client.c

#include <string.h>
#include "headers.h"

int main(int argc, char const *argv[])
{
    int id_shared, id_server;
    char *psh_msg;

    sem_t *sem_server;

    const char *message = argv[1];

    key_t sh_key = SHKEY;
    key_t srv_key = SRVKEY;

    SHM_GET_MAT(sh_key, srv_key);

    // message será copiado para psh_msg, inicializado em SHM_GET_MAT
    strcpy(psh_msg, message);

    sem_post(&(*sem_server));

    return 0;
}

headers.h

#include <sys/shm.h>
#include <stdio.h>
#include <semaphore.h>

#define SHM_GET_MAT(sh_key, srv_key) \
    if ((id_shared = shmget(sh_key, SHMSZ, IPC_CREAT | 0666)) < 0) { \
        perror("shmget id_shared"); \
        return 1; \
    } \
 \
    if ((id_server = shmget(srv_key, sizeof(sem_t), IPC_CREAT | 0666)) < 0) { \
        perror("shmget id_server"); \
        return 1; \
    } \
 \
    if ((psh_msg = shmat(id_shared, NULL, 0)) == (char *) -1) { \
        perror("shmat psh_msg"); \
        return 1; \
    } \
 \
    if ((sem_server = (sem_t*) shmat(id_server, NULL, 0)) < 0) { \
        perror("shmat sem_server"); \
        return 1; \
    } \

#define SHKEY  2345
#define SRVKEY 3456
#define SHMSZ  27

server.c

#include "headers.h"

int main(int argc, char const *argv[])
{
    int id_shared, id_server;
    char *psh_msg, *message;

    sem_t *sem_server;

    key_t sh_key = SHKEY;
    key_t srv_key = SRVKEY;

    SHM_GET_MAT(sh_key, srv_key);

    message = psh_msg;
    *message = (long) NULL;

    sem_init(&(*sem_server), 1, 1);

    while (1) {
        sem_wait(&(*sem_server));

        for (message = psh_msg; *message != (long) NULL; message++) {
            putchar(*message);
        }

        printf("\n");
    }

    return 0;
}