Semaphore/Example03-sem.c at master · ariakh55/Semaphore · GitHub

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

#define KEY 0x1111 //to refer to same IPC structure

union semun
{
    int val;
    struct semid_ds* buf;
    unsigned short* array;
};

struct sembuf p = {0, -1, SEM_UNDO}; //semWait
struct sembuf v = {0, +1, SEM_UNDO}; //semSignal

int main(){
    int id = semget(KEY,1,0666 | IPC_CREAT);
    if(id < 0){
        perror("semget");
        exit(11);
    }
    union semun u;
    u.val = 1;
    if(semctl(id,0,SETVAL,u) < 0){
        perror("semctl");
        exit(12);
    }
    int pid;
    pid = fork();
    srand(pid);
    if(pid < 0){
        perror("pid");
        exit(1);
    }else if(pid){
        char *s = "abcdefgh";
        int l = strlen(s);
        for (int i = 0; i < l; ++i){
            if(semop(id,&p,1) < 0){
                perror("semop p");
                exit(13);
            }
            putchar(s[i]);
            fflush(stdout);
            sleep(rand() % 2);
            putchar(s[i]);
            fflush(stdout);
            if (semop(id, &v, 1) < 0)
            {
                perror("semop v");
                exit(14);
            }
            sleep(rand() % 2);
        }
    }else{
        char *s = "ABCDEFGH";
        int l = strlen(s);
        for (int i = 0; i < l; ++i)
        {
            if (semop(id, &p, 1) < 0)
            {
                perror("semop p");
                exit(15);
            }
            putchar(s[i]);
            fflush(stdout);
            sleep(rand() % 2);
            putchar(s[i]);
            fflush(stdout);
            if (semop(id, &v, 1) < 0)
            {
                perror("semop v");
                exit(16);
            }
            sleep(rand() % 2);
        }
    }
    return 0;
}