There are two ways for specifying parameters in C. One is using an identifier list, and the other is using a parameter type list. The identifier list can be omitted, but the type list can not. So, to say that one function takes no arguments in a function definition you do this with an (omitted) identifier list
void f() {
/* do something ... */
}
And this with a parameter type list:
void f(void) {
/* do something ... */
}
If in a parameter type list the only one parameter type is void (it must have no name then), then that means the function takes no arguments. But those two ways of defining a function have a difference regarding what they declare.
Identifier lists
The first defines that the function takes a specific number of arguments, but neither the count is communicated nor the types of what is needed - as with all function declarations that use identifier lists. So the caller has to know the types and the count precisely before-hand. So if the caller calls the function giving it some argument, the behavior is undefined. The stack could become corrupted for example, because the called function expects a different layout when it gains control.
Using identifier lists in function parameters is deprecated. It was used in old days and is still present in lots of production code. They can cause severe danger because of those argument promotions (if the promoted argument type do not match the parameter type of the function definition, behavior is undefined either!) and are much less safe, of course. So always use the void
thingy for functions without parameters, in both only-declarations and definitions of functions.
Parameter type list
The second one defines that the function takes zero arguments and also communicates that - like with all cases where the function is declared using a parameter type list, which is called a prototype
. If the caller calls the function and gives it some argument, that is an error and the compiler spits out an appropriate error.
The second way of declaring a function has plenty of benefits. One of course is that amount and types of parameters are checked. Another difference is that because the compiler knows the parameter types, it can apply implicit conversions of the arguments to the type of the parameters. If no parameter type list is present, that can't be done, and arguments are converted to promoted types (that is called the default argument promotion). char
will become int
, for example, while float
will become double
.
Composite type for functions
By the way, if a file contains both an omitted identifier list and a parameter type list, the parameter type list "wins". The type of the function at the end contains a prototype:
void f();
void f(int a) {
printf("%d", a);
}
// f has now a prototype.
That is because both declarations do not say anything contradictory. The second, however, had something to say in addition. Which is that one argument is accepted. The same can be done in reverse
void f(a)
int a;
{
printf("%d", a);
}
void f(int);
The first defines a function using an identifier list, while the second then provides a prototype for it, using a declaration containing a parameter type list.