进程间通信：使用信号量实现父子进程交互

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

static int set_semvalue(void); static int semaphore_p(void); static int semaphore_v(void); static void del_sem_set(void);

/* 定义自己的semun联合体*/ union semun { int val; struct semid_ds *buf; unsigned short *array; struct seminfo *__buf; };

/* 定义全局变量sem_id保存信号量集的标识符*/ static int sem_id;

int main() { pid_t pid;

/* 创建信号量集*/
sem_id = semget(IPC_PRIVATE, 1, 0666 | IPC_CREAT);
if(sem_id == -1) {
    fprintf(stderr, "Failed to create semaphore set. \n");
    exit(EXIT_FAILURE);
}
if(!set_semvalue()) {	        /* 设置信号量的值*/
    fprintf(stderr, "Failed to initialize semaphore\n");
    exit(EXIT_FAILURE);
}

pid = fork();				    /* 创建子进程*/
switch(pid)
{
case -1:
    perror("fork failed");
    exit(EXIT_FAILURE);
case 0:						    /* 子进程*/
    execl("./program2", "program2", NULL);  /* 执行程序2*/
    break;
default:						/* 父进程*/
    srand((unsigned int)getpid());  /* 为随机数播种*/

    for(int i=0; i < 10; i++){		    /* 使用信号量控制*/
        semaphore_p();			    /* P操作，获取信号量*/
        printf("X");		    /* 在父进程中输出X */
        fflush(stdout);
        sleep(rand() % 4);

        printf("X");
        fflush(stdout);
        sleep(1);
        semaphore_v();			    /* V操作，释放信号量*/
    }

    wait(NULL);				    /* 等待子进程退出*/
    del_sem_set();			    /* 删除信号量集*/

    printf("\n%d - finished\n", getpid());
    exit(EXIT_SUCCESS);
}

}

/* 设置信号量的值*/ static int set_semvalue(void) { union semun sem_union;

sem_union.val = 1;
if(semctl(sem_id, 0, SETVAL, sem_union) == -1)
    return 0;

return 1;

}

/* P操作，获取信号量*/ static int semaphore_p(void) { struct sembuf sem_b;

sem_b.sem_num  = 0;
sem_b.sem_op   = -1;
sem_b.sem_flg  = SEM_UNDO;
if(semop(sem_id, &sem_b, 1) == -1) {
    fprintf(stderr, "semaphore_p failed/n");
    return 0;
}

return 1;

}

/* V操作，释放信号量*/ static int semaphore_v(void) { struct sembuf sem_b;

sem_b.sem_num  = 0;
sem_b.sem_op   = 1;
sem_b.sem_flg  = SEM_UNDO;
if(semop(sem_id, &sem_b, 1) == -1) {
    fprintf(stderr, "semaphore_v failed/n");
    return 0;
}

return 1;

}

/* 删除信号量集*/ static void del_sem_set(void) { union semun sem_union;

if(semctl(sem_id, 0, IPC_RMID, sem_union) == -1)
    fprintf(stderr, "Failed to delete semaphore/n");

}