模拟实现库函数strncpy,strncmp,strncat

①模拟实现strncpy

函数作用：拷贝count个字符从源字符串到目标空间

                 如果源字符串的长度小于count，则拷贝完源字符串之后，在目标的后边追加null，直到count个

参考代码：

#include<stdio.h>
#include<assert.h>
char *my_strncpy(char *dest, const char *src,int count)
{
	assert(dest != NULL);
	assert(src != NULL);
	char *ret = dest;
	while (count && (*dest++ = *src++))
	{
		count--;
	}
	if (count > 0)
	{
		while (count--)
		{
			dest = NULL;
		}
	}//源字符串小于count，在后边追加NULL
	return ret;
}
int main()
{
	char str1[10] = { 0 };
	char *str2 = "abcdef";
	char *ret = my_strncpy(str1, str2, strlen(str2));
	printf("%s\n", ret);
	return 0;
}

②模拟实现strncat

参考代码：

#include<stdio.h>
#include<assert.h>
char *my_strncat(char *dest, const char *src, int count)
{
	assert(dest != NULL);
	assert(src != NULL);
	char *ret = dest;
	while (*dest)
	{
		dest++;
	}//找到目标字符串的'\0'，开始拷贝
	while (count && (*dest++ = *src++))
	{
		count--;
	}
	dest = '\0';
	//拷贝完之后自动不上'\0'
	return ret;
}
int main()
{
	char str1[20] = "hello";
	char *str2 = " world";
	char *ret = my_strncat(str1, str2, strlen(str2));
	printf("%s\n", ret);
	return 0;
}

③模拟实现strncmp

参考代码：

#include<stdio.h>
#include<assert.h>
int my_strncmp(const char *dest, const char*src, int count)
{
	assert(dest != NULL);
	assert(src != NULL);
	while (count && (*dest ==*src))
	{
		if (*dest == '\0')
		{
			return 0;
		}
		dest++;
		src++;
		count--;
	}
	return *dest - *src;
}
int main()
{
	int ret = 0;
	const char *str1 = "abcdt";
	const char *str2 = "cdefu";
	ret = my_strncmp(str1, str2, 4);
	if (0 == ret)
	{
		printf("str1=str2\n");
	}
	else if (ret > 0)
	{
		printf("str1>str2");
	}
	else
	{
		printf("str1<str2");
	}
	return 0;
}