Find dangling pointers

Memory on microcontrollers is scarce, so finding and closing memory leaks is crucial. This script helps you detect dangling pointers in Mbed OS 5 applications.

Enabling memory tracing

First, enable memory tracing via:

1. In mbed_app.json under macros, set:

``json
{
"macros": ["MBED_MEM_TRACING_ENABLED"]
}
`

1. In your application, call:

`
mbed_mem_trace_set_callback(mbed_mem_trace_default_callback);
`

1. Connect a serial monitor to your device - probably set a high baud rate to not slow down the application too much.

Using this script

1. Install the script through npm.

`
$ npm install mbed-find-dangling-pointers -g
`

1. Save the complete serial output of your application to a file, then run:

`
$ mbed-find-dangling-ptrs serial-log-file.txt path-to-your-elf-file.elf
`

Note: You can find the .elf file in BUILD/TARGET_NAME/GCC_ARM/.

1. This will give you the dangling pointers:

`
Extracting symbols from example/example.elf
Extracting symbols OK

Found 5 dangling pointers

--------------------------------------------------
5 dangling pointers (total: 3840 bytes): [ 0x200036c0 (768), 0x200039d0 (768), 0x20003ce0 (768), 0x20003ff0 (768), 0x20004300 (768) ]

int main() {
print_memory_info();
mbed_mem_trace_set_callback(mbed_mem_trace_default_callback);

while (true) {
wait(2.0);

void *ptr1 = malloc(512);
>>> void *ptr2 = calloc(768, 1);
void ptr3 = (void)new DigitalOut(LED1);

// Grab the heap statistics
mbed_stats_heap_t heap_stats;
mbed_stats_heap_get(&heap_stats);
printf("Heap size: %lu / %lu bytes\r\n", heap_stats.current_size, heap_stats.reserved_size);

// Forget to free a pointer
free(ptr1);
free(ptr3);
`

The line that is marked with >>> is the line where the allocation happened.

Example program

`cpp
#include "mbed.h"
#include "mbed_mem_trace.h"

DigitalOut led1(LED1);

void print_memory_info() {
// allocate enough room for every thread's stack statistics
int cnt = osThreadGetCount();
mbed_stats_stack_t stats = (mbed_stats_stack_t) malloc(cnt * sizeof(mbed_stats_stack_t));

cnt = mbed_stats_stack_get_each(stats, cnt);
for (int i = 0; i < cnt; i++) {
printf("Thread: 0x%lX, Stack size: %lu / %lu\r\n", stats[i].thread_id, stats[i].max_size, stats[i].reserved_size);
}
free(stats);

// Grab the heap statistics
mbed_stats_heap_t heap_stats;
mbed_stats_heap_get(&heap_stats);
printf("Heap size: %lu / %lu bytes\r\n", heap_stats.current_size, heap_stats.reserved_size);
}

int main() {
print_memory_info();
mbed_mem_trace_set_callback(mbed_mem_trace_default_callback);

while (true) {
wait(2.0);

void *ptr1 = malloc(512);
void *ptr2 = calloc(768, 1);
void ptr3 = (void)new DigitalOut(LED1);
void *ptr4 = malloc(256);

ptr4 = realloc(ptr4, 512);

// Grab the heap statistics
mbed_stats_heap_t heap_stats;
mbed_stats_heap_get(&heap_stats);
printf("Heap size: %lu / %lu bytes\r\n", heap_stats.current_size, heap_stats.reserved_size);

// Forget to free a pointer
free(ptr1);
free(ptr3);
free(ptr4);
}
}
``