#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

union semun {                   /* Used in calls to semctl() */
    int                 val;
    struct semid_ds *   buf;
    unsigned short *    array;
#if defined(__linux__)
    struct seminfo *    __buf;
#endif
};

#define MEM_SIZE 4096

/* svshm_string_read.c

   Licensed under GNU General Public License v2 or later.
*/
#include "svshm_string.h"

int
main(int argc, char *argv[])
{
    int semid, shmid;
    union semun arg, dummy;
    struct sembuf sop;
    char *addr;

    /* Create shared memory and semaphore set containing one
       semaphore. */

    shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
    if (shmid == −1)
        errExit("shmget");

    semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
    if (shmid == −1)
        errExit("shmget");

    /* Attach shared memory into our address space. */

    addr = shmat(shmid, NULL, SHM_RDONLY);
    if (addr == (void *) −1)
        errExit("shmat");

    /* Initialize semaphore 0 in set with value 1. */

    arg.val = 1;
    if (semctl(semid, 0, SETVAL, arg) == −1)
        errExit("semctl");

    printf("shmid = %d; semid = %d\n", shmid, semid);

    /* Wait for semaphore value to become 0. */

    sop.sem_num = 0;
    sop.sem_op = 0;
    sop.sem_flg = 0;

    if (semop(semid, &sop, 1) == −1)
        errExit("semop");

    /* Print the string from shared memory. */

    printf("%s\n", addr);

    /* Remove shared memory and semaphore set. */

    if (shmctl(shmid, IPC_RMID, NULL) == −1)
        errExit("shmctl");
    if (semctl(semid, 0, IPC_RMID, dummy) == −1)
        errExit("semctl");

    exit(EXIT_SUCCESS);
}

/* svshm_string_write.c

   Licensed under GNU General Public License v2 or later.
*/
#include "svshm_string.h"

int
main(int argc, char *argv[])
{
    int semid, shmid;
    struct sembuf sop;
    char *addr;
    size_t len;

    if (argc != 4) {
        fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    len = strlen(argv[3]) + 1;  /* +1 to include trailing '\0' */
    if (len > MEM_SIZE) {
        fprintf(stderr, "String is too big!\n");
        exit(EXIT_FAILURE);
    }

    /* Get object IDs from command−line. */

    shmid = atoi(argv[1]);
    semid = atoi(argv[2]);

    /* Attach shared memory into our address space and copy string
       (including trailing null byte) into memory. */

    addr = shmat(shmid, NULL, 0);
    if (addr == (void *) −1)
        errExit("shmat");

    memcpy(addr, argv[3], len);

    /* Decrement semaphore to 0. */

    sop.sem_num = 0;
    sop.sem_op = −1;
    sop.sem_flg = 0;

    if (semop(semid, &sop, 1) == −1)
        errExit("semop");

    exit(EXIT_SUCCESS);
}