Simple example of shared memory between parent and child using mmap() and fork().

shared-memory.c

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

#include <sys/mman.h>


int main(int argc, char *argv[]) {
    size_t size = 4096;
    char *smem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (!smem) {
        perror("mmap() failed");
        exit(1);
    }

    pid_t child = fork();
    if (child == 0) {
        /* child */

        sprintf(smem, "Hello!");
    } else {
        /* parent */

        sleep(1);

        printf("got: '%s'\n", smem);

        if (waitpid(child, NULL, 0) < 0) {
            perror("waitpid() failed");
        }

        munmap(smem, size);
    }
}