C语言三个函数的模拟实现详解

//模拟实现strcpy
#include<stdio.h>
#include<assert.h>
char* my_strcpy(char*dest, char*str)
{
	assert(dest && str);
	char* tmp = dest;
	while (*str != '\0')
	{
		*dest = *str;
		dest++;
		str++;
	}
	*dest = '\0';
	return tmp;
}
int main()
{
	char arr1[20] = "xxxxxxxxxxxxxxx";
	char arr2[] = "study hard!";
	my_strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

//模拟实现strcat
//字符串追加
#include<stdio.h>
#include<assert.h>
char* my_strcat(char* dest, const char* str)
{
	assert(dest && str);
	char* tmp = dest;
	//找到目标的尾巴
	while (*dest != '\0')
	{
		dest++;
	}
	//链接字符串
	while (*dest++ = *str++)
	{
		;
	}
	*dest = '\0';
	return tmp;
}
int main()
{
	char arr1[20] = "hello";
	char arr2[] = " world!";
	my_strcat(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

//模拟实现strcmp 返回的不是1 -1，而是正数，负数
#include<stdio.h>
#include<assert.h>
int my_strcmp(const char* str1, const char* str2)
{
	assert(str1&&str2);
	while (*str1 == *str2)
	{
		if (*str1 == '\0')
		{
			return 0;
		}
		str1++;
		str2++;
	}
	return *str1 - *str2;
}
int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "abcder";
	int ret=my_strcmp(arr1, arr2);
	if (ret < 0)
	{
		printf("<\n");
	}
	else if (ret == 0)
	{
		printf("=\n");
	}
	else
	{
		printf(">\n");
	}
	return 0;
}