[SOLVED] fifo: Unblock pending open calls

fifo: Unblock pending open calls

I use a FIFO (named pipe) for IPC. Now process A calls

mkfifo(path)
open(path)

Naturally open() will block until the file is written to by process B. Now I need a way to invalidate a FIFO. Therefore I call

unlink(path)

Now I expected that any blocking open call would return, but they don't and my process hangs indefinitely.

How can I unblock the open call when the FIFO is unlinked? Do I have to resort to O_NONBLOCK?

PS: I tried the suggested write/unlink/close approach to no avail. The open call blocks immediately.

void invalidate() {
    int fd = open(path, O_WRONLY)
    unlink(path)
    close(fd)
}

I think the issue is

The FIFO must be opened on both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.

However invalidate should work without knowing if the FIFO is currently opened for reading or not.

Solution

The invalidator should open the FIFO in non-blocking write mode. This way, its call to open() won't hang if there are no readers waiting for it. It should then close the FIFO immediately.

Any processes that were waiting in a read-only open will return when the FIFO is opened. Then they'll immediately read EOF.

void invalidate() {
    int fd = open(path, O_WRONLY | O_NONBLOCK);
    unlink(path);
    close(fd);
}