Question

这是结构：

typedef struct Queue {
  int data[MAX_SIZE];
  long front;
  long back;
  int size;
  int mysize;
} Queue;

这是代码：

if((queueShmid = shmget(IPC_PRIVATE, sizeof(Queue), IPC_CREAT | IPC_EXCL ))==-1) {
  printf("Shared memory segment exists - opening as client\n");
  /* Segment probably already exists - try as a client*/
  if((queueShmid = shmget(IPC_PRIVATE, 2*sizeof(Queue), 0)) == -1)
  { perror(" bad shmget"); exit(1);}
}
Queue *queue = (Queue*) shmat(queueShmid, NULL, 0);
if(queue=NULL) {
  perror("bad shmat"); exit(1);
}

if ((int) queue < 0) {
  printf("shmat() failed %d \n",(int)queue); exit(1);
}
printf ("shared memory attached at address %p\n", queue);

返回：共享内存附加地址（nil），
任何其他共享内存都运行良好，任何想法？

Answer 1

my comments are prefixed with '// <--'

suggest OP read the documentation on system calls, especially the returned values

 typedef struct Queue
 {
    int data[MAX_SIZE];
    long front;
    long back;
    int size;
    int mysize;
} Queue;



if((queueShmid = shmget(IPC_PRIVATE, sizeof(Queue), IPC_CREAT | IPC_EXCL ))==-1)
{
    printf("Shared memory segment exists - opening as client\n");

    // <-- should verify errno is EEXIST (Segment exists, cannot create)
    // <-- rather than making any assumptions

    /* Segment probably already exists - try as a client*/

    // <-- it was a single queue instance above so why is it 2 times that size below???
    if((queueShmid = shmget(IPC_PRIVATE, 2*sizeof(Queue), 0)) == -1)
    {
        //perror(" bad shmget");
        perror( "shmget failed for 2 times size of queue" );
        exit(1);
    } // end if
} // end if

Queue *queue=(Queue*)shmat(queueShmid, NULL, 0);

//if(queue=NULL) // <-- placing the literal on the left would have enabled the compiler to catch this error
if(queue==NULL)  // <-- on error, shmat returns (void*)-1 (not NULL) and sets errno
{
    perror( "shmat for ptr to instance of queue failed" );
    //perror("bad shmat");
    exit(1);
}

if ((int) queue < 0)
{
    printf("shmat() failed %d \n",(int)queue); exit(1);
}

printf ("shared memory attached at address %p\n", queue);

尝试与Struct共享内存

1 个答案: