Different behavior with multiprocessing

[Tomas Kotal]

Dne pondělí, 4. března 2013 15:31:41 UTC+1 Tomas Kotal napsal(a):
> Hi all.
> 
> 
> 
> Recently I discovered a strange behavior with multiprocessing library and call to function os.system (a different behavior under Linux and Windows to be more specific). I have this simple testing script:
> 
> 
> 
> ############################################
> 
> import sys
> 
> import os
> 
> from multiprocessing import Process
> 
> 
> 
> def do_fork(cmd):
> 
>     ret = os.system(cmd)
> 
>     # print result of call
> 
>     print ret
> 
>     os._exit(ret)
> 
> 
> 
> if __name__ == "__main__":
> 
>     cmds = [ "dir", "xy" ]
> 
> 
> 
>     procs = []
> 
>     for cmd in cmds:
> 
>         proc = Process(target=do_fork, args=(cmd, ))
> 
>         proc.start()
> 
>         procs.append( proc )
> 
> 
> 
>     for proc in procs:
> 
>         proc.join()
> 
>         print "exitcode: %d" % proc.exitcode
> 
> 
> 
>     print "ok" 
> 
> ############################################
> 
> 
> 
> This script just starts 2 processes. Each process executes one command in system shell and exits setting it's exit code same as exit code of a system call. There are 2 commands: "dir" (which works fine on Windows and Linux) and "xy" (which is supposed to fail under both systems). The strange thing is that when I run this script under Windows I get this output:
> 
> 
> 
> 0
> 
> 1
> 
> exitcode: 0
> 
> exitcode: 1
> 
> ok
> 
> 
> 
> The first 0 and 1 are results of os.system call which are printed from child processes. Rest of lines are printed from main process. This is expected output.
> 
> 
> 
> But when I run same script on Linux, what I get is this:
> 
> 
> 
> 0
> 
> 32512
> 
> exitcode: 0
> 
> exitcode: 0
> 
> ok
> 
> 
> 
> Although the second command fails and returns exit code 32512, the exit code from process in parent process gives me 0. I tried to change the script to use fork() instead of Process but the result was same.
> 
> 
> 
> Can anybody explain me what's the problem here?
> 
> 
> 
> (I use Python 2.6 on both Windows and Linux machines)

Seems like I found the problem: os._exit probably takes as parametr unsigned char, so it uses as error code whatever value it gets modulo 256:

os._exit(1)  # process.exitcode == 1
os._exit(255)  # process.exitcode == 255
os._exit(256)  # process.exitcode == 0
os._exit(257)  # process.exitcode == 1
os._exit(32512)  # process.exitcode == 0

So on Linux it's necesary to call something like this:
os._exit( os.system(cmd) >> 8 )

Because the first byte of return value on Linux is number of signal which kills the process and the second one is actual exit code.

http://docs.python.org/2/library/os.html#os.wait