Main with single argument

Question

I've recently come across a C program in which the main function only took a single argument. Is this legal in C89? gcc didn't seem to have any problems with it.

What I think happens is that the signature is ignored and main is called as main(int,char**) anyways, but I'm not sure.

It looks like this in the program: main(argc) { ... }

Answer 1

According to the C89 standard, it is not legal. From section 2.1.2.2 Hosted environment:

The function called at program startup is named `main`. The implementation
declares no prototype for this function. It can be defined with no parameters:

    int main(void) { /*...*/ }

or with two parameters (referred to here as argc and argv , though any names
may be used, as they are local to the function in which they are declared):

    int main(int argc, char *argv[]) { /*...*/ }

The C99 standard states the same in section 5.1.2.2.1 Program startup.

Answer 2

Yes, it works^†, but it is useless.

Remember that in C, any variable that doesn't have a type specified defaults to int, so that means that the function expands to this:

int main(int argc) {
    ...
}

Which is legal in C89. Most of the time, however, if you want to know the number of arguments sent to a program, you probably want the contents of those arguments, so this is mostly useless.

However, GCC (when compiled with -Wall) gives me a warning:

Only one parameter on 'main' declaration.

It's just saying that this code is pretty much useless.

However, technically, as @hmjd noted, this is illegal, in that it is undefined behavior. However, in most implementations of C that I have came across, when you pass extra parameters to a function, they are just ignored for the most part. So, unless you are on a system where it matters if you overflow the amount of variables sent to a function, you should be fine.