[Bug 1349907] Re: gcc, powerpc with C++11 standard does not define __linux

[Ulrich Weigand]

So there's two issues:

First of all, on amd64:

g++ -std=c++0x -dM  -E - < /dev/null | grep linux
cc1: warning: command line option "-std=c++0x" is valid for C++/ObjC++ but not for C
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1

Note the warning?  That means the -std=c++0x option is effectively
ignored.   When you add e.g. -x c++, you'll get instead:

g++ -std=c++0x -dM -x c++ -E - < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1

Which is how it should be; in "standard" mode, "linux" is an identifier
reserved to user code and must not be pre-defined.  However, __linux
*should* be predefined.


The second issue is that on rs6000, the linux macros are defined in a way that's different than on other platforms.  This has the effect that even though we get the warning, the std=c++0x option is still considered to be in effect for the preprocessor defines.  In addition, due to the bug you mention above, __linux is only defined in non-standard mode instead of always ...

However, given that this has been that way (unfortunately) on rs6000
forever, it is probably best not to rely on the presence of __linux in
standard mode, and use __linux__ instead.

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https://bugs.launchpad.net/bugs/1349907

Title:
  gcc, powerpc with C++11 standard does not define __linux

Status in “gcc-4.9” package in Ubuntu:
  Won't Fix

Bug description:
  Envoking:
  $ g++-4.9 -std=c++0x -dM -E - < /dev/null | grep linux

  On amd64 shows:
  #define __linux 1
  #define __linux__ 1
  #define __gnu_linux__ 1
  #define linux 1

  
  On powerpc shows:
  #define __linux__ 1
  #define __gnu_linux__ 1

  Looks like "linux" and "__linux" defines are missing.

  Dropping "-std=c++0x" on powerpc, makes it show all 4 linux defines.

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