Runtime memory tracing

Running out of memory is a common problem with resource constrained systems such as the MCUs on which mbed OS runs. When faced with an out of memory error, you often need to get a picture of how your software uses dynamic memory. The runtime memory tracer in mbed OS 5 is the tool that shows the runtime memory allocation patterns of your software: which parts of the code allocate and free memory, and how much memory they need.

Using the memory tracer

The memory tracer is not enabled by default. To enable it, you need to define the MBED_MEM_TRACING_ENABLED macro. The recommended way to define this macro is to add it to the list of macros defined in your mbed_app.json:

    "macros": ["MBED_MEM_TRACING_ENABLED"]

Tip: See the documentation of the mbed configuration system for more details about mbed_app.json.

After it is enabled, the memory tracer intercepts the calls to the standard allocation functions (malloc, realloc, calloc and free). It invokes a user supplied callback each time one of these functions is called. To let the tracer know which callback it needs to invoke, call mbed_mem_trace_set_callback(callback_function_name) as early as possible (preferably at the beginning of your main function). The full documentation of the callback function can be found in the memory tracer header file. The tracer supplies a default callback function (mbed_mem_trace_default_callback) that outputs trace data on the mbed console (using printf). For each memory operation, the callback outputs a line that begins with #<op>:<0xresult>;<0xcaller>-:

  • op identifies the memory operation (m for malloc, r for realloc, c for calloc and f for free).
  • result (base 16) is the result returned by the memory operation. This is always 0 for free, since free doesn’t return anything.
  • caller (base 16) is the address in the code where the memory operation was called.

The rest of the output depends on the operation being traced:

  • For malloc: size, where size is the original argument to malloc.
  • For realloc: 0xptr;size, where ptr (base 16) and size are the original arguments to realloc.
  • For calloc: nmemb;size, where nmemb and size are the original arguments to calloc.
  • For free: 0xptr, where ptr (base 16) is the original argument to free.


  • #m:0x20003240;0x600d-50 encodes a malloc that returned 0x20003240. It was called by the instruction at 0x600D with the size argument equal to 50.
  • #f:0x0;0x602f-0x20003240 encodes a free that was called by the instruction at 0x602f with the ptr argument equal to 0x20003240.

The source of the default callback can be found here. Besides being useful in itself, it can also serve as a template for user defined callback functions.

Tip: The full documentation of the callback function can be found in the memory tracer header file.


A simple code example that uses the memory tracer on a K64F board:

#include <stdlib.h>
#include "mbed.h"
#include "mbed_mem_trace.h"

int main() {
    while (true) {
        void *p = malloc(50);

It outputs the following trace:



  • The tracer doesn’t handle nested calls of the memory functions. For example, if you call realloc and the implementation of realloc calls malloc internally, the call to malloc is not traced.
  • The caller argument of the callback function isn’t always reliable. It doesn’t work at all on some toolchains, and it might output erroneous data on others.