Question

这里的问题是:。利用<代码>std: 填满(<>>>> 本身获得一定帮助——汇编者可能决定后不得进入记忆组,从而消除这一障碍是时间的浪费,消除了记名。

http://stackoverflow.com/questions/3785582/how-to-write-a-password-safe-class/3785676#3785676>。关键词 :

{
  char buffer[size];
  //obtain and use password
  std::fill_n( (volatile char*)buffer, size, 0);
}

目的是在看到<代码>volatus时。汇编者不会试图取消对<代码>以下的号召:填充(n)()。

volaful 关键词总是阻止汇编者更改消除代码的记忆?

Answer 1

编辑可在because buffer<>/code>上优化你的代码。这不是一个挥发性物体。



《标准》只要求汇编者严格遵守挥发性物体的特性。 这里指的是C++03。


  符合执行要求的要求最少:
  
  
  At sequence points, volatile objects are stable in the sense that previous evaluations are complete and
  subsequent evaluations have not yet occurred.
  [...]
  


以及


  The observable behavior of the abstract machine is its sequence of reads 以及writes to volatile data and
  calls to library I/O functions


In your example, what you have are reads 以及writes using volatile lvalues to non-volatile objects. C++0x removed the second text I quoted above, because it s redundant. C++0x just says


  符合执行要求的要求最少:
  
  
  Access to volatile objects are evaluated strictly according to the rules of the abstract machine.[...]
  
  
  这些方案统称为值得注意的行为。


While one may argue that "volatile data" could maybe mean "data accessed by volatile lvalues", which would still be quite a stretch, the C++0x wording removed all doubts about your code 以及clearly allows implementations to optimize it away.

But as people pointed out to me, It probably does not matter in practice. A compiler that optimizes such a thing will most probably go against the programmers intention (why would someone have a pointer to volatile otherwise) 以及so would probably contain a bug. Still, I have experienced compiler vendors that cited these paragraphs when they were faced with bugreports about their over-aggressive optimizations. In the end, volatile is inherent platform specific 以及you are supposed to double check the result anyway.

Answer 2

从最后的C++0x草案[生效]:

8 The least requirements on a conforming implementation are:

— Access to volatile objects are evaluated strictly according to the rules of the abstract machine.

[......]

12 Accessing an object designated by a volatile glvalue (3.10), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, [......]

因此,即使你提供的法典也不得优化。

Answer 3

您希望删除的记忆内容可能已经从贵邮联/核心内藏中流出到援助团,在那里其他邮联能够继续看到。在撰写文章后,你需要使用双向/记忆屏障指示/原子操作或某种东西,以引发与其他核心的结合。在实践中,贵编者在要求任何外部职能之前,或许会这样做(戈格尔·达维·布滕金在多读过程中挥发的可疑用途上担任职务),这样,如果你在不久之后准备好,不会成为一个重大问题。简言之,需要挥发性。

Answer 4


符合要求的执行在休闲时,可推迟任何挥发性的读写字的实际表现,直至变幻莫测的读写结果会影响执行挥发性书写或I/O操作。

例如:

volatile unsigned char vol1,vol2;
extern unsigned char res[1000];
void test(int scale)
{
  unsigned char ch;

  for (int 0=0; i<10000; i++)
  {
    res[i] = i*vol1*scale;
    vol2 = res[i];
  }
}

a 符合规格的汇编者可在其选择中核对<代码>> > > 比例是128的多种,如果是,在从<代码>vol1做任何读物之前,从<代码>中删除。页: 1 即使汇编者需要从<代码>vol1上读出。在编汇到<代码>第2卷<>>之前,编汇人可推迟两个业务,直至其操作基本不受限制的代码。

友情链接