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;

}