malloc behaviour on an embedded system

Question

I\'m currently working on an embedded project (STM32F103RB, CooCox CoIDE v.1.7.6 with arm-none-eabi-gcc 4.8 2013q4) and I\'m trying to understand how malloc() b

Answer 1

Here you can find how I could "force" malloc() to return NULL, if the heap is too small for allocating based on berendi's previous answer. I estimated the maximum amount of STACK and based on this I could calculate the address where the stack can start in worst case.

#define STACK_END_ADDRESS       0x20020000
#define STACK_MAX_SIZE              0x0400
#define STACK_START_ADDRESS   (STACK_END_ADDRESS - STACK_MAX_SIZE)

void * _sbrk_r(
   struct _reent *_s_r,
   ptrdiff_t nbytes)
{
   char  *base;     /*  errno should be set to  ENOMEM on error */

   if (!heap_ptr) { /*  Initialize if first time through.       */
      heap_ptr = end;
   }
   base = heap_ptr; /*  Point to end of heap.           */
   #ifndef STACK_START_ADDRESS
      heap_ptr += nbytes;   /*  Increase heap.              */
      return base;      /*  Return pointer to start of new heap area.   */
   #else
      /* End of heap mustn't exceed beginning of stack! */        
      if (heap_ptr <= (char *) (STACK_START_ADDRESS - nbytes) ) {  
         heap_ptr += nbytes;    /*  Increase heap.              */
         return base;       /*  Return pointer to start of new heap area.   */
      } else {
         return (void *) -1;         /*   Return -1 means that memory run out  */
      }
   #endif // STACK_START_ADDRESS
}