#include <stdint.h>
void foo(void) {
int8_t  w;    // exactly 8 bits, signed
int16_t x;    // exactly 16 bits, signed
int32_t y;    // exactly 32 bits, signed
int64_t z;    // exactly 64 bits, signed

uint8_t w;    // exactly 8 bits, unsigned
...etc.

}