Using pipe to pass integer values between parent and child

This is about as lame (and no error checking, btw) a sample as I can muster for using a pipe to send int from a parent to a child process, where the child was launched from fork(). It gets more complicated (obviously) for sending and receiving data, but i can’t do everything for you. This just forks and waits for an int (actually, the number of bytes that are used by an int) from the child.

Update: Added send+response two-way communication example after this one. See the second code listing for more information.

Hope it helps.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    int fd[2];
    int val = 0;

    // create pipe descriptors
    pipe(fd);

    // fork() returns 0 for child process, child-pid for parent process.
    if (fork() != 0)
    {
        // parent: writing only, so close read-descriptor.
        close(fd[0]);

        // send the value on the write-descriptor.
        val = 100;
        write(fd[1], &val, sizeof(val));
        printf("Parent(%d) send value: %d\n", getpid(), val);

        // close the write descriptor
        close(fd[1]);
    }
    else
    {   // child: reading only, so close the write-descriptor
        close(fd[1]);

        // now read the data (will block)
        read(fd[0], &val, sizeof(val));
        printf("Child(%d) received value: %d\n", getpid(), val);

        // close the read-descriptor
        close(fd[0]);
    }
    return 0;
}

Output:

Parent(5943) send value: 100
Child(5945) received value: 100

---

Update: Expanded to include send+response using two pipe sets

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

// some macros to make the code more understandable
//  regarding which pipe to use to a read/write operation
//
//  Parent: reads from P1_READ, writes on P1_WRITE
//  Child:  reads from P2_READ, writes on P2_WRITE
#define P1_READ     0
#define P2_WRITE    1
#define P2_READ     2
#define P1_WRITE    3

// the total number of pipe *pairs* we need
#define NUM_PIPES   2

int main(int argc, char *argv[])
{
    int fd[2*NUM_PIPES];
    int val = 0, len, i;
    pid_t pid;

    // create all the descriptor pairs we need
    for (i=0; i<NUM_PIPES; ++i)
    {
        if (pipe(fd+(i*2)) < 0)
        {
            perror("Failed to allocate pipes");
            exit(EXIT_FAILURE);
        }
    }

    // fork() returns 0 for child process, child-pid for parent process.
    if ((pid = fork()) < 0)
    {
        perror("Failed to fork process");
        return EXIT_FAILURE;
    }

    // if the pid is zero, this is the child process
    if (pid == 0)
    {
        // Child. Start by closing descriptors we
        //  don't need in this process
        close(fd[P1_READ]);
        close(fd[P1_WRITE]);

        // used for output
        pid = getpid();

        // wait for parent to send us a value
        len = read(fd[P2_READ], &val, sizeof(val));
        if (len < 0)
        {
            perror("Child: Failed to read data from pipe");
            exit(EXIT_FAILURE);
        }
        else if (len == 0)
        {
            // not an error, but certainly unexpected
            fprintf(stderr, "Child: Read EOF from pipe");
        }
        else
        {
            // report what we received
            printf("Child(%d): Received %d\n", pid, val);

            // now double it and send it back
            val *= 2;

            printf("Child(%d): Sending %d back\n", pid, val);
            if (write(fd[P2_WRITE], &val, sizeof(val)) < 0)
            {
                perror("Child: Failed to write response value");
                exit(EXIT_FAILURE);
            }
        }

        // finished. close remaining descriptors.
        close(fd[P2_READ]);
        close(fd[P2_WRITE]);

        return EXIT_SUCCESS;
    }

    // Parent. close unneeded descriptors
    close(fd[P2_READ]);
    close(fd[P2_WRITE]);

    // used for output
    pid = getpid();

    // send a value to the child
    val = 42;
    printf("Parent(%d): Sending %d to child\n", pid, val);
    if (write(fd[P1_WRITE], &val, sizeof(val)) != sizeof(val))
    {
        perror("Parent: Failed to send value to child ");
        exit(EXIT_FAILURE);
    }

    // now wait for a response
    len = read(fd[P1_READ], &val, sizeof(val));
    if (len < 0)
    {
        perror("Parent: failed to read value from pipe");
        exit(EXIT_FAILURE);
    }
    else if (len == 0)
    {
        // not an error, but certainly unexpected
        fprintf(stderr, "Parent(%d): Read EOF from pipe", pid);
    }
    else
    {
        // report what we received
        printf("Parent(%d): Received %d\n", pid, val);
    }

    // close down remaining descriptors
    close(fd[P1_READ]);
    close(fd[P1_WRITE]);

    // wait for child termination
    wait(NULL);

    return EXIT_SUCCESS;
}

(compile with, e.g., gcc thisfile.c -o test)

Output

Parent(2794): Sending 42 to child
Child(2797): Received 42
Child(2797): Sending 84 back
Parent(2794): Received 84