When does malloc return NULL in a bare-metal environment?

Question

There is a c memory model as following :

      +--------+   Last Address of RAM
      | Stack  |
      |   |    |
      |   v    |
      +--------+
RAM   |        |
      |        |
      +--------+
      |   ^    |
      |   |    |
      | Heap   |
      +--------+
      |   ZI   |
      +--------+
      |   RW   |  
      +========+  First Address of RAM

The stack and heap space increase in opposite directions. They would be overlapped with each other in the middle. So my questions are:

• In a bare-metal environment, when does malloc return NULL?
• In a bare-metal environment, how to prevent stack from overlapping with heap?

Answer 1

@WikiWang is correct if you are using a static, compile-time memory layout (edit although you have to tell your malloc implementation somehow where the end of the heap is).

If not, and assuming you mean bare-metal, it depends on the C library implementation in your board-support package. The library must provide some implementation of brk(2) or a function having a similar effect. malloc works within the area of memory set by brk or sbrk. For example, see the malloc source calling macro MORECORE, which is by default sbrk. Your C library will have to use something other than a kernel call for that :) .

Answer 2

Size of stack is determinded at compile time.

      +--------+   Last Address of RAM
      | Stack  |
      |   |    |
      |   v    |
      +--------+
RAM   |        |
      +--------+   Stack Limit
      |        |
      +--------+
      |   ^    |
      |   |    |
      | Heap   |
      +--------+
      |   ZI   |
      +--------+
      |   RW   |  
      +========+  First Address of RAM

malloc return null if there is not enough space for the requested heap.

And its your duty to to prevent stack overflow!