Try To Share Memory With Struct

Question

This is the struct:

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

and this is the code:

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);

its return: shared memory attached at address (nil),
any other shared memory work well, any idea?

`if ((int) queue < 0)`, `%d` with `(int)queue`? isn't that smell foul? why not use simple `%p`? — Sourav Ghosh, Dec 31 '14 at 19:10
Don't cast the result of functions that return `void*`. http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Barmar, Dec 31 '14 at 19:19
Is `if(queue=NULL)` in the real code or a copying error? If it's the real code, this question should be closed as due to a typo. — Barmar, Dec 31 '14 at 19:20
if(queue=NULL) was mistake and i fixed it, however now i get:shmat() failed -1 — daniel, Dec 31 '14 at 20:02
@daniel so it fails on `if ((int) queue < 0)`. My PC has 4gb memory so a 32 bit pointer won't necessarily be positive when cast to `int`. Although you haven't posted what `shmat()` does. — Weather Vane, Dec 31 '14 at 20:20
shmat() attches to the proscess memory the shared memory created by OS,the action return pointer to the shared memory part,the address that always i get with the Queue is 0xFFFFFFFFFFFFFFFF — daniel, Dec 31 '14 at 20:38
regarding this line: 'if ((int) queue < 0) {' Since 'queue' is a pointer, it could be pointing to an area in the upper half of memory. In which case casting it to 'int' will result in a number that is < 0. — user3629249, Dec 31 '14 at 20:51
even when i ingore: if ((int) queue < 0) { printf("shmat() failed %d \n",(int)queue); exit(1); } and then do: queue->front=0;or any other operation with queue i get segmention fault — daniel, Dec 31 '14 at 21:00
@daniel said "even when i ingore *[queue == -1]* and then do: queue->front=0; or any other operation with queue i get segmention fault". Yes of course you do. What is a returned pointer of `-1` trying to tell you? — Weather Vane, Dec 31 '14 at 21:08

score 0 · Accepted Answer · answered Dec 31 '14 at 21:11

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);

problem solved,i use something similiar to the answer above,thanks! — daniel, Jan 01 '15 at 18:19

Try To Share Memory With Struct

1 Answers1