Strange behavior on function argument in C

Question

I have a problem with a program running in an ARM4 MCU. Debugging the code I found this situation: The function changeBackColor calls drawn with the argument this (this=0x1fffc4d3 first image), inside the function draw, immediately after call, the value of this is different (this=0xffc4d3 second image).

changeBackColor function

drawn function

Adittional information: I have no leak in heap memory; I can not reproduce the problem in unit tests; The problem only appears when running the entire system.

Answer 1

Thanks for all help. About suggests: For this situation, it is not possible to post the entire code; The optimization was disabled.

The solution:

This is the code for the drawn function:

static void drawn(TextField * this)
{
  size_t cps = strlen(this->text);
  size_t cls = this->length - cps;
  uint8_t nx = this->px + (this->font->width + 1) * cps;
  char str[] = SPACES;
  str[cls] = '\0';

  if (this->trimSpaces)
  { \\...
  }
  \\...
}

this->length is a property that holds the length of the textField. I do not know the reason, but I got a miss behavior on strlen(this->text). Its returning a number greater than length, than cls became to high, and the instruction: "str[cls] = '\0';" is overwriting the memory in the same position of the first byte of the variable this.