Is main() function better on top of all other functions? [closed]

It’s largely a matter of personal preference, although some places may make it a formal coding standard. C itself doesn’t care.

For my part, if I’m defining multiple functions in a single source file, I will define the called functions before the caller:

void foo( void )
{
  ...
}

void bar( void )
{
  ...
  foo();
  ...
}

int main( void )
{
  ...
  bar();
  ...
}

instead of

int main( void )
{
   void bar( void );
   ...  
   bar();
   ...
}

void bar( void )
{
  void foo( void );
  ...
  foo();
}

void foo( void )
{
  ...
}

The problem with using forward declarations is you increase your maintenance burden; if you change the function prototype (return type, number and/or types of parameters), you now have to make those changes in two places instead of one.

Of course, if you’re calling functions defined in a different source file, you need separate declarations (preferably grouped into their own header file):

/**
 * functions.c
 */
#include "functions.h"

void foo( void )     // Note that I'm still defining called functions
{                    // before their callers, even though in this case
  ...                // it's not necessary, since "functions.h" contains
}                    // the declaration for foo().

void bar( void )
{
  ...
  foo();
  ...
}

/**
 * functions.h
 */
#ifndef FUNCTIONS_H
#define FUNCTIONS_H

void foo( void );
void bar( void );

#endif

/**
 * main.c
 */
#include "functions.h"

int main( void )
{
  ...
  bar();
  ...
}

Does this apply to all programming languages that uses main() as the entry point of the program?

Again, for most of those languages, it’s strictly a matter of style and personal preference. The language definition itself doesn’t care.

The best place is somewhere that people can find it.

In the middle of a very large file is bad. At the top is good, but others might prefer right at the bottom.

Forward declaring main() doesn’t help, and is often pointless anyway – because it’s very unusual to explicitly call main() from within your code.