Tony Finch – blog

There are a couple of version control commands that deserve wider appreciation: SCCS what and RCS ident. They allow you to find out what source a binary was built from, without having to run it – handy if it is a library! They basically scan a file looking for magic strings that contain version control metadata and print out what they discover.

keyword expansion

SCCS, RCS, cvs, and svn all have a way to expand keywords in a file when it is checked out of version control.

The POSIX SCCS get documentation describes its runes under the “identification keywords” heading. The relevant one is %W% which inserts the magic marker @(#) used by what.

RCS / cvs / svn keyword substitution uses more descriptive markers like $Revision$.

a berkeley example

It was a lonstanding BSD practice to use keyword expansion everywhere. I first encountered it when I got involved in the Apache httpd project in the late 1990s – Apache’s CVS repository was hosted on a FreeBSD box and used a version of FreeBSD’s CVS administrative scripts.

Here’s an example from res_send.c in FreeBSD’s libc resolver.

static const char sccsid[] = "@(#)res_send.c	8.1 (Berkeley) 6/4/93";
static const char rcsid[] = "$Id: res_send.c,v 1.22 2009/01/22 23:49:23 tbox Exp $";
__FBSDID("$FreeBSD$");

There are geological strata of version control ident strings here:

• the sccsid from the Berkeley CSRG SCCS repository
• the rcsid from ISC’s BIND repository (tbox was ISC’s tinderbox build / CI system)
• the FBSDID from FreeBSD’s cvs and later svn repositories (which has not been expanded)

an unifdef example

When unifdef was uplifted to git, I wanted to keep its embedded version control keywords – I have a sentimental liking for this old tradition. If you’ve installed cssc, rcs, and unifdef on a Debian box, you can run,

    :; sccs what /usr/bin/unifdef
    :; ident /usr/bin/unifdef

Both of those will produce similar output to

    :; unifdef -V

On a Mac with the developer command-line tools installed,

    :; what /Library/Developer/CommandLineTools/usr/bin/unifdef

You get the output twice because it’s a fat binary!

versioning three ways

In unifdef.c, the embedded version string looks like,

    static const char copyright[] =
        "@(#) $Version: unifdef-2.12 $\n"
        "@(#) $Date: 2020-02-14 16:49:56 +0000 $\n"
        "@(#) $Author: Tony Finch (dot@dotat.at) $\n"
        "@(#) $URL: http://dotat.at/prog/unifdef $\n"
    ;

Each line is prefixed with an SCCS magic marker @(#) so that what can find it, and wrapped in an RCS-style $Keyword$ so that ident can find it. There’s a fairly trivial version() function that spits out the copyright[] string when you run unifdef -V.

embedding versions from git

My projects have various horrible build scripts for embedding the version number from git. The basic idea is,

• use an annotated or signed tag to mark a release, i.e. git tag -a or git tag -s

• use git describe to get a version string that includes an extra number counting commits since the last release

• maybe use git show --pretty=format:%ai -s HEAD to get a release date

• stuff the outputs from git into the $Version$ and $Date$ RCS keywords

retro cool

I enjoy keeping this old feature working, even though it isn’t very useful if no-one knows about it! Maybe if I blog about it, it’ll become more widespread?