RE: Compiling a specific binary in this package

[Assaf Gordon]

(Answering a question from github:
https://github.com/coreutils/coreutils/issues/13)

alxpettit writes:
> I want to use this code as a basis for my project. I am forking the 
> mv command, since it's relatively simple (~500 lines of dedicated 
> code), and its functions closely match the purpose of my program.

That is an inaccurate assessment.
While "mv.c" itself is about ~500 lines,
it uses "copy.c" which is ~3000 lines,
which in turn uses many gnulib modules.
Also used are "remove.c", "cp-hash.c", "selinux.c", etc.

Compiling the file 'mv.c' alone would not yield an executable with mv's 
functionality.

> I tried doing make help and ./configure --help, but I found no 
> option to simply compile one binary.

There is no such option at the moment.
You could run "./configure" followed by "make src/mv"
which will then compile only the required files to build the 'mv' 
executable, but this will still result in many compiled files.

> I did a trace on the Makefile, and isolated the command that produces
> the mv binary:

> gcc -I. -I./lib -Ilib -I./lib -Isrc -I./src -g -O2 -MT src/mv.o -MD 
> -MP -MF $depbase.Tpo -c -o src/mv.o src/mv.c &&\ echo " CCLD " 
> src/mv;gcc -g -O2 -Wl,--as-needed -o src/mv src/mv.o src/remove.o 
> src/copy.o src/cp-hash.o src/extent-scan.o src/force-link.o 
> src/selinux.o src/libver.a lib/libcoreutils.a lib/libcoreutils.a 
> -lacl -lattr

> Of course, it doesn't work without all the object files already 
> generated. I can't tell which lines are producing them, and after 
> skimming the Makefile and configure.ac.

coreutils uses gnu autoconf and automake to generate the Makefiles.

The file "src/local.mk" contains the following line:

    src_mv_SOURCES = src/mv.c src/remove.c \
                     $(copy_sources) $(selinux_sources)

Which defines the needed files to build 'mv'.

To learn more about using or tweaking autotools projects
see here:
 https://en.wikipedia.org/wiki/GNU_Build_System
 https://www.lrde.epita.fr/~adl/autotools.html
 https://autotools.io/index.html

In projects that use Autotools there's little point in
modifying a "Makefile" directly.

> I've concluded that I have a 
> newfound appreciation for docker build environments. (I'm mostly a 
> Python programmer, and don't have too much experience with the 
> in-depth GNU toolchains.)

(the following is just my personal opinion:)

In most cases you'll find that a "docker build environment" is
just a lazy substitute for properly packaging a software project
using existing frameworks (be it autotools, cmake, ninja, waf,
Makefiles, or in python's case: a properly setuptools configuration).

In many of these cases, the developer decides that instead of spending
the time to use a build infrastructure and document the pre-requisites,
they just "freeze" their build environment and expect others to just 
deploy their copy.

That is a poor way to build scalable reliable systems.

> Unfortunately, I can't figure out how to compile it without, y'know, 
> compiling everything else. I don't want to have to drag 50M of source
> code around just for a variation on a coreutils binary. How can I
> compile just `mv.c?

There is no way to do so, because mv's functionality is spread of over
many files, and they are all needed.

(technically, you can compile 'mv.c' alone to 'mv.o', but that won't get
you the 'mv' executable, which I assume is what you need.)

GNU coreutils's mv has a lot of features, and perhaps you don't need all 
of them.

If you want to build something based on a minimal 'mv' functionality,
perhaps consider using the mv code from FreeBSD, which seems a lot 
smaller:  https://github.com/freebsd/freebsd/blob/master/bin/mv/mv.c
(though depending on which OS you need it for, it might have it's own 
difficulties to be built).

Other possible starting point would be with BusyBox ( 
https://busybox.net/ ) or ToyBox (http://landley.net/toybox/about.html)

Both of these projects are designed to be small and very modular,
to the point that you can build just the 'mv' executable with them.
However even when building just the 'mv' binary, many source code files 
are still compiled.


Lastly,
It's sometimes worth going back to the beginning and re-evaluate what 
functionality do you need for your program.
Is really the entire functionality of 'mv' needed?



regards,
 - Assaf