Critical Section comes in single process environment they
didn't share accross process boundary.

In C if you wish to implement criticalsection you have to
use windows datastructure CRITICAL_SECTION and create a
object to access its members.

Initialize it
  InitializeCriticalSection(&cs);
------------
-----------
function to call();
------------
DeleteCriticalSection(&cs);
.
.
call()
{
}

Now sure what is asked but anyway:
1. If the question is how would you implement critical 
section mechanism then the answer is to use a mutex.
2. If the question is to show how to use the critical 
section then the following code fragment adresses that:
//
//  Global variable or a member of some
//  dynamically allocated structure, usualy within
//  the data access to which it is protecting.
//
CRITICAL_SECTION cs;

VOID Init(VOID) {
   InitializeCriticalSection( &cs );
}

VOID SomeCall(VOID) {
  EnterCriticalSection( &cs );
  //
  //  Thread-safe block of code protected by critical 
section.
  //
  LeaveCriticalSection( &cs );
}

VOID
UnInit(VOID) {
  DeleteCriticalSection( &cs );
}