sample to use posix shared memory

mmap.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/mman.h>
// http://man7.org/tlpi/code/online/dist/mmap/anon_mmap.c

//#define USE_MAP_ANON
//#define USE_MAP_SHARED_WITH_OPEN
#define USE_MAP_SHARED_WITH_SHM_OPEN

int main(int argc, char *argv[])
{
    void *addr;                 /* Pointer to shared memory region */
    pid_t cpid        = 0;
    int   buffer_size = 4 * 1024;

#ifdef USE_MAP_ANON             /* Use MAP_ANONYMOUS */
    addr = mmap(NULL, buffer_size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (addr == MAP_FAILED)
    {
        perror("mmap");
        exit(-1);
    }
#endif

#ifdef USE_MAP_SHARED_WITH_OPEN
    int fd = open("/dev/zero", O_RDWR);  /* map to "/dev/zero"*/
    if (fd == -1)
    {
        perror("open");
    }

    addr = mmap(NULL, buffer_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
    {
        perror("mmap");
        close(fd);
        exit(-1);
    }

    if (close(fd) == -1)        /* No longer needed */
    {
        munmap(addr, buffer_size);
        shm_unlink("/dev/zero");
        perror("close");
        exit(-1);
    }
#endif

#ifdef USE_MAP_SHARED_WITH_SHM_OPEN
    int fd = shm_open("/shmname", O_CREAT | O_RDWR, 0666); /* map to "/dev/zero"*/
    if (fd == -1)
    {
        perror("open");
        exit(-1);
    }

    if (ftruncate(fd, buffer_size) < 0)
    {
        perror("ftruncate");
        close(fd);
        exit(-1);
    }
    addr = mmap(NULL, buffer_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
    {
        perror("mmap");
        close(fd);
        exit(-1);
    }

    if (close(fd) == -1)        /* No longer needed */
    {
        munmap(addr, buffer_size);
        shm_unlink("/dev/zero");
        perror("close");
        exit(-1);
    }

#endif
    cpid = fork();
    if (cpid < 0)
    {
        perror("fork");
    }
    else if (cpid == 0)
    {
        printf("child pid is %d\n", getpid());
        sprintf((char *)addr, "Hello world !!");
    }
    else
    {
        printf("parent pid is %d and child pid is %d\n", getpid(), cpid);
        wait(0);
        printf("Get : %s\r\n", (char *)addr);
    }

    if (addr > 0)
    {
        munmap(addr, buffer_size);
    }

#ifdef USE_MAP_SHARED_WITH_OPEN
    shm_unlink("/dev/zero");
#endif

#ifdef USE_MAP_SHARED_WITH_SHM_OPEN
    shm_unlink("/shmname");
#endif
    return 0;
}