c - Why argument type of `putchar()`, `fputc()` and `putc()` is not `char`?

Question

Does anybody know Why argument type of putchar(), fputc() and putc() is not char, but argument type of putwchar(), fputwc() and putwc() is wchar_t? See also this and this.

Answer 1

The answer is 'legacy' (or 'history'). Before the C90 standard, there were no function prototypes and all arguments to all functions were subject to default promotion rules, so a char was automatically passed as an int (short was promoted to int too, and float to double, and similarly for unsigned types). The standard couldn't afford to break existing code, so it kept that type for these functions. It makes very little difference in practice. The value you pass will be treated as a character type even if you pass a value that's out of range. The specification of fputc(int c, FILE *stream) says:

The fputc function writes the character specified by c (converted to an unsigned char) to the output stream pointed to by stream …

Default promotion rules

§6.5.2.2 Function calls

?6 If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions. …

?7 … The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments.

Integer promotions are defined in §6.3.1

?2 The following may be used in an expression wherever an int or unsigned int may be used:

• An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int.
• A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.⁵⁸⁾ All other types are unchanged by the integer promotions.

?3 The integer promotions preserve value including sign. As discussed earlier, whether a 'plain' char is treated as signed is implementation-defined.

⁵⁸⁾ The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses.

The integer ranks are defined in ?1 of the section in 10 bullet points.