CODEWITHSUNDEEP
cmacosposixnamed-pipesmkfifo
user187676
user187676

Reputation:

fifo: Unblock pending open calls

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

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

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.

Upvotes: 2

Views: 898

Answers (3)

Barmar
Barmar

Reputation: 780994

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);
}

Upvotes: 2

Petr Skocik
Petr Skocik

Reputation: 60068

On Linux you can (nonportably) open a FIFO in O_RDWR mode. When you do and hold the FIFO, other openers won't get blocked in their open calls (regardless of whether those are readonly or writeonly open calls). Then you can unlink the FIFO while still holding the O_RDWR file handle to it.

Example code:

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
int main(int C, char **V)
{
    int fd;
    if(0>(fd=open(V[1], O_RDWR))) return perror("open"),1;
    if(0>unlink(V[1])) return perror("unlink"),1;
}

Upvotes: 0

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48572

Open the FIFO for writing, unlink it, then close it. This will make process A's open succeed, and the resulting FD will immediately be at EOF. (By the way, the unlink is unnecessary if you just want to make open return, but is still a good idea for cleaning up.)

Upvotes: 0

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