mkfifo.c

#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define FIFO_PATH "fifo123"

int main(int argc, char** argv) {
    int fd;
    char buffer[1024];

    if (mkfifo(FIFO_PATH, S_IRWXU) < 0) {
        perror("mkfifo error");
        return 1;
    }

    pid_t pid = fork();

    if (pid < 0) {
        perror("fork error");
        return 1;
    } else if (pid == 0) {
        /* client */
        fd = open(FIFO_PATH, O_NONBLOCK | O_RDONLY);
        if (fd < 0) {
            perror("[child] open error");
            return 1;
        }

        /* note that Linux guarantees either no read message or the complete message on FIFO since
         * message size is less than PIPE_BUF; see man pipe(7) */
        ssize_t bytes = 0;
        while (bytes <= 0) {
            errno = 0;
            bytes = read(fd, &buffer, sizeof(buffer));
            if (bytes < 0 && errno != EAGAIN && errno != EWOULDBLOCK) {
                perror("[child] read error");
                return 1;
            }
            sched_yield();
        }

        buffer[sizeof(buffer) - 1] = '\0';
        if (bytes < sizeof(buffer))
            buffer[bytes] = '\0';

        if (close(fd) < 0) {
            perror("[child] close error");
            return 1;
        }

        printf("read on FIFO: %s\n", buffer);
    } else {
        /* server */
        fd = -1;
        while (fd < 0) {
            /* wait until client is ready for read */
            errno = 0;
            fd = open(FIFO_PATH, O_NONBLOCK | O_WRONLY);
            if (fd < 0 && errno != ENXIO) {
                perror("[parent] open error");
                return 1;
            }
            sched_yield();
        }

        /* note that Linux guarantees sending the complete message on FIFO since message size is
         * less than PIPE_BUF and there are no signals possible in this test; see man pipe(7) */
        snprintf(buffer, sizeof(buffer), "Hello from write end of FIFO!");
        if (write(fd, &buffer, strlen(buffer) + 1) < 0) {
            perror("[parent] write error");
            return 1;
        }

        if (close(fd) < 0) {
            perror("[parent] close error");
            return 1;
        }

        pid = wait(NULL); /* wait for child termination, just for sanity */
        if (pid < 0) {
            perror("[parent] wait error");
            return 1;
        }

        if (unlink(FIFO_PATH) < 0) {
            perror("[parent] unlink error");
            return 1;
        }

        /* Check if we can create a normal file with the same name. */
        fd = open(FIFO_PATH, O_CREAT | O_TRUNC, 0600);
        if (fd < 0) {
            perror("[parent] open error");
            return 1;
        }
        if (close(fd) < 0) {
            perror("[parent] close error");
            return 1;
        }
        if (unlink(FIFO_PATH) < 0) {
            perror("[parent] unlink error");
            return 1;
        }
    }

    return 0;
}