[c] function declaration isn't a prototype

I have a library I created,

mylib.c:

#include <mylib.h>
int
testlib() {
    printf("Hello world\n");
    return (0);
}

mylib.h:

#include <stdio.h>
extern int testlib();

In my program, I've attempted to call this library function:

myprogram.c:

#include <mylib.h>

int
main (int argc, char *argv[]) {
    testlib();
    return (0);
}

When I attempt to compile this program I get the following error:

In file included from myprogram.c:1
mylib.h:2 warning: function declaration isn't a prototype

I'm using: gcc (GCC) 3.4.5 20051201 (Red Hat 3.4.5-2)

My question is, what is the proper way to declare a function prototype?

Try:

extern int testlib(void);

Quick answer: change int testlib() to int testlib(void) to specify that the function takes no arguments.

A prototype is by definition a function declaration that specifies the type(s) of the function's argument(s).

A non-prototype function declaration like

int foo();

is an old-style declaration that does not specify the number or types of arguments. (Prior to the 1989 ANSI C standard, this was the only kind of function declaration available in the language.) You can call such a function with any arbitrary number of arguments, and the compiler isn't required to complain -- but if the call is inconsistent with the definition, your program has undefined behavior.

For a function that takes one or more arguments, you can specify the type of each argument in the declaration:

int bar(int x, double y);

Functions with no arguments are a special case. Logically, empty parentheses would have been a good way to specify that an argument but that syntax was already in use for old-style function declarations, so the ANSI C committee invented a new syntax using the void keyword:

int foo(void); /* foo takes no arguments */

A function definition (which includes code for what the function actually does) also provides a declaration. In your case, you have something similar to:

int testlib()
{
    /* code that implements testlib */
}

This provides a non-prototype declaration for testlib. As a definition, this tells the compiler that testlib has no parameters, but as a declaration, it only tells the compiler that testlib takes some unspecified but fixed number and type(s) of arguments.

If you change () to (void) the declaration becomes a prototype.

The advantage of a prototype is that if you accidentally call testlib with one or more arguments, the compiler will diagnose the error.

(C++ has slightly different rules. C++ doesn't have old-style function declarations, and empty parentheses specifically mean that a function takes no arguments. C++ supports the (void) syntax for consistency with C. But unless you specifically need your code to compile both as C and as C++, you should probably use the () in C++ and the (void) syntax in C.)