C部分库函数的模拟实现

1. 模拟实现strlen

方法1：

int my_strlen( const char*src)
{
	int count = 0;
	assert(src != NULL);
	while(*src != '\0')
	{
		count++;
		src++;
	}
	return count;
}

方法2：

int my_strlen( const char*src)
{
	 char*p = src;
	 assert(src != NULL);
	 while(*src != '\0')
	 {
		 src++;
	 }
	 return src - p;//两个指针指向同一数组空间，两个指针相减指的是相差元素的个数
 }

方法3：

int my_strlen( const char*src)
 {
	 if(*src == '\0')
		 return 0;
	 else
		 return 1+my_strlen(src+1);
 }

方法4：

int my_strlen( const char*src)
 {
	 int i = 0;
	 for(i = 0; src[i] != '\0'; i++)
	 {
		 ;
	 }
	 return i;
 }

2. 模拟实现strcpy

方法1：

my_strcpy(char*dest,const char*src)
{
	while(*src != '\0')
	{
		*dest = *src;
		dest++;
		src++;
	}
}

方法2：[对方法1的优化]

char* my_strcpy(char*dest,const char*src)
{
	char* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while(*dest++ == *src++)
		;
	return ret;
}

3. 模拟实现strcat

    char* my_strcat(char*dest, const char*src)
    {
    	char*ret = dest;
    	assert(dest != NULL);
    	assert(src! = NULL);
    	while(*dest != '\0')
    	{
    		dest++;
    	}
    	while(*dest++ = *src++)
    		;
    	return ret;
    }

4. 模拟实现strstr

char* my_strstr(const char*dest, const char*src)
{
	assert(dest != NULL);
	assert(src != NULL);
	
	char *start = dest;
	char *str1 = dest;
	char *str2 = src;

	while(*start)
	{
		str1 = start;
		str2 = src;
		while(*str1 && *str2 && *str1 == *str2)
		{
			str1++;
			str2++;
		}
		if(str2 == ‘\0’)
			return start;
		start++;
	}
	return NULL;
}

5. 模拟实现strcmp

int my_strcmp(const char*str1, const char*str2)
{
	assert(str1 != NULL);
	assert(str2 != NULL);
	while(*str1 == *str2)
	{
		str1++;
		str2++;
	}
	return *str1 - *str2;
}

6. 模拟实现memcpy

void* my_memcpy(void* dest, const void* src, int n)
{
    assert(dest != NULL);
    assert(src != NULL);
	
	char *str1 = (char *)dest;
	char *str2 = (char *)src;
   
    while (n--)
    {
        *str1++ = *str2++;
    }
    return dest;
}

7. 模拟实现memmove

void* my_memmove(void*dest, const void*src, int count)
{
	assert(dest != NULL);
	assert(src != NULL);
	char *str1 = (char *)dest;
	char *str2 = (char *)src;

	if(str1 <= str2)
	{
		while(count--)
		{
			*str1++ = *str2++;
		}
	}
	else
	{
		while(count--)
		{
			*(str1 + count) = *(str2 + count);
		}
	}
	return dest;
}