in perl, how do we detect a segmentation fault in an external command

Question

Following is C code that is destined to crash:

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

int main() {
    char *p = NULL;
    printf("Value at P: %c
", *p);
    return 0;
}

When I compile and run it (RH4 machine with gcc 4.5.2), it predictably gives a segmentation fault:

%  ./a.out
Segmentation fault

%  echo $status
139

If I run it with Perl v5.8.5, this happens:

%  perl -e 'system("./a.out") and die "Status: $?"'
Status: 11 at -e line 1.

The perlvar documentation for $? says that

Thus, the exit value of the subprocess is really ($?>> 8 ), and $? & 127 gives which signal, if any, the process died from, and $? & 128 reports whether there was a core dump.

11 >> 8 is 0, and 11 & 127 is 11.

Why the different exit statuses? If we cannot depend on the exit status, what should be the way to detect segmentation fault in an external command?

Answer 1

Reading the documentation for system might answer your question:

system('a.out');

if ($? == -1) {
    print "failed to execute: $!
";
}
elsif ($? & 127) {
    printf "child died with signal %d, %s coredump
",
        ($? & 127),  ($? & 128) ? 'with' : 'without';
}
else {
    printf "child exited with value %d
", $? >> 8;
}

Output:

child died with signal 11, without coredump

The shell just encodes the signal in the status in a different way: 139 - 128 = 11. For example, man bash says:

The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.