strcpy,strcat,strstr,strcmp,memmove的实现

1.实现strcpy

把从src地址开始且含有'\0'结束符的字符串复制到以dest开始的地址空间

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

char* my_strcpy(char* dest, const char* src)
{
	assert(dest != NULL);
	assert(src != NULL);

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

int main()
{
	char arr1[] = "hello";
	char arr2[] = "world";
	my_strcpy(arr1, arr2);
	printf("%s\n", arr1);

	system("pause");
	return 0;
}
           

2.strcat

用于连接两个字符串

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

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

int main()
{
	char arr1[] = "hello ";
	char arr2[] = "world";
	my_strcat(arr1, arr2);
	printf("%s", arr1);
	system("pause");
	return 0;
}

           

3.strstr

strstr(dest,src) 函数用于判断字符串src是否是dest的子串。如果是，则该函数返回src在dest中首次出现的地址；否则，返回NULL。

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

char* my_strstr(const char* dest, const char* src)
{
	assert(dest);
	assert(src);
	const char* cur = dest;
	while (*cur)
	{
		const char* s1 = cur;
		const char* s2 = src;
		while ((*s1) && (*s2) && (*s1 == *s2))
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
			return (char*)cur;
		}
		cur++;
	}
	return NULL;
}

int main()
{
	char arr1[] = "hello ";
	char arr2[] = "world";
	char* ret = my_strstr(arr1, "l");
	if (ret == NULL)
	{
		printf("no\n");
	}
	else
	{
		printf("%s\n", ret);
	}
	system("pause");
	return 0;
}
           

4.strcmp

strcmp用于比较两个字符串s1和s2，若s1>s2,返回1; s1==s2,返回0；若s1<s2,返回-1；

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

int my_strcmp(const char*s1, const char* s2)
{
	assert(s1);
	assert(s2);
	while (*s1 == *s2)
	{
		if (*s1 == '\0')
			return 0;
		s1++;
		s2++;
	}
	if (*s1 > *s2)
		return 1;
	else
		return -1;
}

int main()
{
	char arr1[] = "hello ";
	char arr2[] = "world";
	int ret = my_strcmp(arr1, arr2);
	printf("%d\n", ret);
	system("pause");
	return 0;
}

           

5.memmove

memmove用于从src拷贝count个字节到dest，如果目标区域和源区域有重叠的话，memmove能够保证源串在被覆盖之前将重叠区域的字节拷贝到目标区域中。但复制后src内容会被更改。但是当目标区域与源区域没有重叠则和memcpy函数功能相同。

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

void* my_memmove(const void* dest, const void* src,int count)
{
	assert(dest);
	assert(src);
	char* cur = (char*)dest;
	while (*cur)
	{
		char* pdest = (char*)dest;
		char* psrc = (char*)src;
		if ((pdest > psrc) && (pdest < psrc + count))
		{
			while (count--)
				*(pdest+count) = *(psrc+count);
		}
		else
		{
			while (count--)
			{
				*pdest = *psrc;
			}
		}
		cur++;
	}
	return dest;
}

int main()
{
	int arr1[20] = { 1,2,3,4,5,6,7,8,9,0 };
	int arr2[10] = { 0 };
	my_memmove(arr1 + 3, arr1, 20);
	system("pause");
	return 0;
}
           

如发现错误，请及时告知我谢谢~