Difference between identifier list and parameter list in function?

Question

For this quote:

An identifier list in a function declarator that is not part of a definition of that function shall be empty.

what is the difference between identifier list and parameter list and can someone provide an example for this quote.

Answer 1

The "identifier list" is only used in obsolete "K&R style" functions. New code written today would never use it. You can see more details here: https://stackoverflow.com/a/3092074/4323

It's something like this:

void func(identifier-list)
declaration-list
{
    body
}

When they say it shall be empty, they mean that even admitting the possibility of ancient code, you are not allowed to have this in a declaration which does not define a function. So for example this is not allowed:

void func(x) int x;

Answer 2

Identifier list without identifiers' definitions says nothing about the types of function parameters. So it does not make sense to specify an identifier list for a function declaration when it is not at the same time a function definition.

So this restriction of the cited quote is used.

Here is an example

#include <stdio.h>

void f();

int main(void) 
{
    int x = 10;
    f( x );

    return 0;
}

void f( x ) 
int x;
{
    printf( "x = %d\n", x );
}

When a parameter list is used the compiler can check a call of a function that valid arguments are passed to the function. So it is better always to use parameter list instead of identifier list.