库函数的模拟实现

• • 模拟实现strcpy
  • 模拟实现strcat
  • 模拟实现strstr
  • 模拟实现strcmp

模拟实现strcpy

#include<stdio.h>
#include<windows.h>
#include<assert.h>

size_t myStrlen(const char* str)
{
	assert(str);
	//1
	/*int length = 0;
	while (*str)
	{
		str++;
		length++;
	}
	return length;*/
	//2
	const char* start = str;
	const char* end = str;
	while (*end){
		end++;
	}
	return end - start;
}
int main()
{
	const char *str = "Hello world!";
	printf("%d\n", myStrlen(str));
	
	system("pause");
	return 0;
}

模拟实现strcat

#include<stdio.h>
#include<windows.h>
#include<assert.h>

char* myStrcat(char* dst, const char* src)
{
	assert(dst);
	assert(src);

	char* ret = dst;
	while (*dst != '\0')
	{
		dst++;
	}
	while (*src != '\0')
	{
		*dst++ = *src++;
	}
	return ret;
}
int main()
{
	const char *src = "Hello world!";
	char dst[32];
	myStrcpy(dst, src);
	myStrcat(dst, src);

	system("pause");
	return 0;
}

模拟实现strstr

#include<stdio.h>
#include<windows.h>
#include<assert.h>

const char* myStrstr(const char* str, const char* sub)
{
	assert(str);
	assert(sub);

	const char* str_start = str;
	const char* str_p = str;

	const char* sub_p = sub;

	while (*str_start)
	{
		str_p = str_start;
		sub_p = sub;
		while (*sub_p && *str_p == *sub_p)
		{
			str_p++; sub_p++;
		}
		if (*sub_p == '\0')
		{
			return str_start;
		}
		str_start++;
	}
	return NULL;
}
int main()
{
	char* str = "hello 123 abc!";
	const char* sub = "123";
	
	printf("%s\n", myStrstr(str, sub));
	system("pause");
	return 0;
}

模拟实现strcmp

#include<stdio.h>
#include<windows.h>
#include<assert.h>

int myStrcmp(const char* str1, const char* str2)
{
	assert(str1);
	assert(str2);

	int ret = 0;
	while (!(ret = *(unsigned char*)str1 - *(unsigned char*)str2) && *str1)
	{
		str1++; str2++;
	}
	if (ret > 0)
	{
		return 1;
	}
	else if (ret < 0)
	{
		return -1;
	}
	else
		return 0;
}
int main()
{
	const char* str1 = "hello 123 abc!";
	const char* str2 = "hello 123 abc!";

	printf("ret = %d\n", myStrcmp(str1, str2));

	system("pause");
	return 0;
}