[cfe-dev] [libcxx] Why is <sys/sysctl.h> included by default?

[Craig, Ben via cfe-dev]

Summary:
I want to switch some code from black-listing platforms to white-listing 
platforms instead.
Does anyone know what OSes / platforms want to use sys/sysctl.h in 
thread.cpp?

Longer post:
I suspect that this is rapidly going to turn into a discussion on how 
libcxx does preprocessor platform detection, but that's kind of what I 
want to talk about anyway.

Libcxx's thread.cpp has this chunk of code:
   #if !defined(_WIN32)
   # if !defined(__sun__) && !defined(__linux__) && !defined(_AIX) && 
!defined(__native_client__) && !defined(__CloudABI__)
   #   include <sys/sysctl.h>
   # endif // !defined(__sun__) && !defined(__linux__) && !defined(_AIX) 
&& !defined(__native_client__) && !defined(__CloudABI__)
   # include <unistd.h>
   #endif // !_WIN32

So if your OS is not one of the six listed (_WIN21, __sun__, __linux__, 
AIX, __native_client__, __CloudABI__), then you get to include 
sys/sysctl.h.  sysctl.h isn't defined by the standard C library, and 
isn't part of POSIX.

I'm trying to add some new embedded OS and C library support to libcxx, 
and I don't have a sys/sysctl.h.  I think the correct and maintainable 
thing to do here is to switch away from black-listing certain OSes, and 
instead white-list the OSes that want to use sys/sysctl.h here.  I don't 
know what that list is though.

I'm a little more forgiving of unistd.h, as POSIX at least partially 
defines that header.

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