When fgetc succeeds, the int it returns has a value based on interpreting the byte as an unsigned char. On a platform with an 8-bit char, that means it’s in the range 0 to 255. This is required by the C spec regardless of whether char is signed or unsigned. Meanwhile, EOF is required to be negative. Thus, you can always distinguish the cases as long as you look at the original int return value rather than casting to char.
Fun fact: This approach causes trouble on obscure embedded platforms where char and int are the same size (and therefore an unsigned char value can’t fit inside a signed int). Such platforms are allowed by the C standard as freestanding implementations that don’t implement the full standard library, but they can’t conformantly implement fgetc. https://stackoverflow.com/questions/3860943/can-sizeofint-ev...
Fun fact: This approach causes trouble on obscure embedded platforms where char and int are the same size (and therefore an unsigned char value can’t fit inside a signed int). Such platforms are allowed by the C standard as freestanding implementations that don’t implement the full standard library, but they can’t conformantly implement fgetc. https://stackoverflow.com/questions/3860943/can-sizeofint-ev...