如何同步两个应用程序对内存映射的访问

#include <pthread.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int fd = open("sharedfile", O_RDWR);
    void *addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

    pthread_mutex_t mutex;
    pthread_mutex_init(&mutex, NULL);

    // 加锁
    pthread_mutex_lock(&mutex);
    // 访问共享内存
    *(int*)addr = 42;
    // 解锁
    pthread_mutex_unlock(&mutex);

    munmap(addr, sizeof(int));
    close(fd);
    return 0;
}

#include <sys/sem.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

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

int main() {
    int fd = open("sharedfile", O_RDWR);
    void *addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

    int semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0666);
    union semun arg;
    arg.val = 1;
    semctl(semid, 0, SETVAL, arg);

    struct sembuf sb = {0, -1, SEM_UNDO}; // P操作
    semop(semid, &sb, 1);

    // 访问共享内存
    *(int*)addr = 42;

    sb.sem_op = 1; // V操作
    semop(semid, &sb, 1);

    munmap(addr, sizeof(int));
    close(fd);
    return 0;
}