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Re: 39-exit-status.patch

From: Akim Demaille
Subject: Re: 39-exit-status.patch
Date: 26 Feb 2001 18:57:38 +0100
User-agent: Gnus/5.0808 (Gnus v5.8.8) XEmacs/21.1 (Cuyahoga Valley)

Pavel Roskin <address@hidden> writes:

> Hello, Akim!
> I'm not a Perl guru, and maybe I don't quite understand you doubts, but
> I'll try to anser the questions that I can answer.
> > Yep, I know this, but the documentation for die was saying it should
> > be automatic.  And in fact, it is automatic: it works fine when -d is
> > given, i.e., when END does not run unlink.
> You are missing one important point.
> >        die LIST
> >                Outside an `eval', prints the value of LIST to
> >                `STDERR' and exits with the current value of `$!'
> >                (errno).  If `$!' is `0', exits with the value of
> If `$!' is `0'!!! This is not the case if system() fails. In this case the
> exit status is in the upper byte of $!,

This is not my reading :(

       system LIST
       system PROGRAM LIST
               Does exactly the same thing as `exec LIST', except
               that a fork is done first, and the parent process
               waits for the child process to complete.  Note
               that argument processing varies depending on the
               number of arguments.  If there is more than one
               argument in LIST, or if LIST is an array with more
               than one value, starts the program given by the
               first element of the list with arguments given by
               the rest of the list.  If there is only one scalar
               argument, the argument is checked for shell
               metacharacters, and if there are any, the entire
               argument is passed to the system's command shell
               for parsing (this is `/bin/sh -c' on Unix plat­
               forms, but varies on other platforms).  If there
               are no shell metacharacters in the argument, it is
               split into words and passed directly to `execvp',
               which is more efficient.

               Beginning with v5.6.0, Perl will attempt to flush
               all files opened for output before any operation
               that may do a fork, but this may not be supported
               on some platforms (see the perlport manpage).  To
               be safe, you may need to set `$|' ($AUTOFLUSH in
               English) or call the `autoflush()' method of
               `IO::Handle' on any open handles.

               The return value is the exit status of the program
               as returned by the `wait' call.  To get the actual
               exit value divide by 256.  See also the exec entry
               elsewhere in this document.  This is not what you
               want to use to capture the output from a command,
               for that you should use merely backticks or
               `qx//', as described in the section on "`STRING`"
               in the perlop manpage.  Return value of -1 indi­
This is not => cates a failure to start the program (inspect $!
our case    => for the reason).

               Like `exec', `system' allows you to lie to a pro­
               gram about its name if you use the `system PROGRAM
               LIST' syntax.  Again, see the exec entry elsewhere
               in this document.

               Because `system' and backticks block `SIGINT' and
               `SIGQUIT', killing the program they're running
               doesn't actually interrupt your program.

                   @args = ("command", "arg1", "arg2");
                   system(@args) == 0
                        or die "system @args failed: $?"

This is  =>    You can check all the failure possibilities by
what I   =>    inspecting `$?' like this:
meant to use.
This is not $!.    $exit_value  = $? >> 8;
                   $signal_num  = $? & 127;
                   $dumped_core = $? & 128;

               When the arguments get executed via the system
               shell, results and return codes will be subject to
               its quirks and capabilities.  See the section on
               "`STRING`" in the perlop manpage and the exec
               entry elsewhere in this document for details.

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