面试：C/C++常见库函数实现

 1 void *mymemcpy(void *dest, const void* src, size_t n) {
 2     assert(dest != NULL && src != NULL);
 3 
 4     //考虑内存重叠
 5     if (src < dest && (char*) src + n > dest) {
 6 
 7         //void*类型强制转换为char*
 8         char* psrc = (char*) src + n - 1;
 9         char* pdst = (char*) dest + n - 1;
10 
11         while (n--) {
12             *pdst-- = *psrc--;
13         }
14     } else {
15         char* psrc = (char*) src;
16         char* pdst = (char*) dest;
17         while (n--) {
18             *pdst++ = *psrc++;
19         }
20     }
21     return dest;
22 }

1 void* mymemset(void* s, int c, size_t n) {
2     if (s == NULL) return NULL;
3     char* ps = (char*) s;
4     while (n--) {
5         *ps++ = (char) c;
6     }
7     return s;
8 }

 1 char* mystrcpy(char* dest, const char* src) {
 2     if (dest == NULL || src == NULL) return NULL;
 3 
 4     int n = strlen(src);
 5     //memory overlap
 6     if (src < dest && src + n > dest) {
 7         char* pdst = dest + n;
 8         char* psrc = const_cast<char*>(src) + n;
 9         *pdst-- = *psrc--;
10         while (n--) {
11             *pdst-- = *psrc--;
12         }
13     } else {
14         char* pdst = dest;
15         while ((*pdst++ = *src++) != ‘\0‘)
16             ;
17     }
18     return dest;
19 }

 1 char* mystrncpy(char* dest, char* src, size_t n) {
 2     if (dest == NULL || src == NULL)
 3         return NULL;
 4 
 5     if (src < dest && src + n > dest) {
 6         char* pdst = dest + n - 1;
 7         char* psrc = src + n - 1;
 8         *(pdst + 1) = ‘\0‘;
 9         while (n--) {
10             *pdst-- = *psrc--;
11         }
12     } else {
13         char* pdst = dest;
14         while (n--) {
15             *pdst++ = *src++;
16         }
17         *pdst = ‘\0‘;
18     }
19     return dest;
20 }

 1 char* mystrcat(char* dest, const char* src) {
 2     if (dest == NULL || src == NULL)
 3         return NULL;
 4 
 5     int n = strlen(src);
 6     char* pdst = dest;
 7     while (*pdst != ‘\0‘) {
 8         pdst++;
 9     }
10     while (n--) {
11         *pdst++ = *src++;
12     }
13     pdst = ‘\0‘;
14     return dest;
15 }

1 int mystrcmp(const char* s1, const char* s2) {
2     assert(s1 != NULL && s2 != NULL);
3     while (s1 && s2 && *s1 == *s2) {
4         s1++;
5         s2++;
6     }
7     return *s1 - *s2;
8 }

1 int mystrlen(const char* s) {
2     assert(s != NULL);
3     int len = 0;
4     while (*s++ != ‘\0‘) {
5         len++;
6     }
7     return len;
8 }