[Advanced C language] Simulation implementation of major common library functions

foreword

What Hengchuan brings to you today is the library functions that are commonly used. Hengchuan will simulate and implement them for everyone. I hope it will be helpful to everyone! !

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1. Simulate the implementation of strlen

Three ways:
Way 1:

//计数器方式
int my_strlen(const char* str)
{
    
    
	int count = 0;
	while (*str)
	{
    
    
		count++;
		str++;
	}
	return count;
}
int main()
{
    
    
	int len = my_strlen("abcdef");
	printf("%d\n", len);

	return 0;
}

Method 2:

//不能创建临时变量计数器
//递归
int my_strlen(const char* str)
{
    
    
	if (*str == '\0')
		return 0;
	else
		return 1 + my_strlen(str + 1);
}
int main()
{
    
    
	int len = my_strlen("abcdef");
	printf("%d\n", len);

	return 0;
}

Method 3:

//指针-指针的方式
int my_strlen(char* str)
{
    
    
	char* start = str;
	while (*str != '\0')
		str++;
	return str - start;//指针-指针得到的是元素个数
}

int main()
{
    
    
	int len = my_strlen("abcdef");
	printf("%d\n", len);

	return 0;
}

2. Simulate the implementation of strcpy

Reference Code:

//1.参数顺序
//2.函数的功能，停止条件
//3.assert
//4.const修饰指针
//5.函数返回值
//6.题目出自《高质量C/C++编程》书籍最后的试题部分
char* my_strcpy(char* dest, const char* src)
{
    
    
	char* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);

	while ((*dest++ = *src++))
	{
    
    
		;
	}
	return ret;
}

3. Simulate the implementation of strcat

Reference Code:

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

4. Simulate the implementation of strstr

char* strstr(const char* str1, const char* str2)
{
    
    
	char* cp = (char*)str1;
	char* s1, * s2;
	if (!*str2)
		return((char*)str1);
	while (*cp)
	{
    
    
		s1 = cp;
		s2 = (char*)str2;
		while (*s1 && *s2 && !(*s1 - *s2))
			s1++, s2++;
		if (!*s2)
			return(cp);
		cp++;
	}
	return(NULL);
}

5. Simulate the implementation of strcmp

Reference Code:

int my_strcmp(const char* src, const char* dst)
{
    
    
	int ret = 0;
	assert(src != NULL);
	assert(dest != NULL);
	while (!(ret = *(unsigned char*)src - *(unsigned char*)dst) && *dst)
		++src, ++dst;
	if (ret < 0)
		ret = -1;
	else if (ret > 0)
		ret = 1;
	return(ret);
}

6. Simulate the implementation of memcpy

Reference Code:

void* memcpy(void* dst, const void* src, size_t count)
{
    
    
	void* ret = dst;
	assert(dst);
	assert(src);
	       /*
	        * copy from lower addresses to higher addresses
	        */
	while (count--) {
    
    
		*(char*)dst = *(char*)src;
		dst = (char*)dst + 1;
		src = (char*)src + 1;
	}
	return(ret);
}

7. Simulate memmove

Reference Code:

void* memmove(void* dst, const void* src, size_t count)
{
    
    
	void* ret = dst;
	if (dst <= src || (char*)dst >= ((char*)src + count)) {
    
    
		               /*
		                * Non-Overlapping Buffers
						*  * copy from lower addresses to higher addresses
                 */
		while (count--) {
    
    
			*(char*)dst = *(char*)src;
			dst = (char*)dst + 1;
			src = (char*)src + 1;
		}
	}
	else {
    
    
		               /*
		                * Overlapping Buffers
		                * copy from higher addresses to lower addresses
		                */
		dst = (char*)dst + count - 1;
		src = (char*)src + count - 1;
		while (count--) {
    
    
			*(char*)dst = *(char*)src;
			dst = (char*)dst - 1;
			src = (char*)src - 1;
		}
	}
	return(ret);
}

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