一篇文章带你了解C语言的一些重要字符串与内存函数

size_t my_strlen(const char*str)
{ 
  size_t count=0;
  while(*str)
  {
   str++;
   count++;
  }

int my_strcmp(const char*str1,const char*str2)
{
   while (*str1 == *str2)
   {
   	if (*str1 && *str2)//判断'\0'
   		return *s2 - *s1;
   	*str1++;
   	*str2++;
   }//循环结束标志*str1！=*str2
   return *str2 - *str1;
}

char *my_strcpy(char *dest, const char*src)
{ 
 char *ret = dest;
 while((*dest++ = *src++))
 {
  ;
 }
 return ret;
}

char* my_strcat(const char* str1, const char* str2)
{
		while (*str1) str1++;
		while (*(char*)str1++ = *(char*)str2++) {
			;
		}
}

char* my_strstr(const char* str1, const char* str2)
{
	if (!*str2)//判断字符串是否为空
		return (char*)str1;
	 char* ret1= (char*)str1;//将str类型转换
	 char *cp = ret1;
	while (*ret1)
	{
		cp = ret1;
		char* ret2 =(char*) str2;
		while (*cp== *str2) {//遍历字符串
		   if (!*ret2)//判断str2是否到达‘0'的位置
			   return cp;
		   cp++, ret2++;
		}
		ret1++;//循环结束没有返回，从下一个字符开始查找
		if (!*ret1)
			return NULL;
	}
	return NULL;
}

void * my_memcpy ( void * dst, const void * src, size_t count)
{
  void * ret = dst;
  while (count--) {
    *(char *)dst = *(char *)src;
    dst = (char *)dst + 1;
    src = (char *)src + 1;
  }
  return(ret);
}

int main()
{
	int arr1[10] = { 1,2,3,4,5,6,7,8,9,0 };
	// 预期结果       1 2 1 2 3 4 7 8 9 0
	my_memcpy(arr1+2, arr1, 16);
		for (int i = 0; i < 10; i++)
		{
			printf("%d ", arr1[i]);
	}
}

int main()
{
   int arr1[10] = { 1,2,3,4,5,6,7,8,9,0 };
   my_memcpy(arr1, arr1+2, 16);
   	for (int i = 0; i < 10; i++)
   	{
   		printf("%d ", arr1[i]);
   }
}

**void* my_memmove(void* dest, void* src, size_t num)
{
	char* ret = dest;
	//如果指向同一块空间 判断地址大小，避免数据在被复制前被改变
	if ( (char*)dest-(char*)src< 0){
		while (num)
		{
			*((char*)dest)++ = *((char*)src)++;
			num--;
		}
	}
	else {
		while(num--){
			*((char*)dest+num) = *((char*)src+num);
		}
	}
	return ret;
}