17.6 Differentiating operating systems and OS versions

You may come across code that needs modifications or conditional compilation based upon what version of Unix it is running under. If you need to make such changes to the code for conditional compilation, make sure you make the changes as general as possible so that we can back-port code to older FreeBSD systems and cross-port to other BSD systems such as 4.4BSD from CSRG, BSD/386, 386BSD, NetBSD, and OpenBSD.

The preferred way to tell 4.3BSD/Reno (1990) and newer versions of the BSD code apart is by using the BSD macro defined in sys/param.h. Hopefully that file is already included; if not, add the code:

#if (defined(__unix__) || defined(unix)) && !defined(USG)
#include <sys/param.h>

to the proper place in the .c file. We believe that every system that defines these two symbols has sys/param.h. If you find a system that does not, we would like to know. Please send mail to the FreeBSD ports mailing list.

Another way is to use the GNU Autoconf style of doing this:

#include <sys/param.h>

Do not forget to add -DHAVE_SYS_PARAM_H to the CFLAGS in the Makefile for this method.

Once you have sys/param.h included, you may use:

#if (defined(BSD) && (BSD >= 199103))

to detect if the code is being compiled on a 4.3 Net2 code base or newer (e.g. FreeBSD 1.x, 4.3/Reno, NetBSD 0.9, 386BSD, BSD/386 1.1 and below).


#if (defined(BSD) && (BSD >= 199306))

to detect if the code is being compiled on a 4.4 code base or newer (e.g. FreeBSD 2.x, 4.4, NetBSD 1.0, BSD/386 2.0 or above).

The value of the BSD macro is 199506 for the 4.4BSD-Lite2 code base. This is stated for informational purposes only. It should not be used to distinguish between versions of FreeBSD based only on 4.4-Lite vs. versions that have merged in changes from 4.4-Lite2. The __FreeBSD__ macro should be used instead.

Use sparingly:

In the hundreds of ports that have been done, there have only been one or two cases where __FreeBSD__ should have been used. Just because an earlier port screwed up and used it in the wrong place does not mean you should do so too.

For questions about the FreeBSD ports system, e-mail <ports@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.

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