Detect close of named pipe

Question

I have a C program that writes data to a named pipe and a Python program that reads data from the named pipe, like this:

p = open('/path/to/named/pipe', 'r')
...
data = p.read(size)

When the C program exits, it closes the pipe.

How can I detect this from the Python side? I've tried installing a handler for SIGPIPE, but it seems that SIGPIPE only happens when attempting to write to a closed pipe, not read from it. I also expected that p.read(size) might return a length-zero string because of the EOF at the other end, but actually it just hangs waiting for data.

How can I detect this situation and handle it?

Answer 1

You can use the select module to monitor the state of your pipe. On Linux (where select.poll() is available, the following code will detect the presence of a closed pipe:

import select

# ...

poller = select.poll()
# Register the "hangup" event on p
poller.register(p, select.POLLHUP)

# Call poller.poll with 0s as timeout
for descriptor, mask in poller.poll(0):
    # Can contain at most one element, but still:
    if descriptor == p.fileno() and mask & select.POLLHUP:
        print('The pipe is closed on the other end.')
        p.close()

Analogous methods exist for other OS as well that can detect such situations.

The reason why it hangs when calling read , is because the IO is blocking. You can turn it into non-blocking (and have read return an empty string) by using os.set_blocking , but this would still not allow you to detect when the pipe on the other end is closed.