目录

memcpy

模拟实现memcpy

memmove

模拟实现memmove

memcmp

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memcpy

它的函数原型为：

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。

例子：

#include <stdio.h>
#include <string.h>
struct
{char name[40];int age;
} person, person_copy;
int main()
{char myname[] = "Pierre de Fermat";//用memcpy拷贝字符串memcpy(person.name, myname, strlen(myname) + 1);person.age = 46;//用memcpy拷贝结构体memcpy(&person_copy, &person, sizeof(person));printf("person_copy: %s, %d \n", person_copy.name, person_copy.age);return 0;
}

输出结果为：

模拟实现memcpy

void* memcpy(void* dst, const void* src, size_t count)
{void* ret = dst;assert(dst);assert(src);//拷贝从低地址到高地址while (count--) {*(char*)dst = *(char*)src;dst = (char*)dst + 1;src = (char*)src + 1;}return(ret);
}

memmove

memmove的函数原型是：

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。

如果源空间和目标空间出现重叠，就得使用memmove函数处理。

例子：

#include <stdio.h>
#include <string.h>
int main()
{char str[] = "memmove can be very useful......";memmove(str + 20, str + 15, 11);puts(str);return 0;
}

输出结果为：

模拟实现memmove

void* memmove(void* dst, const void* src, size_t count)
{void* ret = dst;if (dst <= src || (char*)dst >= ((char*)src + count)) {/** Non-Overlapping Buffers* copy from lower addresses to higher addresses
*/while (count--) {*(char*)dst = *(char*)src;dst = (char*)dst + 1;src = (char*)src + 1;}}else {/** Overlapping Buffers* copy from higher addresses to lower addresses*/dst = (char*)dst + count - 1;src = (char*)src + count - 1;while (count--) {*(char*)dst = *(char*)src;dst = (char*)dst - 1;src = (char*)src - 1;}}return(ret);
}

memcmp

它的函数原型为：

int memcmp( const void *buf1, const void *buf2, size_t count );

作用是比较从ptr1和ptr2指针开始的num个字节。

查找它的文档：

Return Value

The return value indicates the relationship between the buffers.

Return Value	Relationship of First count Bytes of buf1 and buf2
< 0	buf1 less than buf2
0	buf1 identical to buf2
> 0	buf1 greater than buf2

由此可知buf1小于buf2时返回值小于0，buf1等于buf2时返回值为0，buf1大于buf2时返回值大于0。

例子：

void* memmove(void* dst, const void* src, size_t count)
{void* ret = dst;if (dst <= src || (char*)dst >= ((char*)src + count)) {while (count--) {*(char*)dst = *(char*)src;dst = (char*)dst + 1;src = (char*)src + 1;}}else {dst = (char*)dst + count - 1;src = (char*)src + count - 1;while (count--) {*(char*)dst = *(char*)src;dst = (char*)dst - 1;src = (char*)src - 1;}}return(ret);
}