Reading from FIFO after unlink()

Question

I have created a FIFO, wrote to it and unlinked it. To my surprise I was able to read data from the fifo after unlinking, why is that?

#include <fcntl.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#define MAX_BUF 256
int main()
{
    int fd;
    char * myfifo = "/tmp/myfifo";

    /* create the FIFO (named pipe) */
    mkfifo(myfifo, 0666);

    int pid = fork();
    if (pid != 0)
    {
        /* write "Hi" to the FIFO */
        fd = open(myfifo, O_WRONLY);
        write(fd, "Hi", sizeof("Hi"));
        close(fd);

        /* remove the FIFO */
        unlink(myfifo);
    }
    else 
    {
        wait(NULL);
        char buf[MAX_BUF];

        /* open, read, and display the message from the FIFO */
        fd = open(myfifo, O_RDONLY);
        read(fd, buf, MAX_BUF);
        printf("Received: %s\n", buf);
        close(fd);

        return 0;
    }


    return 0;
}

Answer 1

Unless you pass the O_NONBLOCK flag to open(2), opening a FIFO blocks until the other end is opened. From man 7 fifo:

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.

A process can open a FIFO in nonblocking mode. In this case, opening for read only will succeed even if no-one has opened on the write side yet, opening for write only will fail with ENXIO (no such device or address) unless the other end has already been opened.

Which is to say, your parent / child processes are implicitly synchronized upon opening the FIFO. So by the time the parent process calls unlink(2), the child process opened the FIFO long ago. So the child will always find the FIFO object and open it before the parent calls unlink(2) on it.

A note about unlink(2): unlink(2) simply deletes the filename from the filesystem; as long as there is at least one process with the file (FIFO in this case) open, the underlying object will persist. Only after that process terminates or closes the file descriptor will the operating system free the associated resources. FWIW, this is irrelevant in the scope of this question, but it seems worth noting.

A couple of other (unrelated) remarks:

• Don't call wait(2) on the child. It will return an error (which you promptly ignore), because the child didn't fork any process.
• mkfifo(3), fork(2), open(2), read(2), write(2), close(2) and unlink(2) can all fail and return -1. You should gracefully handle possible errors instead of ignoring them. A common strategy for these toy programs is to print a descriptive error message with perror(3) and terminate.
• If you just want parent to child communication, use a pipe: it's easier to setup, you don't need to unlink it, and it is not exposed in the filesystem (but you need to create it with pipe(2) before forking, so that the child can access it).