You haven't mentioned what $! say

In this posting, I was mainly interested why system() does not adhere to the docs in that it does not return -1 on some platforms even the execution failed.

But - and maybe I should have mentioned this in my original post - the interest in this issue arose from a discussion with a co-worker about the proper usage of $!. My co-worker argued that it is never necessary to explicitly set $!=0 in a Perl program. His argument sounded convincing: $! (which is just errno) is only set if a low-level system call encounters a problem; otherwise, the value is not changed. Therefore, his argument goes, we first need to check if a function was successful, and only if it is not, we investigate $!. Example: 

if(open(FOO,'<',$foo)) {
  ...
} else {
  print "open $foo: $!\n";
}

[download]
This sounded convincing, but for the safe side, I tried to find a counter example, and came up with the example of system('myprog.exe'), where I can't, without prior setting of $!=0, reliably distinguish, whether or not myprog.exe has been executed with an exit code of 1, or could not be executed because it does not exist:

If myprog.exe can not be run, system(...) returns on my platform 256 (which is exit code 1, no child-error), and, as you point out, sets $! to some error code. But if myprog.exe can be run, and exits with code 1, I also get 256 as return code, and $! has an undefined value (it is set to whatever it had been set before the call to system). Only by explicitly setting it to 0 prior to running system(), is it that I can distinguish between those cases.

-- 
Ronald Fischer <ynnor@mm.st>

In reply to Re^2: system and $? by rovf
in thread system and $? by rovf

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