#include
     
      char *mystrcpy(char *dest,char *src);unsigned int mystrlen(char *src);char *mystrcat(char *dest,char *src);int main(int argc, const char *argv[]){    char str[20] = { 0 };    char *p = NULL;    p = "hello";    char *p1 = "world";    mystrcpy(str,p);    mystrcat(str,p1);    puts(str);    printf("len = %d\n",mystrlen(str));        return 0;}unsigned int mystrlen(char *src){    char *head = src;    if(src == NULL)        return 0;    while(*src != '\0')        src++;    return src - head;}char *mystrcpy(char *dest,char *src){    char *old = dest;    if(dest == NULL||src == NULL)        return NULL;    while(*src != '\0')        *dest ++ = *src++;        *dest ='\0';    return old;    }char *mystrcat(char *dest,char *src){    char *old = dest;    if (dest == NULL||src == NULL)        return NULL;    while(*dest != '\0')        dest++;    while(*src != '\0')        *dest++ = *src++;    *dest = '\0';    return old;}