Cannot use malloc in Contiki - c

Question

I am trying to write a simple program using the Contiki-ng operating system for IoT platforms. First tests were ok, but something weird happens when trying to allocate dynamic memory. The test code I'm using is the following:
#include "contiki.h"
#include <stdio.h>
#include <stdlib.h>
PROCESS(main_process, "main_process");
AUTOSTART_PROCESSES(&main_process);
PROCESS_THREAD(main_process, ev, data) {
uint8_t *nonce;
int i;
PROCESS_BEGIN();
nonce = (uint8_t *) malloc(32 * sizeof(uint8_t));
if (nonce != NULL) {
for (i = 0; i < 32; i++)
nonce[i] = 0;
printf("Todo OK\n");
} else {
printf("Todo mal\n");
}
PROCESS_END();
}
When building the project, an error rises stating that "In funtion _sbrk_r: undefined reference to _sbrk". After looking some forums on internet, I managed to solve this problem by including the following line into the Makefile of the project:
MODULES += os/lib/newlib
This solves the problem because inside the path os/lib/newlib there is a file, syscalls.c, which implements the mentioned _sbrk function. However, the following error rises now:
I have not found anyone over the internet with the same problem, so I got stuck here. Additionally, I don't know why but since then, removing the line "MODULES += os/lib/newlib" from the Makefile does nothing, and the problem with _eheap and _heap persists without returning to the undefined _sbrk error. However, I believe that this weird error is due to something wrong with Eclipse IDE cleaning the project. The code from syscalls.c, if needed, is the following:
#include <sys/types.h>
#include <errno.h>
#include <stdint.h>
#include <stdio.h>
/*---------------------------------------------------------------------------*/
#define DEBUG 0
#if DEBUG
#define PRINTF(...) printf(__VA_ARGS__)
#else
#define PRINTF(...)
#endif
/*---------------------------------------------------------------------------*/
/**
* \brief Enlarges the allocated heap space
* \param incr Number of bytes by which to increase the heap space
* \return The previous end of heap on success (which is also a pointer to the
* start of the newly allocated memory if \p incr is positive), or
* <tt>(caddr_t)-1</tt> with \c errno set to \c ENOMEM on error
*/
caddr_t
_sbrk(int incr)
{
/*
* Newlib's _sbrk_r() assumes that this global errno variable is used here,
* which is different from the errno definition provided by <errno.h>.
*/
#undef errno
extern int errno;
/* Heap boundaries from linker script. */
extern uint8_t _heap;
extern uint8_t _eheap;
static uint8_t *heap_end = &_heap;
uint8_t *prev_heap_end = heap_end;
if(heap_end + incr > &_eheap) {
PRINTF("Out of heap space!\n");
errno = ENOMEM;
return (caddr_t)-1;
}
heap_end += incr;
return (caddr_t)prev_heap_end;
}
I hope someone could help me. I have read that Contiki gives more ways of using dynamic memory, but I need to do it this way because other parts of the code rely on the use of malloc and cannot be changed.
Thanks.

Answer 1

The short is that you almost certainly don't need and don't want to use malloc() on a Contiki-NG microcontroller. Use Contiki's memb module instead.
The long answer is that you can add the _heap symbol by using a linker script. Check example linker scripts provided by Contiki-NG on how to do that. For example, the script arch/cpu/cc26x0-cc13x0/cc26xx.ld (for the cc26x0-cc13x0 platform) places _heap immediately after the BSS segment:
.bss :
{
/* ... */
} > SRAM
_end = .; /* End of the .bss segment. */
/* ... */
_stack = .;
_heap = _stack;