常见函数实现

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strcpy实现:

char *mycopy(char *strDest,const char *strSrc)
{

		if (strDest == NULL || strSrc == NULL )
		{
			return NULL;
		}

		char *strDestCopy = strDest;

		while ((*strDestCopy++ = *strSrc++) != '');
		
		return strDestCopy;
}

　　bigint:

# include<stdio.h>
# include<string.h>
# include<stdlib.h>
void multiply(const char* a,const char* b,char* c)
{
	int i,j,ca,cb,* s;
	ca=strlen(a);
	cb=strlen(b);
	s=(int*)malloc(sizeof(int)*(ca+cb));
	for (i=0;i<ca+cb;i++)
		s[i]=0;
	for (i=0;i<ca;i++)
		for (j=0;j<cb;j++)
			s[i+j+1]+=(a[i]-'0')*(b[j]-'0');     //s[0]保留为最后的进位
	for (i=ca+cb-1;i>=0;i--)
		if (s[i]>=10)                               
		{
			s[i-1]+=s[i]/10;                        
			s[i]%=10;
		}
		i=0;
		while (s[i]==0)
			i++;
		for (j=0;i<ca+cb;i++,j++)
			c[j]=s[i]+'0';
		c[j]='';
		free(s);
}
int main(void)
{
	char a[]="123511156112122132312235465611";
	char b[]="4561561233211213213213213131232665";
	char *c=(char *)malloc(strlen(a)+strlen(b)+1);
	multiply(a,b,c);
	printf("%s",c);
	free(c);
	getchar();
	return 0;
}

　　参考链接:http://hi.baidu.com/ok558/item/9c5d78e22cd798a9c00d7534

 strstr

char * __cdecl Mystrstr (
	const char * str1,
	const char * str2
	)
{
	char *cp = (char *) str1;
	char *s1, *s2;

	if ( !*str2 )
		return((char *)str1);

	while (*cp)
	{
		s1 = cp;
		s2 = (char *) str2;

		while ( *s1 && *s2 && !(*s1-*s2) )
			s1++, s2++;

		if (!*s2)
			return(cp);

		cp++;
	}

	return(NULL);

}