在g ++ 4.4.3上获得一个负NaN，是这个标准吗？ [英] Getting a negative NaN on g++ 4.4.3, is this standard?

问题描述

-nan

$ b $在Ubuntu Lucid Lynx的Linux上有g ++ 4.4.3结果是b

。在g ++ 4.3.1上，我得到了所有的

nan

这导致我的文本差异回归失败，因为我正在使用cout来打印这个数字结果。

一个有符号的nan的含义是什么，有没有办法告诉编译器：无符号的nan就足够了？

解决方案

行为的改变可能是由于库而不是编译器导致的。 glibc在glibc源代码中肯定会发生一些变化 - 从2009-08-23 2528行： ChangeLog.17 p>

  ... 
 * stdio-common / printf_fp.c：ISO C期望打印NaN 
以及。 
 ... 
  

I have g++ 4.4.3 on Linux with Ubuntu Lucid Lynx, and I am getting a:

-nan

as a result. On Hardy Heron with g++ 4.3.1, I am getting all

nan

This is causing my text diff regression to fail, since I am using cout to print this numerical result.

What is the meaning of a signed nan, and is there a way to tell the compiler that an unsigned nan is sufficient?

解决方案

The change in behaviour may be due to libraries rather than the compiler. There is certainly a change in glibc around the right sort of time - from the entry for 2009-08-23 at line 2528 in ChangeLog.17 in the glibc source:

...
     * stdio-common/printf_fp.c: ISO C expects to print the sign of NaN
     as well.
...

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