c - Why putchar, toupper, tolower, etc. take a int instead of a char?

Question

In C, strings are arrays of char (char *) and characters are usually stored in char. I noticed that some functions from the libC are taking as argument integers instead of a char.

For instance, let's take the functions toupper() and tolower() that both use int. The man page says:

If c is not an unsigned char value, or EOF, the behavior of these functions is undefined.

My guess is that with a int, toupper and tolower are able to deal with unsigned char and EOF. But in fact EOF is in practice (is there any rule about its value?) a value that can be stored with a char, and since those functions won't transform EOF into something else, I'm wondering why toupper does not simply take a char as argument.

In any case why do we need to accept something that is not a character (such as EOF)? Could someone provide me a relevant use case?

This is similar with fputc or putchar, that also take a int that is converted into an unsigned char anyway.

I am looking for the precise motivations for that choice. I want to be convinced, I don't want to answer that I don't know if someone ask me one day.

Answer 1

C11 7.4

The header <ctype.h> declares several functions useful for classifying and mapping characters. In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

C11 7.21.1

EOF

which expands to an integer constant expression, with type int and a negative value, ...

The C standard explicitly states that EOF is always an int with negative value. And furthermore, the signedness of the default char type is implementation-defined, so it may be unsigned and not able to store a negative value:

C11 6.2.5

If a member of the basic execution character set is stored in a char object, its value is guaranteed to be nonnegative. If any other character is stored in a char object, the resulting value is implementation-defined but shall be within the range of values that can be represented in that type.