gcc: division by zero

Question

I am getting division by zero error at this line:

if (tim2_st_ovf < T2_PREK_250)

These values are defines like this:

volatile uint8_t tim2_st_ovf = 0;

#define T2_PREK_250    ((250 * (F_CPU / 1000)) / ((UINT8_MAX + 1) * 1024))
#define F_CPU    16000000UL

And UINT8_MAX equals to 255.

Why am I getting this? I calculated it several times on calculator and am getting ~15. Also, if I change 1024 to 1023 it doesnt show any error.

Answer 1

((UINT8_MAX + 1) * 1024) may become 0, because UINT8_MAX + 1 is usually 256, and 256 * 1024 is 0 modulo 2¹⁶. So if sizeof(int) == 2 on your achitecture, then you get 0.

On the typical modern desktop architectures with GCC, sizeof(int) == 4, and you wouldn't get the division by 0.

To fix it, replace 1024 with 1024UL. That will work, because unsigned long is guaranteed to go up to 4294967295. (Thanks to Pascal Cuoq for explaining it.)