Is this code valid under any C standard?

问题:

Does this code follow C standards (e.g. C89, C99, C10x)?

void 
main(int a,int b, int c, int d,char *msg){
    if(d==1){
        printf("%s\n",msg);
    }else{
        main(1,2,3,1,&"Hello Stackoverflow");
    }
}

If not, why?

回答1:

There's one error: &"Hello Stackoverflow" does not have type char*, so you shouldn't pass that to a function expecting that type.

Apart from that, this program is allowed by the Standard as an implementation-specific extension, but a compiler has the freedom to decline it.

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and 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[]) { /* ... */ }

or equivalent; or in some other implementation-defined manner.

(2011 Standard, latest draft section 5.1.2.2.1, emphasis added.)

There is no ban on recursive calls to main in the C Standard. This is a difference with C++, which does outlaw that.

回答2:

You mean beside it won't run? main is defined to take int, char** as arguments.

Depending on the compiler, this either will fail to start up as the run-time can't find main(int, char**), or on older compilers it'll just crash because it piddles on the stack.

回答3:

It's only valid under C99 and later if the implementation explicitly documents that main may take 5 parameters (4 int and 1 char *) and return void (that's the "or in some other implementation-defined manner" clause that larsmans referenced in his now-un-deleted answer, and I don't think that clause was present in C89).

Otherwise the behavior is undefined, meaning the compiler may or may not choke on it.