内存函数的

1. memcpy使⽤和模拟实现

 void * memcpy ( void * destination, const void * source, size_t num );

• 函数memcpy从source的位置开始向后复制num个字节的数据到destination指向的内存位置。

• 这个函数在遇到 '\0' 的时候并不会停下来。

• 如果source和destination有任何的重叠，复制的结果都是未定义的 

 #include <stdio.h>
 #include <string.h>
 int main()
 {
 int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };
 int arr2[10] = { 0 };
 memcpy(arr2, arr1, 20);
 int i = 0;
 for (i = 0; i < 10; i++)
 {
 printf("%d ", arr2[i]);
 }
 return 0;
 }

 模拟

 void * memcpy ( void * dst, const void * src, size_t count)
 {
 void * ret = dst;
 assert(dst);
 assert(src);
 /*
 * copy from lower addresses to higher addresses
 */
 while (count--) {
 *(char *)dst = *(char *)src;
 dst = (char *)dst + 1;
 src = (char *)src + 1;
 }
 }
 return(ret);

2. memmove使⽤和模拟实现

 void * memmove ( void * destination, const void * source, size_t num );

• 和memcpy的差别就是memmove函数处理的源内存块和⽬标内存块是可以重叠的。

 #include <stdio.h>
 #include <string.h>
 int main()
 {
 int arr1[] = { 1,2,3,4,5,6,7,8,9,10 };
 memmove(arr1+2, arr1, 20);
 int i = 0;
 for (i = 0; i < 10; i++)
 {
 printf("%d ", arr1[i]);
}
 return 0;
 }

• 如果源空间和⽬标空间出现重叠，就得使⽤memmove函数处理。 

模拟实现

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
void* my_memove(void* dest, void* sou, size_t num)
{
	void* ret = dest;
	if (dest <= sou || ((char*)dest > (char*)sou + num))
	{
		while (num--)
		{
			*(char*)dest = *(char*)sou;
			dest = (char*)dest+1;
			sou = (char*)sou + 1;

		}
	}
	else
	{
		dest = (char*)dest + num - 1;
		sou = (char*)sou + num - 1;
		while (num--)
		{
			*(char*)dest = *(char*)sou;
			dest = (char*)dest - 1;
			sou = (char*)sou - 1;
		}
		return ret;
	}
}
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,0 };
	my_memove(arr + 2, arr, 20);
	for (int i = 0; i < 10; i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

3. memset函数的使⽤

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

memset是⽤来设置内存的，将内存中的值以字节为单位设置成想要的内容。 

 #include <stdio.h>
 #include <string.h>
 int main ()
 {
 char str[] = "hello world";
 memset (str,'x',6);
 printf(str);
 return 0;
 }

4. memcmp函数的使⽤

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

⽐较从ptr1和ptr2指针指向的位置开始，向后的num个字节 

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

 int main()
 {
 char buffer1[] = "DWgaOtP12df0";
 char buffer2[] = "DWGAOTP12DF0";
 int n;
 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;
 }