Named pipe, read fail with bad file descriptor

Question

I'm beginning with pipes under Linux and I have a problem with my code. I wanted to test sending an integer through the fifo so I code a small program to test it.

First I open the read only descriptor and then the write only one as precised in the documentation. I send the number to the pipe and close the write descriptor.

However, when I'm trying to read from the pipe, it says that I have a bad file descriptor, I don't understand why it's not working because as the descriptor is the good one and is oppened with good option (O_RDONLY | O_NONBLOCK).

Here is my code :

void main(void)
{
    int modePipeWrite, modePipeRead;

    if(mkfifo("test.fifo", 0777) == -1)
    {
        perror("mkfifo");
        exit(EXIT_FAILURE);
    }
    if(modePipeRead = open("test.fifo", O_RDONLY | O_NONBLOCK) == -1)
    {
        perror("openRead");
        exit(EXIT_FAILURE);
    }
    if(modePipeWrite = open("test.fifo", O_WRONLY | O_NONBLOCK) == -1)
    {
        perror("openWrite");
        exit(EXIT_FAILURE);
    }


    int n = 0;

    if(write(modePipeWrite, &n, sizeof(n)) == -1)
    {
        perror("write");
    }
    printf("Send value: %d\n", n);

    close(modePipeWrite);


    int mode;

    while(1)
    {
        if(read(modePipeRead, &mode, sizeof(mode)) == -1)
        {
            perror("read");
        }
        printf("Received value: %d\n", getpid(), mode);
        sleep(1);
    }
}

And the output :

./a.out
Send value: 0

read: Bad file descriptor
Received value: -1877110288

read: Bad file descriptor
Received value: -1877110288

read: Bad file descriptor
Received value: -1877110288

I don't understand what could be wrong here. If someone have some advices I would be glad to hear it.

Answer 1

Your code has a couple of issues, which you would've found if you compiled it with all warnings enabled. If you use gcc, the following options are nice/mandatory: -Wall -Wextra -Werror -pedantic

One issue is that you don't assign the return value of open() to the file descriptors. You need to add braces around the assignment, or move it out of the if-statements.

Here's a working example, using a thread to read:

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

#include <pthread.h>

static void *reader(void *arg)
{   
    int fd, val;

    if ((fd = open("test.fifo", O_RDONLY)) == -1) {
        perror("openRead");
        exit(EXIT_FAILURE);
    }

    if (read(fd, &val, sizeof val) == -1)
        perror("read");
    else
        printf("Received value: %d\n", val);

    close(fd);
    return NULL;
}

int main(void)
{   
    int modePipeWrite, modePipeRead;
    pthread_t readerid;


    if (mkfifo("test.fifo", 0777) == -1) {
        perror("mkfifo");
        exit(EXIT_FAILURE);
    }
    pthread_create(&readerid, NULL, reader, NULL);
    sleep(1);

    if ((modePipeWrite = open("test.fifo", O_WRONLY)) == -1) {
        perror("openWrite");
        exit(EXIT_FAILURE);
    }

    int n = 1234;

    if (write(modePipeWrite, &n, sizeof n) == -1)
        perror("write");
    else
        printf("Sent value: %d\n", n);

    sleep(1);
    close(modePipeWrite);

    return 0;
}