strcpy_s

　　

char src[5]="abcd";

char *des=new char[str.length(src)+1];   // length()不计\0

strcpy_s(des,length(src),src);    //  崩溃

strcpy_s(des,length(src)+1,src);  // √

因为strcpy_s的边界保护机制      size(src)<=复制size<=size(des)

strcpy_s源码：

 errno_t __cdecl _FUNC_NAME(_CHAR *_DEST, size_t _SIZE, const _CHAR *_SRC)
 {
     _CHAR *p;
     size_t available;

     /* validation section */
     _VALIDATE_STRING(_DEST, _SIZE);
     _VALIDATE_POINTER_RESET_STRING(_SRC, _DEST, _SIZE);

     p = _DEST;
     available = _SIZE;
     while ((*p++ = *_SRC++) !=  && --available > )
     {
     }

     if (available == 0)
17     {
         _RESET_STRING(_DEST, _SIZE);
         _RETURN_BUFFER_TOO_SMALL(_DEST, _SIZE);
     }
     _FILL_STRING(_DEST, _SIZE, _SIZE - available + );
     _RETURN_NO_ERROR;
 }

当源字符串_SRC不到\0字符，复制长度available减到0时，有边界测试会引发断言。  是strcpy的安全版本，其他的str类型函数也有安全版本。

strcpy复制到src的\0字符停止，但是当size(dest)<size(src)时，只要两个指针指向的内存可读写，循环就可以继续下去,会超出Dest的边界覆盖其他内存。

复制char[]无\0字符时，会一直复制到遇到下一个\0字符。 用char[]保存字符串，在最后加上\0,

否则strcpy边界不对时，会有隐形错误。或者在strcpy之后给char[]加上\0

char* p="how are you ?";
char name[10];

strcpy(name,p);      
name[sizeof(name)-1]='"0'

strncpy源码：

 char *strncpy(dest, source, count) - copy at most n characters
 ;
 ;Purpose:
 ;       Copies count characters from the source string to the
 ;       destination.  If count is less than the length of source,
 ;       NO NULL CHARACTER is put onto the end of the copied string.
 ;       If count is greater than the length of sources, dest is padded
 ;       with null characters to length count.
 ;
 ;       Algorithm:
 ;       char *
 ;       strncpy (dest, source, count)
 ;       char *dest, *source;
 ;       unsigned count;
 ;       {
 ;         char *start = dest;
 ;
 ;         while (count && (*dest++ = *source++))
 ;             count--;
 ;         if (count)   //如果count大于零 即count is greater than the length of sources,'\0'已写入，不够的字符数都填充\0
 ;             while (--count)
 ;                 *dest++ = '\0';
 ;         return(start);
 ;       }
 ;
 ;Entry:
 ;       char *dest     - pointer to spot to copy source, enough space
 ;                        is assumed.
 ;       char *source   - source string for copy
 ;       unsigned count - characters to copy
 ;
 ;Exit:
 ;       returns dest, with the character copied there.
 ;

strcpy(des,src,size);

size<src长度时，des最后字符无\0，打印时会乱码直到遇到\0

size>src长度时，不够的位全部补充\0