Why _PROTOTYPE is used header files.

Question

Why do we use _PROTOTYPE e.g. _PROTOTYPE( void *memset, (void *_s, int _c, size_t _n) I saw it in MINIX3 source code

Answer 1

To accommodate both K&R and ANSI C function declaration styles. Take a look at Wikipedia article about C evolution.

Answer 2

The source code you linked to explains it:

00009 * If _ANSI ends up being defined, a macro
00010 *
00011 * _PROTOTYPE(function, params)
00012 *
00013 * is defined. This macro expands in different ways, generating either
00014 * ANSI Standard C prototypes or old-style K&R (Kernighan & Ritchie)
00015 * prototypes, as needed. Finally, some programs use _CONST, _VOIDSTAR etc
00016 * in such a way that they are portable over both ANSI and K&R compilers.
00017 * The appropriate macros are defined here.

Old-style K&R prototypes have the argument names first, then the types:

int foobar (x, y)
    int x;
    float *y;
{
    /* code */
}

ANSI standard prototypes combine them both at the beginning:

int foobar (int x, float *y) {
    /* code */
}

The _PROTOTYPE macro creates an appropriate signature of either type depending on whether or not _ANSI is defined. In this specific case, K&R signatures are used for function implementations, but the function declarations either include or omit their arguments depending on whether _ANSI is defined.

It is worth noting that K&R-style declarations date from 1978 and most C code currently available will use modern ANSI style signatures. It is rare that you need to support both.

Answer 3

Because as the header block says, they didn't know if an ANSI compiler or K&R was going to be used, and this marco allows them to keep the parameters in an ANSI build, and throw them away in a K&R build.

00033   /* Keep everything for ANSI prototypes. */
00034   #define _PROTOTYPE(function, params)    function params

verse

00045   /* Throw away the parameters for K&R prototypes. */
00046   #define _PROTOTYPE(function, params)    function()

which means

00483   _PROTOTYPE( void _exit, (int _status)    );

becomes under ANSI:

void _exit(int _status);

and under K&R:

void _exit();