使用共享内存的IPC

Question

我正在编写一个简单的代码，其中父进程和子进程使用共享内存方法共享一个队列。但是当子进程修改共享数据时，它不会反映父进程和父进程仍在访问之前的值。 这里我附上一个示例代码我试图做的事：提前

int main() 
{ 
    /* Declare fork variables */ 
    pid_t childpid; 

    /* Declare shared memory variables */ 
    key_t key; 
    int shmid; 
    int *front; 
    int *rear; 
    int i; 
    /* Declare semaphore variables */ 
    sem_t sem; 
    int pshared = 1; 
    unsigned int value = 1; 

    /* Initialize Shared Memory */ 
    key = ftok("thread1.c",'R'); 
    shmid = shmget(key, 1024, 0644 | IPC_CREAT); 

    /* Attach to Shared Memory */ 
    rear = shmat(shmid, (void *)0, 0); 
    if(rear == (int *)(-1)) 
     perror("shmat"); 

    front = shmat(shmid, (void *)0, 0); 
    if(front == (int *)(-1)) 
     perror("shmat"); 

    /* Write initial value to shared memory */ 
    rear= front; 
    /* Write to Shared Memory */ 
    for(i=0; i<5; i++) 
    { 
     rear[i] = 5000+(i+1); 
    } 
    printf("value of front is: %d", *front); 
    rear = &rear[i-1]; 
    /* Initialize Semaphore */ 
    if((sem_init(&sem, pshared, value)) == 1) 
    { 
     perror("Error initializing semaphore"); 
     exit(1); 
    } 

    if((childpid = fork()) < 0) // error occured 
    { 
     perror("Fork Failed"); 
     exit(1); 
    } 
    else if(childpid == 0) // child process 
    { 
     printf("came in child process\n"); 
     sem_wait(&sem); 
     for(i=0; i<3; i++){ 
      printf("rear is: %d", *rear); 
      rear--; 
     } 
     sem_post(&sem); 
     shmdt(rear); 

    } 
    else // parent process 
    { 
     /* Write to Shared Memory */ 
     wait(1); 
     sem_wait(&sem); 
     printf("came in parent process and rear is: %d\n", *rear); 
     sem_post(&sem); 
     shmdt(rear); 
    } 
    return 0; 
} 

谢谢！

Answer 1

rear = shmat(shmid, (void *)0, 0); 
front = shmat(shmid, (void *)0, 0); 

您试图创建2个具有相同共享内存ID的独立变量。

尝试使用2个键和2个共享记忆。

key1 = ftok("thread1.c",'A'); 
key2 = ftok("thread1.c",'B'); 
shmid1 = shmget(key, 1024, 0644 | IPC_CREAT); 
shmid2 = shmget(key, 1024, 0644 | IPC_CREAT); 
rear = shmat(shmid1, (void *)0, 0); 
front = shmat(shmid2, (void *)0, 0);