15

I am trying to return an array using malloc in a function:

char* queueBulkDequeue(queueADT queue, unsigned int size)
{
    unsigned int i;
    char* pElements=(char*)malloc(size * sizeof(char));
    for (i=0; i<size; i++)
    {
        *(pElements+i) = queueDequeue(queue);
    }
    return pElements;
}

The problem is that I need to free it because my MCU's heap size is limited. But I want to return it so I cannot free it in the function, right?. Can I free the allocated memory outside the function (where I call the function). Is there any best practices for this? Thank you in advance!

KiL
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4 Answers4

11

As the memory allocated by malloc() is on the heap and not on the stack, you can access it regardless of which function you are in. If you want to pass around malloc()'ed memory, you have no other option than freeing it from the caller. (in reference counting terms, that's what is called an ownership transfer.)

10

1) Yes, you can free() the malloc'ed memory outside the function

2) No, you cannot free it inside the function and have the data passed outside the function, so you must do 1) here

3) If you're concerned about scarce memory, you need to check for failure from memory allocations always, which you fail to do here, which is then likely to lead to a segfault

tbert
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    "...a segfault" Or worse, in an embedded system. – Greg Hewgill Aug 01 '12 at 04:06
  • yes, I do concern about this issue when you free a malloc'd memory outside the function. How can you fix it? – KiL Aug 01 '12 at 04:19
  • @KiL the "fix" is to handle the case where your memory allocation routines are unable to allocate the memory you want, and fail gracefully, instead of whatever thread of control is executing getting b0rkened by a NULL pointer. – tbert Aug 01 '12 at 07:19
9

Ofcourse you can free the memory allocated in a function outside of that function provided you return it.

But, an alternative would be to modify your function like below, where the caller only allocates & frees the memory. This will be inline with concept of the function which allocates the memory takes responsibility for freeing the memory.

void queueBulkDequeue(queueADT queue, char *pElements, unsigned int size) 
{     
   unsigned int i;     
   for (i=0; i<size; i++)     
   {         
      *(pElements+i) = queueDequeue(queue);     
   }     
   return; 
}

//In the caller

char *pElements = malloc(size * sizeof(char));
queueBulkDequeue(queue, pElements, size);
//Use pElements
free(pElements);
Jay
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5

Yes, you can free memory allocated in a function that you call outside the function; this is precisely what you need to do in this case.

Alternatives include passing a buffer and its length into the function, and returning the actual length to the caller, the way fgets does. This may not be the best alternative, because the callers would need to call your function in a loop.

Sergey Kalinichenko
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