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Defined in rt_misc.h, the __user_heap_extend() function can be defined to return extra blocks of memory, separate from the initial one, to be used by the heap.

If defined, this function must return the size and base address of an eight-byte aligned heap extension block.


extern size_t __user_heap_extend(int var0, void **base, size_t requested_size);


There is no default implementation of this function. If you define this function, it must have the following characteristics:

  • The returned size is one of the following:
    • In AArch32 state, a multiple of 8 bytes of at least the requested size.
    • In AArch64 state, a multiple of 16 bytes of at least the requested size.
    • 0, denoting that the request cannot be honored.
  • The returned base address is aligned on:
    • In AArch32 state, an 8-byte boundary.
    • In AArch64 state, a 16-byte boundary.
  • Size is measured in bytes.
  • The function is subject only to Procedure Call Standard for the ARM® Architecture (AAPCS) constraints.
  • The first argument is always zero on entry and can be ignored. The base is returned in the register holding this argument.


This function places a pointer to a block of at least the requested size in *base and returns the size of the block. 0 is returned if no such block can be returned, in which case the value stored at *base is never used.