3.模拟实现strcpy

define _CRT_SECURE_NO_WARNINGS 1

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

void my_strcpy(char *dst,const char *src)
{
	assert(dst);
	assert(src);
	while(*dst++ == *src++ && *src != '\0')
	{
		;
	}
}
int main()
{
	char src[] = "def";
	char dst[] = "abc";
	my_strcpy(dst,src);
	printf("%s",dst);
	return 0;
}

4.模拟实现strcat

define _CRT_SECURE_NO_WARNINGS 1

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

void my_strcat(char *dst,const char *src)
{
	char *tmp = dst;
	assert(dst);
	assert(src);
	while(*tmp != '\0'){tmp++;}
	while(*src != '\0')
	{
		*tmp++ = *src++ ;
	}
	*tmp = '\0';
}

int main()
{
	char src[] = "efg";
	char dst[20] = "abcd";
	my_strcat(dst,src);
	printf("%s\n",dst);
	return 0;
}

3.实现strstr

#define _CRT_SECURE_NO_WARNINGS 1

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

//3.实现strstr 
char* my_strstr(const char *str1,const char *str2)
{
	char *cur =(char *)str1	;
	char *s1,*s2;
	assert(str1 != NULL && str2 != NULL);
	while(*cur)
	{
		s1 = cur;
		s2 = (char *)str2;
		while(*s1 && !(*s2 - *s1))
		{
			s1++;
			s2++;
		}
		if(!*s2)
			return cur;
		cur++;
	}
	return NULL;
}

int main()
{
	char src[] = "abcdaeaf";
	char *dst = "cde";
	if(my_strstr(src,dst))
		printf("true!\n");
	else
		printf("flase!\n");
	return 0;
}

4.实现strchr

char* my_strchr(char* str, char a)
{
	assert(str);
	while(*str != '\0' && *str != a)
	{
		str++;
	}
	return *str==a?str:NULL;
}
int main()
{
	char arr[] = "abcdef";
	char a = 'd';
	printf("%s",strchr(arr,a));
	return 0;
}

5.实现strcmp

int my_strcmp(char const  *p,char const  *q)
{
	assert(p != NULL && q != NULL);
	while(*p == *q)
	{
		if(*p == '\0')
			return 0;
		p++;
		q++;
	}
	if(*p>*q)
		return 1;
	else
		return -1;
}
int main()
{
	char *p = "abcdef";
	char *q = "abcdf";
	int ret = my_strcmp(p,q);
	if(ret > 0)
		printf("前者大于后者!\n");
	else if(ret < 0)
		printf("前者小于后者!\n");
	else
		printf("两者一样!\n");
	return 0;
}

6.实现memcpy 与memmove相似

#define _CRT_SECURE_NO_WARNINGS 1

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


void my_memcpy(char *dst,char *src,int count)
{
	char *p = dst;
	char *q = src;
	assert(dst);
	assert(src);

	while(count-- && *q != '\0')
	{
		*p++ = *q++;
	}
	while(*p++)
	{
		;
	}
		*p = '\0';
}

int main()
{
	char *src = "dsd";
	char dst[100] = "abcd";
	my_memcpy(dst,src,3);
	printf("%s\n",dst);
	return 0;
}

7.实现memmove为前者的优化

    #define _CRT_SECURE_NO_WARNINGS 1
    
    #include <stdio.h>
    #include <assert.h>
    #include <string.h>
    
  void my_memmove(void *dst, void *src, size_t tLen)
{
	char *pDst = (char*)dst;
	char *pSrc = (char*)src;
	assert(pDst != NULL && pSrc != NULL);
    if((pDst>pSrc) && (pDst<(pSrc+tLen)))
    {
        // 特殊处理
        char* p = pSrc + tLen - 1;
        char* q = pDst + tLen - 1;
        while(p >= pSrc)
        {
            *q-- = *p--;
        }
    }
    else
    {
        // 不需要特殊处理可直接用memcpy
        memcpy(pDst, pSrc, tLen);
    }
}
    int main()
    {
    	char *src = "dsd";
    	char dst[100] = "abcdefgh";
    	my_memmove(dst+3,dst+2,4);
    	printf("%s\n",dst);
    	return 0;
    }