The first argument of these functions is of type
int, but only a very restricted subset of values are actually valid. The argument must either be the value of the macro
EOF (which has a negative value), or must be a non-negative value within the range representable as
unsigned char. Passing invalid values leads to undefined behavior.
Values of type
int that were returned by
getc(3),
fgetc(3), and similar functions or macros are already in the correct range, and may be safely passed to these
ctype functions without any casts.
Values of type
char or
signed char must first be cast to
unsigned char, to ensure that the values are within the correct range. The result should then be cast to
int to avoid warnings from some compilers. Casting a negative-valued
char or
signed char directly to
int will produce a negative-valued
int, which will be outside the range of allowed values (unless it happens to be equal to
EOF, but even that would not give the desired result).