sorry! this is the correct version ;-)

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import mmap, os, select

NUM_CHILDREN = 30
MSG_LEN = 9
BUF_LEN = NUM_CHILDREN * MSG_LEN

buf = mmap.mmap(-1, BUF_LEN)
p = select.poll()

def write_buffer(i):
    msg = '%s\t%d\n' % (i, os.getpid())
    offset = MSG_LEN * i
    buf.seek(offset)
    buf.write(msg)

def child(i, pipeout):
    write_buffer(i)
    os.write(pipeout, 'OK\0'.encode())
    os._exit(0)

def fork(i, p):
    pipein, pipeout = os.pipe()
    if os.fork() == 0:
        child(i, pipeout)
    else:
        p.register(pipein)

def loop(msgs, p):
    msgs = NUM_CHILDREN
    while msgs:
        for fd, event in p.poll():
            p.unregister(fd)
            msgs = msgs - 1


for i in range(NUM_CHILDREN):
    fork(i, p)

loop(NUM_CHILDREN, p)

buf.seek(0)
print buf.read(BUF_LEN)