void * memcpy ( void * dest, const void * src, size_t num );

void* my_memcpy(void* dest, const void* src, size_t num) {
    void *ret = dest;    //保存dest，用于最終輸出
    assert(dest);
    assert(src);
	while (num--) {
		*(char*)dest = *(char*)src1;    //(char*): 取出每一個(gè)字節(jié)(8 bit)的值，以單個(gè)字節(jié)為單位進(jìn)行賦值
		dest = (char*)dest + 1;    
		src = (char*)src + 1;
	}    //void * 類型是不能直接運(yùn)算的，因?yàn)闆](méi)有步長(zhǎng)。(char*)將dest與src轉(zhuǎn)換為以char為步長(zhǎng)，再向后移動(dòng)
	return ret;
}

int main()
{
	int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr2[10] = { 0 };
	memcpy(arr2, arr1, 20);    //拷貝20個(gè)字節(jié)，即5個(gè)int元素
	float arr3[] = { 1.0f,2.0f,3.0f,4.0f };
	float arr4[5] = { 0.0 };
	memcpy(arr3, arr4, 8);    //拷貝8個(gè)字節(jié)，即2個(gè)float元素
	return 0;
}

//示例來(lái)自cplusplus官網(wǎng)
/* memcpy example */
#include <stdio.h>
#include <string.h>
struct {
  char name[40];
  int age;
} person, person_copy;
int main ()
{
  char myname[] = "Pierre de Fermat";    //定義一個(gè)字符串
  /* 用 memcpy 拷貝字符串 */
  //每個(gè)char類型占一個(gè)字節(jié)，因此要拷貝的字節(jié)數(shù)即strlen()+1，加一是因?yàn)橐?\0'也拷貝過(guò)去。
  memcpy ( person.name, myname, strlen(myname)+1 );
  person.age = 46;
  /* 用 memcpy 拷貝結(jié)構(gòu)體 */
  //sizeof操作符，可以直接得到結(jié)構(gòu)體變量在內(nèi)存中所占的字節(jié)數(shù)。
  memcpy ( &person_copy, &person, sizeof(person) );
  //直接完成了結(jié)構(gòu)體之間的數(shù)據(jù)拷貝：從person拷貝到person_cpy，不用手動(dòng)轉(zhuǎn)義，非常方便
  printf ("person_copy: %s, %d \n", person_copy.name, person_copy.age );
  return 0;
}

void * memmove ( void * dest, const void * src, size_t num );

#include<stdio.h>
#include<string.h>
//模擬實(shí)現(xiàn)memmove()
void* my_memmove(void* dest, const void* src, size_t num) 
{
	void* ret = dest;	//保存結(jié)果用于輸出
	//從前向后拷貝，也可以寫(xiě)成if(dest <= src || (char*)dest >= (char*)src + num)
	if (dest <= src)
	{
		while (num--) {
			*(char*)dest = *(char*)src;    //拷貝
			dest = (char*)dest + 1;   
			src = (char*)src + 1;     //指針從前向后移動(dòng)（從低地址向高地址移動(dòng)）
		}
	}
	else    //從前向后拷貝
	{
		dest = (char*)dest + num - 1;    
		src = (char*)src + num - 1;    //初始化兩指針至各自范圍的最后
		while (num--) {
			*(char*)dest = *(char*)src;    //拷貝    
			dest = (char*)dest - 1;    
			src = (char*)src - 1;    //指針從后向前移動(dòng)
		}
	}
	return ret;    //返回值為dest
}
//測(cè)試代碼///
int main() {
	int arr1[] = { 2,3,4,5,6 };
	my_memmove(arr1+2, arr1, 8);
	//預(yù)計(jì) 2 3 2 3 6
	for (int i = 0; i < 5; i++) {
		printf("%d ", arr1[i]);
	}
	printf("\n-------------------\n");
	int arr2[] = { 2,3,4,5,6 };
	memmove(arr2 + 2, arr2, 8);
	for (int i = 0; i < 5; i++) {
		printf("%d ", arr2[i]);
	}
	return 0;
}

//錯(cuò)誤的代碼
void* my_memmove(void* dest, const void* src, size_t num)
{
    void * ret = dest;
    while (num--) {
	//從前向后拷貝
	    if (dest <= src)
	    {
		    *(char*)dest = *(char*)src;
		    dest = (char*)dest + 1;
	    	src = (char*)src + 1;
	    }
	    else
	    {
		    dest = (char*)dest + num - 1;    //錯(cuò)誤
		    src = (char*)src + num - 1;    //錯(cuò)誤
		    *(char*)dest = *(char*)src;
		    dest = (char*)dest - 1;
		    src = (char*)src - 1;
	    }
    return ret;
}

//示例來(lái)自cplusplus官網(wǎng)
/* memmove example */
#include <stdio.h>
#include <string.h>
int main ()
{
  char str[] = "memmove can be very useful......";
  memmove (str+20,str+15,11);    //表示將包括str+15向后11個(gè)字節(jié)的內(nèi)容移動(dòng)到str+20位置
  puts (str);
  return 0; 
}

void * memset ( void * ptr, int value, size_t num );

//示例來(lái)自cplusplus官網(wǎng)
/* memset example */
#include <stdio.h>
#include <string.h>
int main ()
{
  char str[] = "almost every programmer should know memset!";
  memset (str,'-',6);    //表示將從str開(kāi)始，包括str向后6個(gè)字節(jié)的內(nèi)存內(nèi)容置為'-'
  puts (str);
  return 0;
}

int memcmp ( const void * ptr1, const void * ptr2, size_t num );

//示例來(lái)自cplusplus官網(wǎng)
/* memcmp example */
#include <stdio.h>
#include <string.h>
int main ()
{
  //創(chuàng)建兩個(gè)要用作比較的數(shù)組
  char buffer1[] = "DWgaOtP12df0";    
  char buffer2[] = "DWGAOTP12DF0";
  //接受比較的結(jié)果
  int n;
  //要比較的字節(jié)數(shù)為buffer1的長(zhǎng)度
  //兩字符串的比較可以用strcmp(buffer1,buffer2)函數(shù)實(shí)現(xiàn)，原理大致相同。
  n = memcmp ( buffer1, buffer2, sizeof(buffer1) );
  if (n>0) printf ("'%s' is greater than '%s'.\n",buffer1,buffer2);
  else if (n<0) printf ("'%s' is less than '%s'.\n",buffer1,buffer2);
  else printf ("'%s' is the same as '%s'.\n",buffer1,buffer2);
  return 0;
}