X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/e8c8d959315d53d951cc75427f9a58a15d8cf780..3d0346a5d1004526830c70905c56755aecc6a442:/pod/perlfaq8.pod diff --git a/pod/perlfaq8.pod b/pod/perlfaq8.pod index e2ea3e7..f9f3584 100644 --- a/pod/perlfaq8.pod +++ b/pod/perlfaq8.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq8 - System Interaction ($Revision: 1.17 $, $Date: 2003/01/26 17:44:04 $) +perlfaq8 - System Interaction ($Revision: 1.25 $, $Date: 2005/06/04 04:12:40 $) =head1 DESCRIPTION @@ -276,8 +276,8 @@ next. If you expect characters to get to your device when you print() them, you'll want to autoflush that filehandle. You can use select() and the C<$|> variable to control autoflushing (see L> -and L, or L, ``How do I flush/unbuffer an -output filehandle? Why must I do this?''): +and L, or L, "How do I flush/unbuffer an +output filehandle? Why must I do this?"): $oldh = select(DEV); $| = 1; @@ -411,33 +411,32 @@ You don't actually "trap" a control character. Instead, that character generates a signal which is sent to your terminal's currently foregrounded process group, which you then trap in your process. Signals are documented in L and the -section on ``Signals'' in the Camel. +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. -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). + # as an anonymous subroutine -For example: + $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) }; - $Interrupted = 0; # to ensure it has a value - $SIG{INT} = sub { - $Interrupted++; - 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. -However, because syscalls restart by default, you'll find that if -you're in a "slow" call, such as , 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 or the section on ``Signals'' -in the Camel book. =head2 How do I modify the shadow password file on a Unix system? @@ -528,7 +527,7 @@ though, so if you use END blocks you should also use Perl's exception-handling mechanism is its eval() operator. You can use eval() as setjmp and die() as longjmp. For details of this, see the section on signals, especially the time-out handler for a blocking -flock() in L or the section on ``Signals'' in +flock() in L or the section on "Signals" in the Camel Book. If exception handling is all you're interested in, try the @@ -749,10 +748,10 @@ but leave its STDOUT to come out our old STDERR: while () { } # plus a read To read both a command's STDOUT and its STDERR separately, it's easiest -and safest to redirect them separately to files, and then read from those -files when the program is done: +to redirect them separately to files, and then read from those files +when the program is done: - system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); + system("program args 1>program.stdout 2>program.stderr"); Ordering is important in all these examples. That's because the shell processes file descriptor redirections in strictly left to right order. @@ -1018,7 +1017,7 @@ the current process group of your controlling terminal as follows: Use the alarm() function, probably in conjunction with a signal handler, as documented in L and the section on -``Signals'' in the Camel. You may instead use the more flexible +"Signals" in the Camel. You may instead use the more flexible Sys::AlarmCall module available from CPAN. The alarm() function is not implemented on all versions of Windows. @@ -1063,8 +1062,76 @@ O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with sysopen(): use Fcntl; - sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) - or die "can't open /tmp/somefile: $!": + 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<< >> + +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; + $x / 0; + __END__ + +The perl message comes out with "Perl" in front. The BEGIN block +works at compile time so all of the compilation errors and warnings +get the "Perl:" prefix too. + + Perl: Useless use of division (/) in void context at ./test line 9. + Perl: Name "main::a" used only once: possible typo at ./test line 8. + Perl: Name "main::x" used only once: possible typo at ./test line 9. + Perl: Use of uninitialized value in addition (+) at ./test line 8. + Perl: Use of uninitialized value in division (/) at ./test line 9. + Perl: Illegal division by zero at ./test line 9. + Perl: Illegal division by zero at -e line 3. + +If I don't see that "Perl:", it's not from perl. + +You could also just know all the perl errors, and although there are +some people who may know all of them, you probably don't. However, they +all should be in the perldiag manpage. If you don't find the error in +there, it probably isn't a perl error. + +Looking up every message is not the easiest way, so let perl to do it +for you. Use the diagnostics pragma with turns perl's normal messages +into longer discussions on the topic. + + use diagnostics; + +If you don't get a paragraph or two of expanded discussion, it +might not be perl's message. =head2 How do I install a module from CPAN? @@ -1110,8 +1177,8 @@ just need to replace step 3 (B) with B and you will get a new F binary with your extension linked in. See L for more details on building extensions. -See also the next question, ``What's the difference between require -and use?''. +See also the next question, "What's the difference between require +and use?". =head2 What's the difference between require and use? @@ -1163,7 +1230,7 @@ See Perl's L for more information. use lib "$FindBin::Bin"; use your_own_modules; -=head2 How do I add a directory to my include path at runtime? +=head2 How do I add a directory to my include path (@INC) at runtime? Here are the suggested ways of modifying your include path: @@ -1185,8 +1252,8 @@ but other times it is not. Modern programs C instead. =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997-2003 Tom Christiansen and Nathan Torkington. -All rights reserved. +Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and +other authors as noted. All rights reserved. This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.