c语言各种字符串操作函数实现

1，strncpy实现

按照man strncpy官方实现版本

 char *strncpy(char *dest, const char *src, size_t n)
 {
      size_t i;

      for (i = 0; i < n && src[i] != ''; i++)
           dest[i] = src[i];

      for ( ; i < n; i++)
             dest[i] = '';

      return dest;
 }

一般别人实现

char *strncpy(char *dest, const char *src, int n)
{
    char *ret = dest;
    assert(dest);
    assert(src);

    while(n && (*dest++ = *src++) != '')
        n--;

    *dest = '';

    return ret;
}

2, strncat实现

官方版本

char * strncat(char *dest, const char *src, size_t n)
{
   size_t dest_len = strlen(dest);
   size_t i;

   for (i = 0 ; i < n && src[i] != '' ; i++)
	   dest[dest_len + i] = src[i];
   dest[dest_len + i] = '';

   return dest;
}

别人实现版本

char *mystrncat(char *dest, const char *src, int n)
{
	char *ret = dest;
	assert(dest);
	assert(src);
	while (*dest != '')
	{
		dest++;
	}
	while (n && (*dest++ = *src++) != '')
	{
		n--;
	}
	dest = '';
	return ret;
}

3， strncmp实现

int mystrcmp(const char *str1, const char *str2, int n)
{
	assert(n);
	while(--n && *str1 && *str1 == *str2)
	{
		str1++;
		str2++;
	}
	return *str1 - *str2;
}

4, strrchr实现

char *mystrrchr(const char *s, int c)
{
	assert(s);
	int i = 0;
	char *ch = NULL;
	char *start = (char *)s;
	
	while(*s != '')
		s++;
	
	
	for(; start != --s; )
	{
		if(*s == c)
		{
			return (char *)s;
		}
	}
	return NULL;
}

方便一点的

char *mystrrchr2(const char *s, int c)
{
	assert(s);
	int i = 0;
	char *start = (char *)s;
	
	while(*s != '')
		s++;
	
	
	while(--s != start && *s != c)
		;
	if( *s == c)
		return (char *)s;
	return NULL;
}