function pointer

Reference: "Embedded Linux, C-language programming practice" 

1. The concept of function pointers: 
   In the C language: the nature of the pointer is a memory address, function pointer is a pointer to a function of the code in the code segment address of the pointer. 
   For the function, its address and function name represents the same meaning 
   In the C language programming, data structures and algorithms are two basic elements, through a function pointer, the algorithm can be embedded into data structure. 

  

2. The usage of function pointers 

  *the basic usage
    int add (int a, int b) 
    {....} 
    int sub (int a, int b) 
    {....} 
    int main ()
    ( 
      int (* pf) (int, int); 
      pf = add; 
      result = pf (100,200); 
      pf = sub; 
      result = pf (100,200); 
     )

   * Type conversion: 
   int add (int a, int b) 
    {....} 
    int sub (int a, int b) 
    {....} 
    int main () 
    ( 
      void * pf; 
      pf = add; 
      result = (int (*) (int, int) pf) (100,200); 
      pf = sub; 
      result = (int (*) (int, int) pf) (100,200); 
     )

    * Function pointer type definition 
   typedef int (* fun_t) (int, int); 
   fun_t pf; 
   pf = add; 
   result = pf (100,200);

    * Function pointer as a structure member 
struct source 
( 
   int a; 
   int b; 
   fun_t operation; / / equivalent to int * fun_t (int, int); 
); 
int main (int argc, char * argv []) 
( 
   struct source data; 
   int result; 
   data.a = 200; 
   data.b = 100; 
   data.operation = add; 
   result = data.operation (data.a, data.b); 
) 

   * Function pointer being as a parameters of the function 
int calculate (int a, int b, fun_t operation) 
( 
   int result; 
   result = operation (a, b); 
   return result; 
) 
  int main (int argc, char * argv []) 
( 
   int a, b, result; 
   a = 200; 
   b = 100; 
   result = calculate (a, b, add); 
   ) 

   * Function pointer as the function return value 
fun_t getoperation (char a) 
( 
   fun_t result; 
   switch (a) 
  ( 
   case "+"; 
       result = add; 
       break; 
   case "-"; 
       result = sub; 
       break; 
   ) 
    return result; 
) 
int main (int argc, char * argv []) 
( 
   int a, b, result; 
   char oper; 
   a = 200; 
   b = 100; 
   oper ="+"; 
   result = getoperation (oper) (a, b); 
)

   * Array of function pointers 
enum ( 
       oper_add = 0, 
       oper_sub 
) 
static const fun_t oper_table [oper_num] = 
( 
   add, 
   sub 
) 
int main (int argc, char * argv []) 
( 
   int a, b, result; 
   a = 200; 
   b = 100; 
   result = oper_table [oper_add] (a, b); 
)