ardupilot/libraries/memcheck/memcheck.cpp

/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-

/*
  note that we use a 32 bit sentinal to reduce the chance
  of false positives with uninitialised stack variables
 */

#include <AP_Common.h>

static const uint32_t *stack_low;
extern unsigned __brkval;

#define STACK_OFFSET 2
#define SENTINAL 0x28021967

/*
  return the current stack pointer
 */
static __attribute__((noinline)) const uint32_t *current_stackptr(void) 
{
    return (const uint32_t *)__builtin_frame_address(0);
}

/*
  this can be added in deeply nested code to ensure we catch 
  deep stack usage. It should be caught by the sentinal, but this
  is an added protection
 */
void memcheck_update_stackptr(void)
{
    if (current_stackptr() < stack_low) {
        unsigned s = (unsigned)(current_stackptr() - STACK_OFFSET);
        stack_low = (uint32_t *)(s & ~3);
    }
}

/*
  initialise memcheck, setting up the sentinals
 */
void memcheck_init(void)
{
    uint32_t *p;
    free(malloc(1)); // ensure heap is initialised
    stack_low = current_stackptr();
    memcheck_update_stackptr();
    for (p=(uint32_t *)(stack_low-1); p>(uint32_t *)__brkval; p--) {
        *p = SENTINAL;
    }
}

/*
  this returns the real amount of free memory by looking for
  overwrites of the stack sentinal values
 */
unsigned memcheck_available_memory(void)
{
    memcheck_update_stackptr();
    while (*stack_low != SENTINAL && stack_low > (const uint32_t *)__brkval) {
        stack_low--;
    }
    return (unsigned)(stack_low) - __brkval;
}