python wrong subprocess.call return code

Question

Look at this 2 simples programs in C:

#include <stdio.h>
int main(int argc, char *argv[]) {
    return -1;
}

#include <stdio.h>
int main(int argc, char *argv[]) {
    return 1337;
}

Now look this very basic python script:

>>> import subprocess
>>> r=subprocess.call(['./a.out'])

I do not understand why but the python script r value contains:

Have i something wrong ?

Thanks

return codes are a single unsigned byte -1 == 255 unsigned byte ... 1337&255= 57 — Joran Beasley
– Joran Beasley, Commented Jul 22, 2018 at 19:47
return codes range from 0 to 255 / 128 (system dependent): 1337 % 256 => 57 — Jean-François Fabre
– Jean-François Fabre ♦, Commented Jul 22, 2018 at 19:47

Jean-François Fabre · Accepted Answer · 2018-07-22 19:55:30Z

1

Python has nothing to do with it. This is system dependent.

On Unix/Linux systems, the return code is stored on 1 byte, and is unsigned, so truncation occurs outside 0-255 range.

So -1 becomes 255, 1337 becomes 57 (can be checked by applying modulus 256).

note that in Windows, return codes can be higher than 255 (was able to pass 100000000, but still not negative)

The conclusion is: don't rely on return codes too much to pass information. Print something on the console instead.

answered Jul 22, 2018 at 19:55

Jean-François Fabre♦

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