15.5 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 FreeBSD 1.x 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>
    #endif

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 <freebsd-ports@FreeBSD.org>.

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

    #ifdef HAVE_SYS_PARAM_H
    #include <sys/param.h>
    #endif

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).

Use:

    #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:

• __FreeBSD__ is defined in all versions of FreeBSD. Use it if the change you are making only affects FreeBSD. Porting gotchas like the use of sys_errlist[] vs strerror() are Berkeleyisms, not FreeBSD changes.

• In FreeBSD 2.x, __FreeBSD__ is defined to be 2. In earlier versions, it is 1. Later versions will bump it to match their major version number.

• If you need to tell the difference between a FreeBSD 1.x system and a FreeBSD 2.x or 3.x system, usually the right answer is to use the BSD macros described above. If there actually is a FreeBSD specific change (such as special shared library options when using ld) then it is OK to use __FreeBSD__ and #if __FreeBSD__ > 1 to detect a FreeBSD 2.x and later system. If you need more granularity in detecting FreeBSD systems since 2.0-RELEASE you can use the following:

      #if __FreeBSD__ >= 2
      #include <osreldate.h>
      #    if __FreeBSD_version >= 199504
               /* 2.0.5+ release specific code here */
      #    endif
      #endif
  

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.