C 也可以寫的很OOP 簡單範例與簡易Makefile

tags: C LANGUAGE oop Object-oriented programming C99 ring buffer

Author: CrazyMonkey
email: [e-mail]
Date: 20230214
Copyright: CC BY-NC-SA

1. 了解這例子可以更深刻了解
   物件導向程式設計(OOP :Object-oriented programming) , 例如 java, javascript, …等
2. 了解 this 的意義, 這裡也用到 function pointer
3. object 的生成(constructor)與回收(deconstructor)需要付出 garbage collection 的代價, 如果於迴圈中大量使用將可能大量消耗記憶體與CPU資源.

//crect.c
/*
*
*Copyright (C) 2019 My-Company-Name
*Version:     1.101 2019/01/01
*Authors:     WhoAmI,  <[e-mail]>
*Complie and Link:
*                   gcc -Wall -c crect.c  -o crect.o
*
*TODO:      Homework, DUE DATE: 2019/01/01
*         
*BUGS:
*/





#include <stdlib.h>
#include "crect.h"

static void SetParameter(CRectangle *rect,int a, int b)
{

//"this" pointer
rect->width=a;
rect->height=b;
}
static int GetArea(CRectangle *rect)
{
return (rect->width)* (rect->height);
}



//global functions

//constructor
CRectangle *init_CRectangle(int width,int height)
{
CRectangle *pThis ;

//allocte object storage
pThis=(CRectangle *)malloc(sizeof(CRectangle));

if(pThis){


//assign function pointers and properties

pThis->set_parameters = SetParameter;
pThis->area=GetArea;
pThis->width=width;
pThis->height=height;

}


return pThis;

}

//deconstructor
int exit_CRectangle(CRectangle *pThis){

if(pThis){
     free(pThis);       //release object
     return 0;
}
return -1;
}


//crect.h

typedef struct Rectangle{
int width, height;
int (*area) (struct Rectangle *);
void (*set_parameters )( struct Rectangle *,int,int);

}CRectangle;
CRectangle *init_CRectangle(int width,int height);
int exit_CRectangle(CRectangle *pThis);



/* app.c

demo
gcc app.c crect.c -o app

*/
#include <stdio.h>
#include "crect.h"


int main () {
CRectangle *rect;

//init object
rect=init_CRectangle(5,3); //allocate memory and initialize  object
if(!rect){

    printf("Cannot init CRectangle\n");
}
rect->set_parameters (rect,2,4);
printf("area:%d\n", rect->area(rect));


//exit
exit_CRectangle(rect);   //release object
return 0;
}

HW. 設計一 CPolygon (多邊形)
CPolygon *init_CPolygon(int edges,…);
//計算周長
int (*Perimeter) (struct Polygon *);
int exit_CRectangle(CPolygon *pThis);

Hint:
stdarg, va_start, va_arg, va_end, va_copy - variable argument lists

簡易的 Makefile

當您改變 CC, CFLAGS, LDFLAGS, …等可以改變為不同 CPU, 如此以後才方便移植至其他 SoC!

    
CC =  gcc
AR =  ar
RANLIB = ranlib
STRIP =  strip
CFLAGS =    -Wall -DDEBUG0
LDFLAGS=    -lpthread
CSOURCE=    crect.c app.c

all: homework2

homework2: ${CSOURCE}
 ${CC} ${CSOURCE} ${CFLAGS} -o app
 ${STRIP} app

clean:
 rm app
    

---

ar - create, modify, and extract from archives
ranlib - generate an index to an archive

使用 gcc 自製 C/C++ 靜態、共享與動態載入函式庫教學, by G. T. Wang

Futher Reading:
-fPIC, PIC = position independent code
Ref.
Creating a shared and static library with the gnu compiler (gcc), by Alberto Fanjul (albfan).

---

C99, Linux kernel 大都使用此寫法

#include <stdio.h>
#include <stdlib.h>
/*

C99

gcc xxx -std=c99

*/


typedef struct student{
    int id;
    int (*year)(struct student * );

    int score;
} Student;

static  int year (struct student *pthis){

  return pthis->id;
}
int main(int argc, char *argv[]){


  Student s= (Student)  {.id=10,.year=year,};
  printf("year=%d\n",s.year(&s));


}
    

drivers/char/misc.c

​  static struct file_operations misc_proc_fops = {
​   .owner	 = THIS_MODULE,
​   .open    = misc_seq_open,
​   .read    = seq_read,
​   .llseek  = seq_lseek,
​   .release = seq_release,
};

ring buffer

簡單但實用的 ring buffer (circular buffer, circular queue and cyclic buffer) , 請自行測試
尤其是記憶體受限時, 例如 單晶片, embedded system 或資料變動快速與大量時(使用ring buffer (fixed maximum size) 不需要 經常 malloc, free, 如此可減少 OS 進行 garbage collection! )

有次 研發人員 處理 快速大量資料時(每次大小不一定) 用 linked list, 改建議 他使用 ring buffer.

Ring Buffer OOP version

/*
*
*Copyright CC BY-NC-SA
*Version:     
*Authors:     WhoAmI
*Complie and Link:
*                  
*
*TODO:      
*         
*BUGS:
*/   
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>


typedef struct ring_buffer
{
    void *buffer;     // data buffer
    void *buffer_end; // end of data buffer
    size_t capacity;  // maximum number of items in the buffer
    size_t count;     // number of items in the buffer
    size_t sz;        // size of each item in the buffer
    void *head;       // pointer to head
    void *tail;       // pointer to tail
} ring_buffer;

ring_buffer * ring_init( size_t capacity, size_t sz)
{
    ring_buffer *ring;
    ring=malloc(sizeof(ring_buffer));

    if(ring==NULL)return NULL;

    ring->buffer = malloc(capacity * sz);
    if(ring->buffer == NULL){

       free(ring);
       return NULL;
    }
     
    ring->buffer_end = (unsigned char *)ring->buffer + capacity * sz;
    ring->capacity = capacity;
    ring->count = 0;
    ring->sz = sz;
    ring->head = ring->buffer;
    ring->tail = ring->buffer;
}

void ring_exit(ring_buffer *ring)
{
    if(ring){
      free(ring->buffer);
      free(ring);
    }
 
}

int ring_push(ring_buffer *ring, const void *item)
{
    if(ring->count == ring->capacity){
        return 0;
    }
    memcpy(ring->head, item, ring->sz);
    ring->head =  (unsigned char*)ring->head + ring->sz;
    if(ring->head == ring->buffer_end)
        ring->head = ring->buffer;
    ring->count++;
    return ring->count;
}

int ring_pop(ring_buffer *ring, void *item)
{
    if(ring->count == 0){
        // handle error
        return 0;
       
    }
    memcpy(item, ring->tail, ring->sz);
    ring->tail = (unsigned char*)ring->tail + ring->sz;
    if(ring->tail == ring->buffer_end)
        ring->tail = ring->buffer;
    ring->count--;
    return ring->count;
}

進一步研究 Document Object Model (DOM) 培養 OOP 的觀念