out-of-tree builds with BSD
If you've ever built parts of OpenBSD from source, you may know that the sequence of commands recommended by release(8) is:
$ make obj $ make # make install
If, like me, you've forgotten the ‘
obj’ step, you'll find yourself with many derived files in the
current directory of whatever program you're building. By running
make obj’ first, a directory called
obj appears and the derived files (usually
*.o files) are placed there instead. Cleverly, the
obj directory is actually a symlink to another
filesystem under /usr/obj, making it truly an
Up until recently, I understood what the
obj’ target did and why it was
useful. However, it wasn't until I tried to replicate it with the build for
text.alexkarle.com that I discovered how it worked. I figured I'd document
it here in case it helps anyone else.
My discovery of the inner workings of this target was a classic lesson in RTFM. After 10-15 minutes of trying to parse the makefiles in /usr/share/mk, I finally searched for obj in the make(1) man page, and sure enough the answer was the first hit! I've copied it for convenience below (licensed under the BSD-3 clause):
makesearches for an alternate directory to place target files.
maketries to chdir(2) into
MAKEOBJDIR(or obj if
MAKEOBJDIRis not defined), and sets .OBJDIR accordingly. Should that fail, .OBJDIR is set to .CURDIR.
With this new knowledge, getting an out-of-tree build was almost
as simple as running ‘
The one catch was that, having chdir'd in, I had to canonicalize the paths to any scripts used in the build recipes. For instance, I have a genpost.sh script in the bin/ directory of this repo. To call it from the obj directory, I needed to use its absolute path via the .CURDIR variable:
$(.CURDIR)/bin/genpost.sh < $< > $@
While I mostly build my site on OpenBSD, it's important to me that it builds with GNU make too.
Unfortunately, the .OBJDIR chdir'ing appears to be an extension in OpenBSD's make (and possibly NetBSD too). The good news is that, with one more trick, GNU make support is easy to add (albeit without out-of-tree builds).
The one final hack to support GNU make was to define a portable version of .CURDIR. Since .CURDIR isn't defined in GNU make (which uses CURDIR instead), I had to define the DIR variable that's the concatenation of the two:
DIR = $(.CURDIR)$(CURDIR)
I hope this sheds some light on why ‘
obj’ is common practice on OpenBSD as well as how to add
similar support to your own projects!
While not as flexible as GNU make's pattern matching inference rules (that allow builds in subdirectories), I find the chdir-ing into obj a cleverly simple way to obtain a similar end result.
|September 28, 2021||OpenBSD 7.0|
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