c language: function pointer, callback and event-driven

tags: linux C LANGUAGE callback event driven

Authors: WhoAmI, CrazyMonkey
email: [e-mail]
Date: 20230416
Copyright: CC BY-NC-SA

HW: 如果 每個 action 有 多種狀態 (輸入 event sequence e0 e1 … e2) 例如 stat 0, stat 1, stat 2 您會如何設計?
例如 e0 e1 e2 則正確, 否則 錯誤 (例如 e0 e0 e1) 錯誤
Time Dependent FSM(Fine State Machine) 問題

function pointer 是c 語言的重要技巧, 一般初學者比較少接觸, 然而在很多實務設計, 例如 AJAX(Asynchronous JavaScript and XML), FSM(Fine State Machine)… 中會用到, 此外 c 語言運用物件導向觀念亦會使用到.

處理 time-dependent FSM 也是 網路程式的重要問題
例如 pop3 SSL 網路驗證

Linux signal 就是此種 function pointer

​​​​   typedef void (*sighandler_t)(int);

sighandler_t 是一新 type

sighandler_t signal(int signum, sighandler_t handler);

此種方式可以讓設計者自行設計 handler

另一例子 qsort

void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));

Event loop (JavaScript execution flow, Win32 proramming) 是類式的觀念, 也都可能用到callback 與 OOP(Object-oriented programming) 的方法.

//callfunc.c
/*

*Copyright (C) GPL

*Version: 1.0 2018/09/18

*Authors: WhoAmI
*
*DEMO:
*             gcc -Wall callfunc.c -o callfunc
*              Enable stdout
*              gcc -Wall -DTEST callfunc.c -o callfunc
*

*/

#include <stdio.h>

#include <stdlib.h>

#include <string.h>


typedef int (*myFunc)(int, int); //myFunc is a new type

static int modulus(int a,int b);

static int add(int a,int b);

static int myCallback(myFunc *pthis,int a,int b);

static int myCallbacks(myFunc *pthis,int a,int b);

#ifdef TEST

#define debugx(x) x

#else

#define debugx(x)

#endif



static int add(int a,int b)

{

return a+b;

}



static int modulus(int a,int b)

{

return a%b;

}

static int myCallback(myFunc *pthis,int a,int b)

{

return (*pthis)(a,b);

}

//這也是一種設計技巧

static myFunc my_func[]={add,modulus,NULL}; //functions in array



static int myCallbacks(myFunc *pthis,int a,int b)

{

int i;

myFunc *p;

for(i=0,p= pthis;*p!=NULL;i++)

{

debugx( printf("[%d] return=%d\n",i,(*p)(a,b)));

p++;

}

return 0;

}



int main (int argc,char *argv[])

{

myFunc myAdd=&add;

printf("function pointer: myAdd(8,3)=%d\n",(*myAdd)(8,3));

printf("callback: %d\n",myCallback((myFunc *) &myAdd,8,3));

myCallbacks(my_func,8,3);

exit(EXIT_SUCCESS);

}

另一例子讓讀者熟悉 callback, event listener, …這些特別的寫程式方法

在此例子中只要增加 Event 中的元素即可執行依照 name 執行 action!

例如 一位設計團隊一員設計一action, 最後再合起來, 如此更能容易除錯與設計

//hander.c
/*

*Copyright (C) GPL

*Version: 1.0 2018/09/18

*Authors: WhoAmI([e-mail])
*DEMO:callback,  event, listener
*
*     gcc -Wall hander.c -o hander

*   


*/

#include <stdio.h>

#include <stdlib.h>

#include <string.h>


typedef int (*myFunc)(int, int); //myFunc is a new type

static int modulus(int a,int b);

static int add(int a,int b);


typedef struct levent {

 char *name;
    myFunc *action;
}Event;



static Event eventlistener[] = {
   {"add", add},
   {"modulus",modulus},
   {NULL,NULL}

};

static int add(int a,int b)

{

return a+b;

}


static int modulus(int a,int b)

{

return a%b;

}



static int CallEvent(char *name, Event *event)

{
   char *p;
   Event *e;
   myFunc pthis;
   int a,b;

   a=5;b=2;

   for(e=event; e->name!=NULL; e++){

       if(strcmp(e->name,name)==0){

            pthis=e->action;

            printf("Find and do %s(%d,%d):%d\n",e->name,a,b,(*pthis)(a,b));
       }

   }
 
   return 0;


}




int main (int argc,char *argv[])

{


CallEvent("add",eventlistener);

CallEvent("modulus",eventlistener);

exit(EXIT_SUCCESS);

}

Event-Driven 的設計方法

:::

在此例子中只要增加 Event 中的元素即可執行依照 name 執行 action!

例如 一位設計團隊一員設計一action, 最後再合起來, 如此更能容易除錯與設計
variable argument lists+function pointer+variadic function 可以彈性設計 action

您也可 定義 action 的 return value 方便除錯

/* demo

WhoAmI 2022

*/


#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>

#include <string.h>



typedef int (*pAction)(void *fmt,...); //pAction is a new type
typedef struct levent {

 char *name;
 pAction *action;
}Event;

static int prnstr(void *str,...)

{

    return printf("%s\n",str);

}
static int prnid(void *str,...)

{
 char *p;
 int i;
 va_list arguments;
 
 i=0;
  
 
 
 va_start ( arguments, str );
 
 p=va_arg ( arguments, void * );

 for( ;p!=NULL; ){
  
   if(p){
    printf("string <%s>:",p);
    p=va_arg ( arguments, void * );
    i++;
   } 
   else break;
   
 }  
 
 va_end ( arguments );
 return i;

}

static Event eventlistener[] = {
   {"print", prnstr},
   {"pid", prnid},
   {NULL,NULL}

};
static int CallEvent(char *name, Event *event)

{
   
   Event *e;
   pAction pthis;
 
   char *p="ha ha ha!";
   char *p2="ha ha ha!!";

  

   for(e=event; e->name!=NULL; e++){

       if(strcmp(e->name,name)==0){

            pthis=e->action;

            printf("Find and do '%s':%d\n",e->name,(*pthis)(e->name,p,p2,NULL));
       }

   }
 
   return 0;


}

int main (int argc,char *argv[])

{


CallEvent("print",eventlistener);
printf("\n\n");
CallEvent("pid",eventlistener);

exit(EXIT_SUCCESS);

}

Homework: 思考此方法的優點
Appendix: State Machine Design in C, by David Lafreniere

---

善用 Macros

//gcc -Wall demodef.c -o demodef


#define VAL(x)  val##x
#define STR(x)  #x


//do{ ...}while(0);

#define prn(x)  do{\
printf("%s val=%d\n",STR(x),60);\
printf("Hello define! line=%d\n",__LINE__);\
}while(0)



//Variadic Macros

#define QUOTE(...) #__VA_ARGS__

#define eprintf(...) fprintf (stderr, __VA_ARGS__)

//multiple lines 

const char *name="OK=Line 1 OK \
Line 2\
Line 3\
";

const char *fname = QUOTE(OK= abc 
" 123 "
" 456 "
def); 




int main()
{
   int val99=10;
   int val911=911;

   prn(Hello C);
   printf("Hello %d  %d\n", VAL(99),VAL(911));   //val99 val911
   eprintf("name=%s\n fname=%s\n",name,fname);

   eprintf("Output to stderr date: %s  line: %d\n",__DATE__,__LINE__);

   return 0;

}

typeof

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
#define max(a,b) \
  ({ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; })

struct fruit{
  int weight;
  int cost;
};

static struct fruit *put_fruit()
{
  struct fruit *_fruit;
  _fruit = malloc(sizeof(struct fruit));
  if(_fruit == NULL)
  {
    printf("malloc error.\n");
    return NULL;
  }

  _fruit->weight = 10;
  _fruit->cost = 2;

  return _fruit;
}

int main(int argc, char *argv[])
{

  typeof(put_fruit()) _fruit; //不會執行 put_info(), 取得型態 struct fruit *

  _fruit = put_fruit(10,20);

  if(_fruit){
    printf("weight:%d\n", _fruit->weight);
    printf("cost:%d\n", _fruit->cost);
  
    printf("max(2,3)=%d\n", max(2,3));
    free(_fruit);
  } 
  

  return 0;
}