Compiler warning - something differs in parameter lists

Question

I'm trying to compile the following source code which successfully compiles on both gcc and Microsoft's cl.exe.

void SomethingBeforeExit();

void SomethingBeforeExit()
{
    // some code
    _exit(0);
}

int main(int argc, char *argv[])
{
    // some code
    atexit(SomethingBeforeExit);
}

However, I'm getting a C4113 warning from cl.exe with the following message:

SomeCode.c(10): warning C4113: 'void (__cdecl *)()' differs in parameter lists from 'void (__cdecl *)(void)'

As I said, the source code still compiles successfully and appears to work. My goal is to prevent this warning from happening in cl, as gcc doesn't generate any warnings upon compilation.

I assume that the declaration of that function is not being treated as void SomethingBeforeExit(void), however, I don't know how to specifically declare a function's parameter list as void.

I'm using VS14 and C/C++ 19.00.23918 for x86 for cl.exe and gcc v5.4.0 compilers to compare generated warnings.

Answer 1

In C, empty parentheses in a function declaration doesn't mean "no parameters." Instead, it means any number of parameters (similar to ... in C++). Perhaps what you meant to declare was void SomethingBeforeExit(void).

Answer 2

OP is not using a compliant C99/C11 compiler. The code is valid C (C99/C11).

// declares function SomethingBeforeExit and no info about it parameters.
void SomethingBeforeExit();

// declares/defines function SomethingBeforeExit and 
// indirectly specifies the parameter list is `(void)`
// This does not contradict the prior declaration and so updates the parameter signature.
void SomethingBeforeExit() {
   ...  
}

int main(int argc, char *argv[]) {
  ...
  // `atexit() expects a `void (*f)(void))`
  atexit(SomethingBeforeExit);

In C89 the void SomethingBeforeExit() { ...} definition still says nothing about the parameter list. This is likely the cause of OP's problem.

To fix:

void SomethingBeforeExit(void) { 
  ...
}

The prior void SomethingBeforeExit(); declaration need not update, yet would be in good to also update that so parameter checking can happen with code that does not see the definition.