#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>int semget(key_t key, int nsems, int semflg);

int semop(int semid, struct sembuf *sops, size_t nsops);

/* Structure used for argument to `semop' to describe operations.  */
struct sembuf
{unsigned short int sem_num;   /* semaphore number */short int sem_op;     /* semaphore operation */short int sem_flg;        /* operation flag */
};

int semctl(int semid, int semnum, int cmd, ...);

           union semun {int              val;    /* Value for SETVAL */struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */unsigned short  *array;  /* Array for GETALL, SETALL */struct seminfo  *__buf;  /* Buffer for IPC_INFO(Linux-specific) */};

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/sem.h>//这个联合体需要手动定义
union semun
{int              val;    /* Value for SETVAL */struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */unsigned short  *array;  /* Array for GETALL, SETALL */struct seminfo  *__buf;  /* Buffer for IPC_INFO(Linux-specific) */
};void ErrExit(const char* reason)
{fprintf(stderr, "%s: %d, %s\n", reason, errno, strerror(errno));exit(1);
}int initsem(int key = 0)
{int semid = -1;if (-1 == (semid = semget(key, 1, 0666 | IPC_CREAT))){ErrExit("semget");}// 信号量初始值为1
    union semun sem_un;sem_un.val = 1;if (-1 == semctl(semid, 0, SETVAL, sem_un)){ErrExit("semctl");}return semid;
}void destroysem(int semid)
{if (-1 == semctl(semid, 0, IPC_RMID)){ErrExit("semctl del");}
}// -1 为 p 操作
void P(int semid)
{struct sembuf op;op.sem_num = 0;op.sem_op = -1;op.sem_flg = SEM_UNDO;if (-1 == semop(semid, &op, 1)){ErrExit("semop p");}
}// 1 为 v 操作
void V(int semid)
{struct sembuf op;op.sem_num = 0;op.sem_op = 1;op.sem_flg = SEM_UNDO;if (-1 == semop(semid, &op, 1)){ErrExit("semop v");}
}int main(int argc, char const *argv[])
{int semid = initsem();pid_t pid = fork();if (pid > 0){P(semid);printf("in parent process...\n");sleep(1);V(semid);waitpid(pid, NULL, 0);// 删除信号量集
        destroysem(semid);}else if (0 == pid){P(semid);printf("in child process...\n");sleep(1);V(semid);}else{ErrExit("fork");}return 0;
}