Question

I have an issue attempting to access shared memory using mmap for complex types.

So I allocate my memory as so in my parent process:

/* Create mmap file */
fid = open(TMP_FILE_NAME, O_RDWR | O_CREAT | O_EXCL, (mode_t) 0755);
if (fid < 0)
{
  printf("Bad Open of mmap file <%s>\n", TMP_FILE_NAME);
  die(-1);
}

/* Make mmap file Buffer Size */
status = ftruncate(fid, INPUT_BUFFER_SIZE); 
if (status)
{
    printf("Failed to ftruncate the file <%s>, status = %d\n", TMP_FILE_NAME, status);
    die(-1);
}

/* Initialize Shared Memory */
mmap_ptr = mmap((caddr_t) 0,
      INPUT_BUFFER_SIZE,        // Default Buffer Size
      PROT_WRITE | PROT_READ,   // R/W Permissions
      MAP_SHARED, // No file backing
      fid,  
      (off_t) 0);
if (mmap_ptr == MAP_FAILED)
{
    printf("Failed to perform mmap, Exiting\n");
    die(-1);
}

Now the Struct that I’m passing in memory to my child process is as follows:

/* Data structue for IPC */
typedef struct {
    int current_active_id;
    int consume_remaining;
    Queue buffer;
} input_buffer;

where Queue is a data structure class from the following:
http://www.idevelopment.info/data/Programming/data_structures/c/Queue/Queue.shtml

In my child process it’s okay when I do this, it returns the correct value:

printf("Got here... Shared Mem: %d\n", input_queue->consume_remaining);

but when I do something like:

IsEmpty(input_queue->buffer)

it crashes and in the code of the Queue it’s only doing this:

return Q->Size == 0;

Any help would be appreciated, thanks!!

Answer 1

Queue is a pointer to struct QueueRecord, and should be allocated as such, presumably using the same shared memory segment. note that this should also be mapped at the same address in both parent and child, or you will not be able to dereference it.