Python Fork() Exit Status

Most computer programs return an exit code to the operating system’s shell. Shell scripting tools can use the exit status of a program to indicate if the program exited normally or abnormally. In either case, the shell script can react depending on the outcome of the child process.

Python programs can use os.fork() to create child processes. Since the child process is a new instance of the program, it can be useful in some cases to inspect if the child process exited normally or not. For example, a GUI program may spawn a child process and notify the user if the operation completed successfully or not.

This post shows an example program taken from Programming Python: Powerful Object-Oriented Programming that demonstrates how a parent process can inspect a child process’ exit code. I added comments to help explain the workings of the program.

Code

import os

exitstat = 0


# Function that is executed after os.fork() that runs in a new process
def child():
    global exitstat
    exitstat += 1
    print('Hello from child', os.getpid(), exitstat)
    
    # End this process using os._exit() and pass a status code back to the shell
    os._exit(exitstat)


# This is the parent process code
def parent():
    while True:
        # Fork this program into a child process
        newpid = os.fork()
        
        # newpid is 0 if we are in the child process
        if newpid == 0:
            # Call child()
            child()
            
        # otherwise, we are still in the parent process
        else:
            # os.wait() returns the pid and status and status code
            # On unix systems, status code is stored in status and has to
            # be bit-shifted
            pid, status = os.wait()
            print('Parent got', pid, status, (status >> 8))
            if input() == 'q':
                break


if __name__ == '__main__':
    parent()

Explanation

This program is pretty basic. We have two functions, parent() and child(). When the program starts on line 38, it calls parent() to enter the parent() function. The parent() function enters and infinite loop that forks this program on line 20. The result of os.fork() is stored in the newpid variable.

Our program is executing in the child process when newpid is zero. If that case, we call our child() function. The child() function prints a message to the console on line 10 and then exits by calling os._exit() on line 13. We pass the exitstat variable to os._exit() whose value becomes the exit code for this process.

The parent process continues in the meantime. On line 32, we use os.wait() to return the pid and status of the child process. The status variable also containes the exitstat value passed to os._exit() in the child process, but to get this code, we have to perform a bit shift operation by eight bits. The following line prints the pid, status, and the child process’ exit code to the console. When the user presses ‘q’, the parent process ends.

References

Lutz, Mark. Programming Python. Beijing, OReilly, 2013.

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