Cannot use malloc in Contiki

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;