strlen、strcpy、strcat、strcmp函数编写

关于四个strlen，strcpy，strcat，strcmp函数的编写方法

#include <assert.h>

size_t str_len(const char* str)
{
  assert(NULL != str);
  const char* tmp = str;
  while(*tmp) tmp++;
  return tmp - str;
}

char* str_cpy(char* dest,const char* src)
{
  assert(NULL!=dest && NULL!=src);
  char* tmp = dest;
  while(*tmp++ = *src++);
  return dest;
}

char* str_cat(char* dest,const char* src)
{
  assert(NULL!=dest && NULL!=src);
  char* tmp = dest;
  while(*tmp)tmp++;
  while(*tmp++ = *src++);
  return dest;
}

int str_cmp(const char* s1,const char* s2)
{
  assert(NULL!=s1 && NULL!=s2);
  while(*s1 == *s2 && *s1) s1++,s2++;
  if(*s1 > *s2)
    return 1;
  if(*s1 < *s2)
    return -1;
  return 0;
}