[Scons-dev] Tutorial for Linux versioned libraries

[anatoly techtonik]

Hi,

If I understand the problem correctly, the issue with
versioned libraries is not fixed yet:
http://scons.tigris.org/issues/show_bug.cgi?id=2869


>From times I used to code on low C level my primary
system was Windows, so I miss all the background
about about versioned libraries domain, but I am
interested to resolve this issue in SCons.

I have a good use case for that. I am enhancing a
voice synthesizer for blind computer users. It was
created by a blind girl and she used SCons as a build
system (which says something about it).

First, I am trying to provide a Python interface for it
using ctypes. The windows part works fine - I place
RHVoice.dll near RHVoice.py and then execute

> RHVoice.py -o say.wav "say something beautifully"

You can try it yourself (wine?):
https://ci.appveyor.com/project/techtonik/rhvoice/build/1.0.13/artifacts


But on Linux it is complicated.

$ git clone https://github.com/techtonik/RHVoice.git
$ cd RHVoice
$ git checkout d526ba60
$ scons
$ python src/nvda-synthDriver/RHVoice.py -o say.wav "say something"
detected dll/so path: /home/user/RHVoice/build/linux/lib/libRHVoice.so
...
OSError: libRHVoice_core.so.0: cannot open shared object file: No such
file or directory

The .so file loaded with ctypes is libRHVoice.so, the
other .so file - libRHVoice_core.so - is a dependency
and it was built in sibling core/ directory.

$ ls build/linux/core/libRHVoice_core.so
build/linux/core/libRHVoice_core.so

However, even if I add this dir to LD_LIBRARY_PATH,
the .so still can not be found, because generated file
doesn't have .0 extension

$ LD_LIBRARY_PATH=/home/user/RHVoice/build/linux/core python
src/nvda-synthDriver/RHVoice.py -o say.wav "say something again"


I can rename file to have .0 extension (or load core .so
with ctypes explicitly), but this is a workaround, and
not a solution, so while I am not cursed with knowledge,
and interested to solve it, the questions are:

1. Extension is an ABI version, who should set it?
a. gcc
b. scons
c. ?

2. The SConstruct files have some Install targets, that
produce both .so files in the same directory:

$ scons install prefix=$PWD/outdir
...
$ ls outdir/lib
libRHVoice_core.so.0.0.0  libRHVoice.so.2.0.0

As you may see, the libs produced by install have
those prefixes. I thought that these should be added by
compiler automatically, but it appears that at least 0.0.0
is in SConscripts file.

Now the problem with those is that even if I load
versioned libRHVoice.so.2.0.0 with ctypes, it still tries to
load dependency libRHVoice_core.so.0 which is again
absent.


So, who should add those suffixes (gcc or scons)?
Why main lib insists that it should load core with
so.0?
Are these versions should be specified in SConstruct
or gcc should figure them out from the sources?


What are the best practices in C on Linux to use
versioned libraries?
What is the current SCons strategy to deal with them?
What is the strategy that should be implemented to
solve the
http://scons.tigris.org/issues/show_bug.cgi?id=2869 ?

-- 
anatoly t.