[riot-notifications] [RIOT-OS/RIOT] cpu/atmega_common: make remaining RAM available as heap (#10934)

Gunar Schorcht notifications at github.com
Sun Feb 3 18:27:58 CET 2019

### Contribution description

This PR make the remaining RAM of AVR MCUs available as heap for `malloc/free` functions of `avr-libc`.

`__malloc_heap_end` is set to 0 by default. Therefore, `malloc` checks `__brkval` against the SP instead of `__malloc_heap_end`.
	cp = __malloc_heap_end;
	if (cp == 0)
		cp = STACK_POINTER() - __malloc_margin;
	if (cp <= __brkval)
In thread mode, however, the SP is is always in `.data` section below `__bss_end` and thus below `__malloc_heap_start`. Since `__brkval` points at least to `__malloc_heap_start`, the check always fails.

This PR sets `__malloc_heap_end` to the current SP before switching to thread mode. Thus, `malloc` checks `__brkval` against `__malloc_heap_end`.

### Testing procedure

Use `tests/malloc` to check `malloc/free` functions of `avr-libc`, for example:
make -C tests/malloc BOARD=arduino-mega2560 PORT=/dev/ttyUSB0 flash

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-- Commit Summary --

  * cpu/atmega_common: make RAM available as heap

-- File Changes --

    M cpu/atmega_common/thread_arch.c (17)

-- Patch Links --


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