库函数memcpy()与memmove()实现
原文:http://www.cppblog.com/zenliang/articles/131782.html
根据MSDN文档,当源区域与目标区域存在重叠时,memcpy()函数报错,而memmove()函数可以处理重叠情况!
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/*
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* 函数名: memcpy
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* 功 能: 从源source中拷贝n个字节到目标destin中
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* 用 法: void *memcpy(void* destin, const void* source, size_t n);
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* 说 明: 内存拷贝
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*/
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#include <stdio.h>
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#include <conio.h> //getch头文件
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#include <assert.h> //assert头文件
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typedef unsigned char byte;
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//typedef unsigned int size_t;
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/*
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memcpy函数,如果内存重叠则报错
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*/
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//src要保留
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void* memcpy(void* dst,const void* src,size_t count)
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{
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byte* pbTo = (byte*)dst;
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byte* pbFrom = (byte*)src;
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assert(dst!= NULL && src != NULL);//不能存在空指针
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assert(pbTo >= pbFrom+count || pbFrom >= pbTo + count);//防止内存重叠(overlap)
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while (count-- > 0)
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{
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*pbTo++ = *pbFrom++;
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}
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return dst;
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}
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/*
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memmove函数,考虑了内存重叠的情况
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*/
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//src可以不保留
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void* memmove(void* dst,const void* src,size_t count)
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{
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byte* pbTo = (byte*)dst;
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byte* pbFrom = (byte*)src;
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assert(dst != NULL && src != NULL);//不能存在空指针
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if (dst <= src || pbTo >= pbFrom + count)//
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{
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while (count-- > 0)
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{
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*pbTo++ = *pbFrom++; //按递增拷贝
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}
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}
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else //
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{
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pbTo = pbTo + count -1;//overlap的情况,从高位地址向低位拷贝
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pbFrom = pbFrom + count -1;
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while (count-- > 0)
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{
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*pbTo-- = *pbFrom--; //按递减拷贝
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}
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}
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return dst;
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}
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