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Date: Sun, 6 Sep 2015 00:06:58 +1000
From: andrew clarke <mail@ozzmosis.com>
To: "William A. Mahaffey III" <wam@hiwaay.net>
Cc: FreeBSD Questions !!!! <freebsd-questions@freebsd.org>
Subject: Re: GCC question
Message-ID: <20150905140658.GA790@ozzmosis.com>
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On Sat 2015-09-05 08:34:27 UTC-0453, William A. Mahaffey III (wam@hiwaay.net) wrote:

>> I have some code which was originally SGI native, then moved to Linux 
> (FC14 x86_64 & CentOS 5). I am now interested in getting it going under 
> FreeBSD 9.3R. Due to differences in system header file includes, I need 
> to tweek some of my app-specific header files. I have poked around the 
> (*COPIOUS* !!!!) GCC man page & I couldn't find (or missed) either how 
> to get it to regurgitate its default compiler-defines or a tabulation of 
> those defines, so I can use them to conditionally include system headers 
> in my own header files. Where is this info :-) ? TIA & have a nice 
> (long) weekend.

You probably want:

gcc -dM -E - < /dev/null

Ordinarily to detect FreeBSD you'd use:

#ifdef __FreeBSD__
...
#endif