Clueless: piping between 2 non-python processes

[Andrew Bennetts]

On Sat, Oct 25, 2003 at 08:16:33PM +0200, Michael Lehmeier wrote:
> 
> So I wanted to start A and B as fork, but all solutions to create a pipe
> between them seem to demand that at least one of the two processes is in
> python itself.
> Or maybe I just understood it wrong.
> 
> So the basic problem is:
> - create two processes A and B
> - pipe from A to B
> - terminate A when B ends

I think you can do something like (untested):

    import os, signal
    
    pathToA = '/usr/bin/A'
    pathToB = '/usr/bin/B'
    
    # Create pipes
    readA, writeB = os.pipe()
    readB, writeA = os.pipe()
    
    # Create process A
    pidA = os.fork()
    if pidA == 0:   # child
        os.dup2(readA.fileno(), 0)  # set read pipe to stdin
        os.dup2(writeA.fileno(), 1) # set write pipe to stdout
        os.execl(pathToA)
    
    # Create process B
    pidB = os.fork()
    if pidB == 0:   # child
        os.dup2(readB.fileno(), 0)  # set read pipe to stdin
        os.dup2(writeB.fileno(), 1) # set write pipe to stdout
        os.execl(pathToB)

    # Close file descriptors in the parent; it doesn't need them anymore
    readA.close(); writeB.close(); readB.close(); writeA.close()
        
    # Wait for B to terminate
    os.waitpid(pidB, 0)

    # Kill A (even if it has finished already... this might need a try/except)
    os.kill(pidA, signal.SIGTERM)

    # Wait for A (to avoid zombies)
    os.waitpid(pidA, 0)

I've probably stuffed up some details, but I'm pretty sure that that's the
basic idea.
    
-Andrew.