Pouziti semaforu a shm

makefile

CC=gcc
CFLAGS=-pthread -lrt -std=gnu99

sem: sem.c

clean:
	rm -f sem

sem.c

#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

const char* name = "/jmeno";

sem_t mutex;


void die(const char *msg) {

    perror(msg);
    exit(1);
}

void reader(int *s) {

    printf("data: %d\n", *s);
    sem_wait(&mutex);
    printf("reading old....\n");

    int x = 5;
    printf("writing... %d\n", x);
    *s = x;

    sleep(3);
    sem_post(&mutex);
    exit(10);
}



int main () {

    // inicializace semaforu
    // 1 - bude na zacatku odemceny
    // 0 - bude moci byt sdileny mezi procesy
    if (sem_init(&mutex, 1, 1))
        die("nepovedlo se inicializovat semafor");


    int fd = shm_open(name, O_RDWR|O_CREAT, 0600);
    if (fd < 0)
        die("nepovedlo se otevreni");

    if (ftruncate(fd, sizeof(int)))
        die("truncate...");

    int *shared;
    shared = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (shared == MAP_FAILED)
        die("mmap failed ");

    printf("hodnota pameti: %d\n", *shared);

    pid_t pid = fork();

    switch (pid) {

        case 0:

            reader(shared);
            break;

        case -1:
            die("nepovedlo se vytvorit potomka");
            break;

        default:

            printf("rodicak...\n");
            // rodic

    };

    
    int stat;
    waitpid(pid, &stat, 0);
    printf("potomek exiting.... <-> %d\n", stat);

    sem_wait(&mutex);
    printf("hodnota pameti po ukonceni potomka: %d\n", *shared);
    *shared = 5500;
    printf("hodnota pameti nova: %d\n", *shared);
    sem_post(&mutex);

    printf("rodic exiting\n");


    if (shm_unlink(name) < 0)
        die("nepovedlo se smazat sdilenou pamet");

    // musime zavrit semafor
    if (sem_destroy(&mutex))
        die("nepovedlo se smazat semafor");

    return 0;
}