Re^2: Calling strace from a perl program

Interesting. I tried this on my Linux system, and it was not at all as I expected. Long story short:

Different behavior when using ps vs. ps ww!
ps was not affected by the change to $0 at all.
ps ww did see the change, but using local didn't unmake the change as I would have expected.

sub ps_line {
    my $ps = shift;
    print join ' ', "$ps:\t", @{ (grep { $_->[0] == $$ }
                                   map { [ split ] }
                                  `$ps | tail -n +2`)[0] };
    print "\n";
}

print "\$0 = $0\n";
ps_line( 'ps' );
ps_line( 'ps ww' );
{
    local( $0 ) = 'nyuck' x 3;
    print "\$0 = $0\n";
    ps_line( 'ps' );
    ps_line( 'ps ww' );
}
print "\$0 = $0\n";
ps_line( 'ps' );
ps_line( 'ps ww' );
__END__
$0 = ./perlmonks.pl
ps:      875 pts/1 00:00:00 perl
ps ww:   875 pts/1 S+ 0:00 perl ./perlmonks.pl
$0 = nyucknyucknyuck
ps:      875 pts/1 00:00:00 perl
ps ww:   875 pts/1 S+ 0:00 nyucknyucknyuck
$0 = ./perlmonks.pl
ps:      875 pts/1 00:00:00 perl
ps ww:   875 pts/1 S+ 0:00 ./perlmonks.pl
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Re^3: Calling strace from a perl program by pc88mxer (Vicar) on Mar 11, 2008 at 21:39 UTC
That's very odd. I tried it using this test program: `#!/usr/bin/perl print "my pid = $$\n"; get_lock(); print "enter return again:"; my $x = <STDIN>; sub get_lock { local($0) = "(acquiring lock)"; print "enter return:"; my $x = <STDIN>; }` [download] In another terminal I get: `$ /bin/ps 27940 PID TTY STAT TIME COMMAND 27940 pts/0 S+ 0:00 (acquiring lock) $ /bin/ps 27940 PID TTY STAT TIME COMMAND 27940 pts/0 S+ 0:00 ./d0` [download] I'm using Ubuntu 7.10. Update: Ok, it does seem that you will lose the initial 'perl' from the `COMMAND` field when `$0` is restored, but the script name should still remain. Don't know why that happens, and I also can explain yet your 'ww' output.	[reply] [d/l] [select]

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Re^3: Calling strace from a perl program
by pc88mxer (Vicar) on Mar 11, 2008 at 21:39 UTC

#!/usr/bin/perl

print "my pid = $$\n";

get_lock();
   print "enter return again:";
   my $x = <STDIN>;

sub get_lock {
   local($0) = "(acquiring lock)";
   print "enter return:";
   my $x = <STDIN>;
}
[download]

In another terminal I get:

$ /bin/ps 27940
  PID TTY      STAT   TIME COMMAND
27940 pts/0    S+     0:00 (acquiring lock)
$ /bin/ps 27940
  PID TTY      STAT   TIME COMMAND
27940 pts/0    S+     0:00 ./d0
[download]

I'm using Ubuntu 7.10.

Update: Ok, it does seem that you will lose the initial 'perl' from the COMMAND field when $0 is restored, but the script name should still remain. Don't know why that happens, and I also can explain yet your 'ww' output.

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