Can we define function prototype in main function in C?

Question

in my university our teacher taught us "we define function prototype before main function. Like:

#include<stdio.h>
void findmax(float, float);
int main()
{
    //code goes here
}

But today my friend showed me they learned they put prototype inside main function. Like:

#include<stdio.h>
int main()
{
    void findmax(float, float);

    float firstnum, secondnum;

    printf("Enter first:");
    scanf("%f", &firstnum);

    printf("Enter second:");
    scanf("%f", &secondnum);

    findmax(firstnum, secondnum);
}

void findmax(float x, float y)
{
    float maxnum;

    if(x>y)
    {
            maxnum=x;
    }

    else
    {
       maxnum=y;
    }

    printf("The max is %f", maxnum);
}

They both works.I wonder if there are differences between them. Thanks.

Answer 1

Can we define function prototype in main function in C?

Yes.

I wonder if there are differences between them

The difference is that in first snippet prototype is global while in second it is local to main. findmax will be visible after its declaration and/or definition.

#include<stdio.h>
void foo();
int main()
{
    void findmax(float, float); 
    foo();
    findmax(10, 20);  // It knows findmax by the prototype declared above
}

void findmax(float x, float y)
{
    float maxnum;

    if(x>y)
        maxnum=x;
    else
        maxnum=y;

    printf("The max is %f", maxnum);
}

void foo(){   // foo is using findmax after its definition.
    findmax(12, 30);
}

Answer 2

If foo is declared in outside a function, it can be called from any function in the same file:

void foo(); // <-- global declaration

int main() {
  foo();    // <-- works, foo() is declared globally
}

void otherfunc() {
  foo();    // <-- works, foo() is declared globally
}

However, if foo is declared inside a function, it can only be used within the same scope:

int main() {
  void foo(); // <-- scoped declaration
  foo();      // works, foo() is declared in same scope
}

void otherfunc() {
  foo();      // ERROR: foo() is not declared in scope of otherfunc()
}

In either case, foo must be declared before it is used.

Answer 3

If you declare the function in main() it is scoped in the main() and you cannot access it in another function. But if you declare it at the start of the file or in a header file you can use it in any function.