Is $! supposed to be set after system?

[p5pRT]

Migrated from rt.perl.org#977 (status was 'resolved')

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[p5pRT]

From @vlbrown

Is $! supposed to be set after system?

From p. 230 of the Camel (2nd edition) one might gather this and a
lot of people seem to believe this to be the case.

But I am thinking it's a mis-assumption and system() (unlike, say,
open) does not set $!.

This should be _explicitly_ documented, especially in light of the
example on p. 230 of Programming Perl which solidly implies that $!
may get set.

However, one of the clp-mod members did some playing with a C program
that indicated errno is being set after a program is run, so perhaps
system() _should_ be setting $! and isn't.

Is this a bug?

(It's either a bug in the book, i.e. Programming Perl should document
that the use of $! in the code on p. 230 is not _directly_ related to
system()... unlike certain other function such as open(), system()
does not actually set $!.

_Or_ system() is supposed to set $! and I can't make it do so.

If it's a documentation bug only I'll follow up with that. If it is a
bug in system() you folks get it.

Vicki Brown <vlb@​cfcl.com> writes​:

From the Camel, p. 230
)
) $rc = 0xffff & system(@​args);
[...]
) elsif ($rc == 0xff00) {
) print "command failed​: $!\n";

I cannot make $! contain anything useful no matter how hard I try,
or anyone else tries

At 21​:06 -0600 07/07/1999, Tom Christiansen wrote in comp.lang.perl.moderated​:

​:If $! is meaningful in any situation after running system(...)

It was when I wrote that.

​:I'd appreciate an example. $! is empty when I try
​: system("cat food"); # no such file food; exit(2) from cat
​: system("cats food"); # no such program cats
​: system("command that seg faults");

Testing...

bsd% perl -le 'system("nonesuch"); print $!'
<blank line>

linux% perl -le 'system("nonesuch"); print $!'
<blank line>

sunos% perl -le 'system("nonesuch"); print $!'
No such file or directory

and

bsd% perl -le 'system("/"); print $!'
<blank line>

linux% perl -le 'system("/"); print $!'
<blank line>

sunos% perl -le 'system("/"); print $!'
Permission denied

Well, something's odd. I'd report it.

One of the other folks on the list quoted the docs thusly​:

C<system> returns C<-1> if it is unable to request that the
command be run. This happens if C<fork()> fails, for example.
C<$!> will say why C<fork()> or some other call failed [though,
on most implementations, I think C<fork()> is the only thing
that can fail to cause this case].

This isn't explicitly documented with Perl, possibly because
it depends on your operating system.

So perhaps $! is not meant to reflect the result of system()and the
book (Programming Perl) should mention this as a comment in the code
on p. 230.

However, another clp-mod member tried this​:

At 19​:00 -0700 07/07/1999, Dirk Myers wrote​:

If you take the
programs that follow (test.c is compiled to test -- gcc test.c -o test --
, and the file "gabba" in the current directory exists) you get​:

harpo 17% ./test.pl
The error was -1, errno was 17, strerror was File exists
17
17 File exists

... it *looks like* $! works, but isn't actually getting set to errno.

Is this a bug? I dunno... the C program is clearly setting errno and
exiting with an error, but $! sure isn't getting it...

--- snip here for test.c ---
/* Test program to blow up with an error
  * ... you gotta touch 'gabba' in the current directory before
  * running it... */

#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>

void main(void) {
  int rv ;
  rv = open("./gabba", O_EXCL | O_CREAT | O_SHLOCK | O_EXLOCK) ;
if (rv != 0 ) {
  printf ("The error was %d, errno was %d, strerror was %s\n", rv,
errno,
strerror(errno)) ;
}
  exit (errno) ;
--- snip for test.c ---

--- snip for test.pl ---
#!/usr/local/bin/perl

$rv = system "./test" ;
if ($rv) {
$rv = $rv >> 8 ;
print "$rv $!\n" ;
$! = $rv ;
print "$rv $!\n" ; }
else { print "nope.\n" ; }
--- snip for test.pl ---

Perl Info

Site configuration information for perl 5.00502:

Configured by rdm at Sun Feb  7 16:25:31 PST 1999.

Summary of my perl5 (5.0 patchlevel 5 subversion 2) configuration:
   Platform:
     osname=freebsd, osvers=2.2.8-release, archname=i386-freebsd
     uname='freebsd cfcl.com 2.2.8-release freebsd 2.2.8-release #0: 
fri jan 15 1
8:39:18 pst 1999 rdm@cfcl.com:usrsrcsyscompilefreebie i386 '
     hint=recommended, useposix=true, d_sigaction=define
     usethreads=undef useperlio=undef d_sfio=undef
   Compiler:
     cc='cc', optimize='-O', gccversion=2.7.2.1
     cppflags='-I/usr/local/include'
     ccflags ='-I/usr/local/include'
     stdchar='char', d_stdstdio=undef, usevfork=true
     intsize=4, longsize=4, ptrsize=4, doublesize=8
     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
     alignbytes=4, usemymalloc=n, prototype=define
   Linker and Libraries:
     ld='ld', ldflags =' -L/usr/local/lib'
     libpth=/usr/local/lib /usr/lib
     libs=-ldb -lm -lc -lcrypt
     libc=/usr/lib/libc.so.3.1, so=so, useshrplib=false, libperl=libperl.a
   Dynamic Linking:
     dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
     cccdlflags='-DPIC -fpic', lddlflags='-Bshareable  -L/usr/local/lib'

Locally applied patches:



@INC for perl 5.00502:
     /usr/local/lib/perl5/5.00502/i386-freebsd
     /usr/local/lib/perl5/5.00502
     /usr/local/lib/perl5/site_perl/5.005/i386-freebsd
     /usr/local/lib/perl5/site_perl/5.005
     .


Environment for perl 5.00502:
     HOME=/home/vlb
     LANG (unset)
     LD_LIBRARY_PATH (unset)
     LOGDIR (unset)
  
PATH=/home/vlb/bin:/usr/local/bin:/usr/local/nib:/usr/bin:/bin:/etc:/s 
bin:/u
sr/sbin:/usr/local/games:/usr/games:.:/home/rdm/bin:/home/vlb/projex/ptf/bin
     PERL_BADLANG (unset)
     SHELL=/ulb/tcsh>Coming over from clp-mod

-- --
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