#include <stdlib.h>

int fun ( int para1, int para2 );

int main ( void )
{
  int a = 2, b = 1, c = 0;

  if ( NULL == &fun )                //遵循1
  {
    return -1;
  }
  else
  {
    int ( *p ) ( int, int ) = &fun;  //遵循2

    c = p ( a, b );
  }

  return ( 0 );
}

int fun ( int para1, int para2 )
{
  return ( para1 - para2 );
}