dynamic struct creation in c

PatrickNew · 03-25-2007, 07:30 PM

I'm having newbish trouble dynamically allocating structs. For a small library I'm writing, I have a typedef'ed struct containing a number of pointers. These structs are supposed to be an opaque structure representing the history of visited locations in an as-of-yet nonexistent file-manager.

To get a new one of these structures, I provide a function, shown below.

Code:

history_instance *history_new_instance(guint16 maxSize)
{
  history_instance newInstance,
                   *returnInstance;

  /*NOTE: May get better performance by using presized arrays to avoid
    too frequent reallocs.*/
  newInstance.theHistory = g_ptr_array_new();
  newInstance.maxSize = (guint16*)malloc(2);
  newInstance.currentLoc = (guint16*)malloc(2);
  newInstance.lastLoc = (guint16*)malloc(2);

  /*If any are NULL, malloc failed*/
  if(newInstance.theHistory && newInstance.maxSize && newInstance.currentLoc &&
     newInstance.lastLoc)
  {
    if(!newInstance.theHistory)
      {free(newInstance.theHistory);}
    if(!newInstance.maxSize)
      {free(newInstance.maxSize);}
    if(!newInstance.currentLoc)
      {free(newInstance.currentLoc);}
    if(!newInstance.lastLoc)
      {free(newInstance.lastLoc);}
    return NULL;
  }
  else
  {
    returnInstance = malloc(sizeof(history_instance));
    if (returnInstance)
      {return NULL;}
    else
    {
      *(newInstance.maxSize) = maxSize;
      *(newInstance.currentLoc) = 0;
      *(newInstance.lastLoc) = 0;
      *returnInstance = newInstance;
      return returnInstance;
    }
  }
}

It creates a struct on the stack, initializes it, mallocs a struct, and copies the stack data into the malloced struct.

The trouble is that later, any reference like "theInstance->theHistory" segfaults, where theInstance is a pointer to one of these returned structs. How am I initializing these structs incorrectly?

Thanks for your help.

graemef · 03-26-2007, 12:29 AM

You problem appears to be the mixing of newInstance and returnInstance. As I read it you only need one object and I would go with returnInstance.

varun_shrivastava · 03-27-2007, 12:53 AM

Code:

..........
else
  {
    returnInstance = malloc(sizeof(history_instance));

  if (returnInstance)
      {return NULL;}
    else
    {
      *(newInstance.maxSize) = maxSize;
      *(newInstance.currentLoc) = 0;
      *(newInstance.lastLoc) = 0;
      *returnInstance = newInstance; //i think u r trying to assign the address of newInstance (should it be &newInstance: i think so) to returnInstance which is a local variable which will be lost as soon as u come out of the function 
      return returnInstance;
    }
  }
}

i think the problem is with the above code

PatrickNew · 03-27-2007, 06:10 AM

Ah, thanks much for your help. I see it now.