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

void up_and_wait(int mux)
{
        struct sembuf up = {0, 1, 0};
        struct sembuf wait = {0, 0, 0};

        if(semop(mux, &up, 1) < 0)
                perror("semop up");
        if(semop(mux, &wait, 1) < 0)
                perror("semop wait");
}

void down (int mux)
{
        struct sembuf down = {0, -1, 0};

        if(semop(mux, &down, 1) < 0)
                perror("semop down");
}

void writer(int mux, int shmid)
{
        char *str = shmat(shmid, NULL, 0);
        if(str == (void*)-1)
                perror("writer shmat");

        down(mux);
        strncpy(str, "Systemy Operacyjne 1", 50);

        if(shmdt(str) < 0)
                perror("writer strdt");
        exit(0);
}

void reader(int mux, int shmid)
{
        char *str = shmat(shmid, NULL, 0);
        if(str == (void*)-1)
                perror("reader shmat");

        up_and_wait(mux);
        printf("Odczytana wiadomosc: %s\n", str);

        if(shmdt(str) < 0)
                perror("reader strdt");
        if(wait(NULL) < 0)
                perror("wait");
}

int main(void)
{
        int mux = semget(IPC_PRIVATE, 1, 0600 | IPC_CREAT | IPC_EXCL);
        if(mux < 0)
                perror("semget");

        int shmid = shmget(IPC_PRIVATE, 4096, 0600 | IPC_CREAT | IPC_EXCL);
        if(shmid < 0)
                perror("shmget");

        int pid = fork();

        if(pid < 0)
                perror("fork");
        if(pid == 0)
                writer(mux, shmid);
        else
                reader(mux, shmid);

        if(shmctl(shmid, IPC_RMID, NULL) < 0)
                perror("shmctl");
        if(semctl(mux, 0, IPC_RMID, NULL) < 0)
                perror("semctl");

        return 0;
}