面试：C/C++常见库函数实现

1. void *mymemcpy(void *dest, const void* src, size_t n)；

内存拷贝函数，memcpy函数的功能是从源src所指的内存地址的起始位置开始拷贝n个字节到目标dest所指的内存地址的起始位置中

 void* memcpy(void* dest,void* src,size_t n){
     assert(dest != NULL && src != NULL);

     if(src < dest && (char*)src + n > dest){
         char* pdst = (char*)dest+n-;
         char* psrc = (char*)src+n-;
         while(n--){
             *pdst-- = *psrc--;
         }
     }else{
         char* pdest = (char*)dest;
         char* psrc = (char*)src;
         while(n--){
             *pdest++ = *psrc++;
         }
     }
     return dest;
 }

2.void *memset(void *s, int c, size_t n);

 void* memset(void* src,int c,size_t n){
     assert(src != NULL);
     char* psrc = (char*)src;
     while(n--){
         *psrc++ = (char)c;
     }
     return src;
 }

3. char* strcpy(char* dest,const char* src);

 char* strcpy(char* dest,const char* src){
     assert(dest != NULL && src != NULL);

     size_t n = strlen(src);
     if(src < dest && src + n > dest){
         char* pdst = dest + n;
         *pdst-- = '\0';
         src = src + n - ;
         while(n--){
             *pdst-- = *src--;
         }
     }else{
         char* pdst = dest;
         while((*pdst++ = *src++) != );
     }
     return dest;
 }

4. char* mystrncpy(char* dest,char* src,size_t n);

 char* strncpy(char* dest,const char* src,size_t n){
     assert(dest != NULL && src != NULL);

     if(src < dest && src + n > dest){
         int m = strlen(src) < n ? strlen(src):n;
         char* pdst = dest+m;
         *pdst-- = '\0';
         src = src + m-;
         while(m--){
             *pdst-- = *src--;
         }
     }else{
         char* pdst = dest;
         size_t i = ;
         while(i++ < n && (*pdst++ = *src++) != '\0');
         if(*(pdst-) != '\0') *pdst = '\0';
     }
     return dest;
 }

5.char* strcat(char* dest,const char* src)

 char* strcat(char* dest,const char* src){
     assert(dest != NULL && src != NULL);
     char* pdst = dest;
     while(*pdst != '\0'){
         pdst++;
     }
     while((*pdst++ = *src++) != '\0');
     return dest;
 }

6.int strcmp(const char* s1,const char* s2)

 int strcmp(const char* s1,const char* s2){
     assert(s1 != NULL && s2 != NULL);
     while(s1 && s2 && *s1 == *s2){
         s1++;
         s2++;
     }
     return *s1 - *s2;
 }

7.size_t strlen(const char* s)

size_t strlen(const char* src){
    assert(src != NULL);
    size_t ret = ;
    while(*src++ != '\0'){
        ret++;
    }
    return ret;
}