Hello. I need a bit of help with my case. I have nRF52832 dev kit. I have set heap size to zero in Makefile:
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nrf52832_xxaa: CFLAGS += -D__HEAP_SIZE=0
nrf52832_xxaa: CFLAGS += -D__STACK_SIZE=8192
nrf52832_xxaa: ASMFLAGS += -D__HEAP_SIZE=0
nrf52832_xxaa: ASMFLAGS += -D__STACK_SIZE=8192
And even with that change I am still able to allocate memory using malloc(). Here is the code:
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#include "SEGGER_RTT.h"
#include <stdlib.h>
int main(void) {
int * p = malloc(16);
char buf[40] = {0};
sprintf(buf, "p: %p\r\n", p);
SEGGER_RTT_WriteString(0, buf);
while (1);
return 0;
}
And this is the result:
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p: 0x20000560
So malloc() returns valid pointer even with no heap. I have checked .map file and I can see that malloc allocates memory outside of heap.
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*(COMMON)
COMMON 0x00000000200004ac 0xa8 _build/nrf52832_xxaa/SEGGER_RTT.c.o
0x00000000200004ac _SEGGER_RTT
COMMON 0x0000000020000554 0x4 /home/era/Downloads/nordic/gcc-arm-none-eabi-7-2018-q2-update/bin/../lib/gcc/arm-none-eabi/7.3.1/../../../../arm-none-eabi/lib/thumb/v7e-m/fpv4-sp/hard/libc_nano.a(lib_a-reent.o)
0x0000000020000554 errno
0x0000000020000558 . = ALIGN (0x4)
0x0000000020000558 __bss_end__ = .
.heap 0x0000000020000558 0x0
0x0000000020000558 __HeapBase = .
0x0000000020000558 __end__ = .
0x0000000020000558 PROVIDE (end = .)
*(.heap*)
.heap 0x0000000020000558 0x0 _build/nrf52832_xxaa/gcc_startup_nrf52.S.o
0x0000000020000558 __HeapLimit = .
.stack_dummy 0x0000000020000558 0x2000
*(.stack*)
.stack 0x0000000020000558 0x2000 _build/nrf52832_xxaa/gcc_startup_nrf52.S.o
0x0000000020010000 __StackTop = (ORIGIN (RAM) + LENGTH (RAM))
0x000000002000e000 __StackLimit = (__StackTop - SIZEOF (.stack_dummy))
So my first question is why malloc returns address to region that is outside of heap?
I checked how big chunk I can allocate and it seems that even if I change the malloc to allocate 1000000 bytes
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int * p = malloc(1000000);
I still get the same 0x20000560 address.
The second question is why malloc() allows to allocate such a big chunk of memory? Size of RAM in nRF52832 is 64kB.