strcpy strcat strlen

char * mystrcat(char *dest,const char *src){    char * p = dest;    while(*dest != '\0')        dest++;    while((*dest++=*src++) != '\0')        ;       return p;}char * mystrcpy(char *dest,const char *src){    char * p = dest;    while((*dest++=*src++) != '\0')      ;       return p;}int mystrlen(char *str){    int len=0;    if(str==NULL)      return 0;    while(*str++!='\0')      len++;    return len;}