strcpy、strncpy、strlen、memcpy、memset、strcat、strncat、strcmp、strncmp，strchr

1、strcpy

 #include<stdio.h>
 #include<assert.h>

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

     char *res = dest;
     while ((*dest++ = *src++) != '\0');

     return res;
 }

 int main()
 {
     char *src = "";
     char dest[];
     printf("%s\n", src);
     char *res = mystrcpy(dest, src);
     printf("%s\n", res);

     return ;
 }

2、strncpy

 #include<stdio.h>
 #include<assert.h>

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

     char *res = dest;
     int i = ;
     while (((*dest++ = *src++) != '\0') && (i++<size));

     *(res + size) = '\0';

     return res;
 }

 int main()
 {
     char *src = "";
     printf("%s\n", src);
     char dest[];
     char *res = mystrncpy(dest, src, );
     printf("%s\n", res);

     return ;
 }

3、strlen

（1）非递归实现

 #include<stdio.h>
 #include<assert.h>

 int mystrlen(const char *str)
 {
     assert(str != NULL);

     int len = ;
     while ((*str++) != '\0')
         ++len;

     return len;
 }

 int main()
 {
     char *str = "";
     int len = mystrlen(str);
     printf("%d\n", len);

     return ;
 }

（2）递归实现

 #include<stdio.h>
 #include<assert.h>

 int strlenRecur(const char *str)
 {
     if (str == NULL || *str == '\0')
         return ;
     return strlenRecur(str + ) + ;
 }

 int main()
 {
     char *str = "";
     int len = strlenRecur(str);
     printf("%d\n", len);

     return ;
 }

4、memcpy

memcpy 和 strncpy 最大的区别是 memcpy不会遇到 '\0' 结束拷贝 。

（1）version1（不能解决数据区重叠问题）：

 #include<stdio.h>
 #include<assert.h>

 char* mymemcpy1(char *dest, const char *src, size_t size)
 {
     assert((dest != NULL) && (src != NULL));

     char *res = dest;
     while (size)
     {
         *(dest++) = *(src++);
         --size;
     }
     *(dest + size) = '\0';

     return res;
 }

 int main()
 {
     char *src = "";
     char dest[];
     char *res = mymemcpy1(dest, src,);
     printf("%s\n", res);

     return ;
 }

（2）version2（解决了数据区重叠问题）：

 #include<stdio.h>
 #include<assert.h>

 char *mymemcpy2(char *dest, const char *src, size_t size)
 {
     assert((dest != NULL) && (src != NULL));
     char *res = dest;

     if (dest > src&&dest < (src + size))
     {
         dest = dest + size - ;
         src = src + size - ;
         while (size--)
             *dest-- = *src--;
     }
     else
         while (size--)
             *dest++ = *src++;
     *(dest + size) = '\0';

     return res;
 }

 int main()
 {
     char *src = "";
     char dest[];
     char *res = mymemcpy2(dest, src,);
     printf("%s\n", res);

     return ;
 }

5、memset

 #include<stdio.h>
 #include<assert.h>

 void *mymemset(void *buf, const char c, size_t size)
 {
     assert(buf != NULL);
     char *res = (char*)buf;
     char *str = res;
     while (size--)
         *str++ = c;

     return res;
 }

 int main()
 {
     char *src = "";
     printf("%s\n", src);
     char dest[];
     void *buf = (char*)dest;
     char *res = (char *)(mymemset(buf, 'a', ));
     printf("%s\n", res);

     return ;
 }

6、strcat

 #include<stdio.h>
 #include<assert.h>

 char *mystrcat(char *dest, const char *src)
 {
     assert((dest != NULL) && (src != NULL));
     char *res=dest;
     while (*dest != '\0')
         ++dest;
     while (*src != '\0')
         *dest++ = *src++;

     return res;
 }

 int main()
 {
     char dest[] = "";
     char *src = "abc";
     char *res = mystrcat(dest, src);
     printf("%s\n", res);

     return ;
 }

7、strncat

 #include<stdio.h>
 #include<assert.h>

 char *mystrncat(char *dest, const char *src,size_t size)
 {
     assert((dest != NULL) && (src != NULL));
     char *res=dest;
     while (*dest != '\0')
         ++dest;
     while ((*src != '\0')&&(size--))
         *dest++ = *src++;
     *dest = '\0';

     return res;
 }

 int main()
 {
     char dest[] = "";
     char *src = "abcdef";
     char *res = mystrncat(dest, src,);
     printf("%s\n", res);

     return ;
 }

8、strcmp

 #include<stdio.h>
 #include<assert.h>

 int mystrcmp(const char *str1, const char *str2)
 {
     assert((str1 != NULL) && (str2 != NULL));
     while (*str1&&*str2 && (*str1++ == *str2++));

     return (*str1 - *str2);
 }

 int main()
 {
     char *str1 = "abcde";
     char *str2 = "abdf";
     int res = mystrcmp(str1, str2);
     printf("%d\n", res);

     return ;
 }

9、strncmp

 #include<stdio.h>
 #include<assert.h>

 int mystrncmp(const char *str1, const char *str2,size_t size)
 {
     assert((str1 != NULL) && (str2 != NULL));
     while (*str1&&*str2 && (*str1++ == *str2++)&&size--);

     return (*str1 - *str2);
 }

 int main()
 {
     char *str1 = "abcde";
     char *str2 = "abdfsd";
     int res = mystrncmp(str1, str2,);
     printf("%d\n", res);

     return ;
 }

10、strchr

 #include<stdio.h>
 #include<assert.h>

 char *mystrchr(const char *str, char c)
 {
     assert(str != NULL);
     while (*str != '\0' && (*str != c))
         ++str;

     if (*str == c)
         return (char*)str;
     else
         return NULL;
 }

 int main()
 {
     char *str = "abcde";
     char *res = mystrchr(str,'c');
     printf("%s\n", res);

     return ;
 }