string.h中一些函数的实现

面试经常会考一些常用函数的实现.尤其是string.h中的函数.准备先!

程序都是在gcc下面调试成功。

 

1 char* strcpy(char *to,const char *from);

 

#include<stdio.h> #include<stdlib.h> #include<string.h> #include<assert.h> /*函数原型char *strcpy(char *to,const char *from); *功能:复制字符串from中的字符到字符串to,包括空值结束符. *返回值为指针to.*/ char* strcpy2(char *strDest,const char *strSrc) { /*使用断言，防止strDest或者strSrc为NULL*/ assert(strDest!=NULL && strSrc!=NULL); char *tmp = strDest; while((*strDest++=*strSrc++) != '/0') ; return tmp; } int main() { char *strSrc = "hello,world"; char *strDest = (char*)malloc(strlen(strSrc)+1); printf("%s",strcpy2(strDest,strSrc)); printf("/n"); return 0; }

 

2 void *memcpy(void *to,const void *from,size_t count);

#include<stdio.h> #include<string.h> #include<assert.h> /*void *memcpy(void *to,const void *from,size_t count); * 功能:函数从from中复制count个字符到to中,并返回to指针. * 如果to和from重叠,则函数行为不确定*/ void* memcpy2(void* pDest,const void* pSrc,size_t size) { assert(pDest!=NULL && pSrc!=NULL); typedef char byte; byte *pTmpD = (byte*)pDest; byte *pTmpS = (byte*)pSrc; /*注意:只是内存之间的拷贝,与字符无关*/ while(size-->0) { *pTmpD++ = *pTmpS++; } return pDest; } int main() { char Dest[100]; char *Src = "hello world"; memcpy2(Dest,Src,sizeof(char)*(strlen(Src)+1)); printf("%s/n",Dest); return 0; }

 

3 void *memmove(void *to,const void *from,size_t count);

#include<stdio.h> #include<string.h> #include<assert.h> /*void *memmove(void *to,const void *from,size_t count) *功能:与memcpy相同,不同的是当to和from重叠时,函数仍能正常工作.*/ void *memmove2(void *to,const void *from,size_t count) { assert( (to != NULL) && (from != NULL)); char *dest = to; const char *source = from; if(source < dest && dest < source + count) { /*copy backwards*/ for(dest += count,source +=count;count > 0;--count) { *--dest = *--source; } } else { /*copy forwards*/ while(count-->0) { *dest++ = *source++; } } return to; } int main() { char source[100] = "hello world!"; char dest[100]; printf("%s/n",memmove2(dest,source,7)); return 0; }