIN { + $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; }; + $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1}; + } + + $a = 1 + undef; + $x / 0; + __END__ + +The perl message comes out with "Perl" in front. The BEGIN block +works at compile time so all of the compilation errors and warnings +get the "Perl:" prefix too. + + Perl: Useless use of division (/) in void context at ./test line 9. + Perl: Name "main::a" used only once: possible typo at ./test line 8. + Perl: Name "main::x" used only once: possible typo at ./test line 9. + Perl: Use of uninitialized value in addition (+) at ./test line 8. + Perl: Use of uninitialized value in division (/) at ./test line 9. + Perl: Illegal division by zero at ./test line 9. + Perl: Illegal division by zero at -e line 3. + +If I don't see that "Perl:", it's not from perl. + +You could also just know all the perl errors, and although there are +some people who may know all of them, you probably don't. However, they +all should be in the perldiag manpage. If you don't find the error in +there, it probably isn't a perl error. + +Looking up every message is not the easiest way, so let perl to do it +for you. Use the diagnostics pragma with turns perl's normal messages +into longer discussions on the topic. + + use diagnostics; + +If you don't get a paragraph or two of expanded discussion, it +might not be perl's message. + =head2 How do I install a module from CPAN? The easiest way is to have a module also named CPAN do it for you. |