Question

I am reading an old book about code obfuscation in C (the book was printed in 1993), and I’ve noticed that the functions with arguments are implemented this way:

real_dump(address, infunc, ofp)
char *address;
int (*infunc)();
FILE *ofp;
{
    /* the code goes here... */
}

Also, no return type is defined.

Is it an old standard? Is it possible to enable gcc to compile this code?

Answer 1

Function definitions in the non-prototype form are valid C89, C99 and C11 code.

It is called the old-style function definition but this feature is marked since C89 as an obsolescent feature.

This form should be not used in new programs.

C99 Rationale says:

“Characterizing the old style as obsolescent is meant to discourage
its use and to serve as a strong endorsement by the Committee of the
new style.”

even K&R2 discourages its use:

“The old style of declaration and definition still works with ANSI C, at least for a transition period, but we strongly recommend that you use the new form when you have a compiler that supports it.”

Now your function also doesn’t have a return type and omitting the return type in a function declaration or in a function definition is no longer valid since C99. Before C99, functions without a return type implicitely returned an int.

Regarding the gcc question, by default gcc compiles with -std=gnu89. It means C89 Standard + gcc extensions. So by default gcc will accept to compile a program with the functions declaration and definition in their old-style form and without a return type.