System V信号量（2）

用信号量实现进程相互排斥演示样例：

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <sys/wait.h>


#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>




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) */
};


#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)


int sem_create(key_t key)
{
int semid;
semid = semget(key, 1, IPC_CREAT | IPC_EXCL | 0666);
if (semid == -1)
ERR_EXIT("semget");


return semid;
}


int sem_open(key_t key)
{
int semid;
semid = semget(key, 0, 0);
if (semid == -1)
ERR_EXIT("semget");


return semid;
}


int sem_setval(int semid, int val)
{
union semun su;
su.val = val;
int ret;
ret = semctl(semid, 0, SETVAL, su);
if (ret == -1)
ERR_EXIT("sem_setval");


return 0;
}


int sem_getval(int semid)
{
        int ret;
        ret = semctl(semid, 0, GETVAL, 0);
        if (ret == -1)
                ERR_EXIT("sem_getval");


        return ret;
}


int sem_d(int semid)
{
int ret;
ret = semctl(semid, 0, IPC_RMID, 0);
if (ret == -1)
ERR_EXIT("semctl");


return 0;
}


int sem_p(int semid)
{
struct sembuf sb = {0, -1, 0};
int ret;
ret = semop(semid, &sb, 1);
if (ret == -1)
ERR_EXIT("semop");


return ret;
}


int sem_v(int semid)
{
        struct sembuf sb = {0, 1, 0};
        int ret;
        ret = semop(semid, &sb, 1);
        if (ret == -1)
                ERR_EXIT("semop");


        return ret;
}


int semid;


void print(char op_char)
{
int pause_time;
srand(getpid());
int i;
for (i=0; i<10; i++)
{
sem_p(semid);
printf("%c", op_char);
fflush(stdout);
pause_time = rand() % 3;
sleep(pause_time);
printf("%c", op_char);
fflush(stdout);
sem_v(semid);
pause_time = rand() % 2;
sleep(pause_time);
}
}


int main(int argc, char *argv[])
{
semid = sem_create(IPC_PRIVATE);
sem_setval(semid, 0);
pid_t pid;
pid = fork();
if (pid == -1)
ERR_EXIT("fork");


if (pid > 0)
{
sem_setval(semid, 1);
print('O');
wait(NULL);
sem_d(semid);
}
else
{
print('X');
}
return 0;
}