Multiple 1-to-1 pipes in C

Question

Is it possible in C to have multiple concurrent 1-to-1 pipes? My example use case is the following:

1. The parent creates x children processes using fork()
2. The parent creates x pipes, one for each child
3. Each child write to its respective pipe, then closes it.
4. The parent reads from the pipes then closes them.

My current draft attempt is as follows however I have doubts about its functionality:

// Create x pipes
int fd[2*x];
pipe(fd);

// In child i do (i from 0 to x-1 inclusive):
close(fd[2*i]);
write(fd[2*i +1], .....);
close(fd[2*i +1]);

// In parent do:
wait() // wait for children to finish
// while any pipe has content do:
// For each i from 0 to x-1 inclusive:
close(fd[2*i +1]);
read(fd[2*i], .....);
close(fd[2*i]);

I would really appreciate it if someone could show me a simple example of this concept at work. The end goal here is one way conversations between children to parent with multiple values that the parent will store into a single array.

Answer 1

This code more or less does what you want. It uses error reporting code that is available in my SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c and stderr.h in the src/libsoq sub-directory.

#include "stderr.h"
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

enum { NUM_CHILDREN = 5 };
enum { MSG_BUFFSIZE = 64 };
static void be_childish(int kid, int *fd);

int main(int argc, char **argv)
{
    int fd[2 * NUM_CHILDREN];
    int pid[NUM_CHILDREN];

    err_setarg0(argv[0]);
    err_setlogopts(ERR_PID);

    if (argc != 1)
        err_usage("");

    for (int i = 0; i < NUM_CHILDREN; i++)
    {
         if (pipe(&fd[2 * i]) != 0)
             err_syserr("failed to create pipe for child %d: ", i);
    }

    for (int i = 0; i < NUM_CHILDREN; i++)
    {
        if ((pid[i] = fork()) < 0)
            err_syserr("failed to fork child %d: ", i);
        else if (pid[i] == 0)
            be_childish(i, fd);
        else
        {
            printf("Child %d has PID %d\n", i, pid[i]);
            fflush(stdout);
        }
    }

    char buffer[MSG_BUFFSIZE];
    for (int i = 0; i < NUM_CHILDREN; i++)
    {
        close(fd[2 * i + 1]);
        int pipe_in = fd[2 * i + 0];
        int nbytes = read(pipe_in, buffer, sizeof(buffer));
        if (nbytes < 0)
            err_syserr("failed to read from FD %2d: ", pipe_in);
        printf("Got %2d bytes [%.*s] from FD %2d, PID %d\n",
               nbytes, nbytes, buffer, pipe_in, pid[i]);
        close(pipe_in);
    }

    for (int i = 0; i < NUM_CHILDREN; i++)
    {
        int status;
        int corpse = wait(&status);
        if (corpse > 0)
            printf("Child with PID %d exited with status 0x%.4X\n", corpse, status);
        else
            err_syserr("Failed to wait for dead children: ");
    }

    return 0;
}

static void be_childish(int kid, int *fd)
{
    for (int i = 0; i < kid; i++)
    {
        close(fd[2 * i + 0]);
        close(fd[2 * i + 1]);
    }

    close(fd[2 * kid + 0]);
    int estat = kid + 32;

    char buffer[MSG_BUFFSIZE];
    int nbytes = snprintf(buffer, sizeof(buffer),
                          "Child %d (PID %d) exiting with status %d",
                          kid, (int)getpid(), estat);
    int pipe_out = fd[2 * kid + 1];
    if (write(pipe_out, buffer, nbytes) != nbytes)
        err_syserr("failed to write to parent: ");
    close(pipe_out);
    exit(estat);
}

Sample run:

Child 0 has PID 36957
Child 1 has PID 36958
Child 2 has PID 36959
Child 3 has PID 36960
Child 4 has PID 36961
Got 42 bytes [Child 0 (PID 36957) exiting with status 32] from FD  3, PID 36957
Got 42 bytes [Child 1 (PID 36958) exiting with status 33] from FD  5, PID 36958
Got 42 bytes [Child 2 (PID 36959) exiting with status 34] from FD  7, PID 36959
Got 42 bytes [Child 3 (PID 36960) exiting with status 35] from FD  9, PID 36960
Got 42 bytes [Child 4 (PID 36961) exiting with status 36] from FD 11, PID 36961
Child with PID 36960 exited with status 0x2300
Child with PID 36959 exited with status 0x2200
Child with PID 36958 exited with status 0x2100
Child with PID 36957 exited with status 0x2000
Child with PID 36961 exited with status 0x2400