Support for malloc

Heap1, the default implementation, implements the smallest and simplest heap manager. The heap is managed as a singly‑linked list of free blocks held in increasing address order. The allocation policy is first‑fit by address.

This implementation has low overheads, but the cost of malloc() or free() grows linearly with the number of free blocks. The smallest block that can be allocated is four bytes and there is an additional overhead of four bytes. If you expect more than 100 unallocated blocks it is recommended that you use Heap2.

Heap2 provides a compact implementation with the cost of malloc() or free() growing logarithmically with the number of free blocks. The allocation policy is first‑fit by address. The smallest block that can be allocated is 12 bytes and there is an additional overhead of four bytes.

Heap2 is recommended when you require near constant‑time performance in the presence of hundreds of free blocks. To select the alternative standard implementation, use either:

The Heap2 real‑time heap implementation must know the maximum address space the heap spans. The smaller the address range, the more efficient the algorithm is.

By default, the heap extent is taken to be 16MB starting at the beginning of the heap (defined as the start of the first chunk of memory given to the heap manager by __rt_initial_stackheap() or __rt_heap_extend()).

The heap bounds are given by:

struct __heap_extent
{
    unsigned base, range;
};

__value_in_regs struct __heap_extent __user_heap_extent(
             unsigned defaultbase, unsigned defaultsize);

The function prototype for __user_heap_extent() is in rt_misc.h.

The Heap1 algorithm does not require the bounds on the heap extent, therefore it never calls this function.

You must redefine __user_heap_extent() if:

If you know in advance that the address space bounds of your heap are small, you do not have to redefine __user_heap_extent(), but it does speed up the heap algorithms if you do.

The input parameters are the default values that are used if this routine is not defined. You can, for example, leave the default base value unchanged and only adjust the size.

To use a heap implementation in an application that does not define main() and does not initialize the C library:

alloca() behaves identically to malloc() except that alloca() has automatic garbage collection (see alloca()).