C Warning : Function was used with no prototype before its definition [closed]

Question

I'm facing a warning while compiling a C project:

myFunct was used with no prototype before its definition.

I do not understand this warning since the prototype is indeed before the call of the function.

Here is my code (simplified):

void myFunct();

int main(void)
{
   myFunct();
}

void myFunct()
{
   // Whatever
}

Answer 1

In C, void myFunct(); is a function declaration, that does not include a prototype. It is an obsolescent feature dated back to K&R (pre-standard) C, where function calls and declarations were not checked.

Replace the declaration with:

void myFunct(void);

Answer 2

Your code provides a forward declaration for function myFunct(), but that declaration is not a prototype because it does not declare the types of the (zero) function parameters (see below). The declaration you provide declares the function to return nothing and to accept a fixed but unspecified number of parameters,^* which you will appreciate is insufficient for the compiler to fully verify the function call. An actual prototype for that function would be this:

void myFunct(void);

Your code is nevertheless completely valid. K&R-style function declarations are still allowed, and they satisfy C's declaration-before requirements. They're just not good style.

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Details:

C2011 6.2.1/2 says,

A function prototype is a declaration of a function that declares the types of its parameters.

C2011 6.7.6/1 distinguishes between a "parameter type list", used in the ANSI declaration style and in particular for function prototypes, and an "identifier list", used in K&R-style declarations. Neither can be empty, but (only) the latter can be omitted altogether, so a function declaration with empty parentheses is a K&R-style declaration, which does not provide a prototype.

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^*C++ differs from C on this.