C基础（41——45）

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#include <stdio.h>#include <stdlib.h>#include <assert.h>char* Strcat(char * s1,char* s2){                 assert(s1 );                 assert(s2 );                 char* tmp=s1 ;                 while(*tmp!='\0' )  //注意此处不能用while(*tmp++!='\0');因为这样的话，到最后tmp指向的是'\0'的下一个位置，而不是'\0'                {                                tmp++;                }                 //出来tmp指向'\0',然后执行下一条语句，就把'\0'覆盖了                 while((*tmp++=*s2 ++)!='\0')                                ;                 return s1 ;}void test(){                 char str1[100]="hello " ;                 char str2[100]="world" ;                printf( "%s\n",Strcat(str1,str2));}int main(){                test();                system( "pause");                 return 0;}

结果：

#include <stdio.h>#include <stdlib.h>#include <assert.h>int Strcmp(const char* str1,const char* str2){                 assert(str1 );                 assert(str2 );                 while(*str1 || *str2)                {                                 if(*str1 >*str2)                                                 return *str1 -*str2;                                 else if (*str1<* str2)                                                 return *str1 -*str2;                                 else                                {                                                 str1++;                                                 str2++;                                }                }                 return 0;}//库函数的实现//int Strcmp(const char* str1,const char* str2)//{//             while(*str1==*str2)//             {//                             if(*str1=='\0')//                                             return 0;////                             str1++;//                             str2++;//             }////             //不相等//             return *str1-*str2;//}void test(){                 char* s1="abcde" ;                 char* s2="abcd" ;                printf( "%d\n",Strcmp(s1,s2));}int main(){                test();                system( "pause");                 return 0;}

结果：

memcpy：从src所指的内存地址的起始位置开始，拷贝n个字节到目标dst所指的内存的起始位置

它没有考虑内存重叠的问题，所以出现了memmove()，memmove考虑到了内存重叠的问题。

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <assert.h>void* Memcpy(void * dst,const void* src,size_t n){                 assert(dst );                 assert(src );                 char* pdst=(char *)dst;                 const char * s = (char*)src;                 while(n --)                {                                *pdst++=*s++;                }                 return dst ;}void test(){                 char dst[100]="hello  world" ;                 char* src="change myself" ;                printf( "%s\n",Memcpy(dst,src,6));}int main(){                test();                system( "pause");                 return 0;}

结果：

memmove()：从src拷贝n个字节到dst，memmove保证在原串被覆盖之前，将重叠区域的字节拷贝到目标区域中。但覆盖后src内容会被修改。但当目标区域和原区域没有重叠时，则和memcopy()函数功能相同

#include <stdio.h>#include <stdlib.h>#include <assert.h>void* Memmove(void * dst,void* src,size_t n){                 assert(dst );                 assert(src );                 char* pdst=(char *)dst;                 char* psrc=(char *)src;                 if(psrc<pdst && psrc+n >pdst)  //内存重叠，倒着拷贝                {                                psrc=psrc+ n-1;                                pdst=pdst+ n-1;                                 while(n --)                                {                                                *pdst--=*psrc--;                                }                }                 else  //内存没有重叠，正常拷贝，等同于memcpy                {                                 while(n --)                                {                                                *pdst++=*psrc++;                                }                }                 return dst ;}void test(){                 char str1[100]="hello" ;                 char str2[100]="hello world" ;                printf( "%s\n",Memmove(str1+2,str1,6));  //测试倒着拷贝hello                printf( "%s\n",Memmove(str2,str2+6,6));  //测试正常拷贝，将'\0'也拷进去了,所以结果是world                 //printf("%s\n",Memmove(str2,str2+6,5));  //测试正常拷贝，没有将'\0'拷贝进去，所以结果是world world}int main(){                test();                system( "pause");                 return 0;}

结果：

memset：将已开辟内存空间的s的前n个字节用ch替换，并返回s

#include <stdio.h>#include <stdlib.h>#include <assert.h>#define N 10void* Memset(void * s,int ch,size_t n){                 assert(s );                 char* ps=(char *)s;                 while(n --)                {                                 if(*ps)                                                *ps++= ch;                }                 return s ;}void test(){                 int arr[N ];                Memset(arr,0, sizeof(arr));                 for(int i=0;i<N;i++)                {                                printf( "%d ",arr[i]);                }                printf( "\n");}int main(){                test();                system( "pause");                 return 0;}

结果：

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