Re: LibGL

[lsorense@csclub.uwaterloo.ca (Lennart Sorensen)]

On Wed, Feb 28, 2007 at 09:55:57PM +0100, Leopold Palomo Avellaneda wrote:
> ummm, for example, without the nvidia-glx, I have:
>
> leo@indiana:~$ ls -la /usr/lib64/libGL*
> lrwxrwxrwx 1 root root     10 2007-02-28 12:42 /usr/lib64/libGL.so -> 
> libGL.so.1
> lrwxrwxrwx 1 root root     12 2007-02-28 12:42 /usr/lib64/libGL.so.1 -> 
> libGL.so.1.2
> -rw-r--r-- 1 root root 584296 2007-02-23 03:21 /usr/lib64/libGL.so.1.2
> 
> ok, is what you are saying. Any program that use the libGL is redirected to 
> the libGL.so.1, etc. The  glxinfo | grep OpenGL && glxinfo | grep direct 
> command:

No any program compiled for libGL is compiled for libGL.so.1 which in
that case there appears to be the mesagl version.

> OpenGL vendor string: NVIDIA Corporation
> OpenGL renderer string: GeForce FX 5600/AGP/SSE2
> OpenGL version string: 1.4 (2.0.2 NVIDIA 87.76)
> OpenGL extensions:
> direct rendering: No

Yeah that looks like the mesa version.

> ok.
> 
> if I install the nvidia-glx package I obtain:
> 
> rwxrwxrwx 1 root root     37 2007-02-28 
> 21:34 /usr/lib64/libGL.so -> /usr/lib/nvidia/libGL.so.1.2.xlibmesa
> lrwxrwxrwx 1 root root     17 2007-02-28 21:34 /usr/lib64/libGL.so.1 -> 
> libGL.so.1.0.8776
> -rw-r--r-- 1 root root 735592 2007-01-05 05:37 /usr/lib64/libGL.so.1.0.8776
> 
> and the libGL.so is pointng to /usr/lib/nvidia/libGL.so.1.2.xlibmesa. and this 
> file is the same as the mesa provides. So, in this case the link is not to 
> libGL.so.1 . libGL.so.1 is pointing correctly to the nvidia lib. But I have:

libGL.so is for compiling, and since you apparently have the
libgl1-mesa-dev package installed, then any compiling is done using the
mesa version of the headers, so libGL.so (used exclusively for
compiling) links to the mesa version of the library.  If you instead
installed nvidia-glx-dev, then it would link to the nvidia version,
since you would be using nvidia's version of those header files.

> leo@indiana:~$ glxinfo | grep OpenGL && glxinfo | grep direct
> OpenGL vendor string: NVIDIA Corporation
> OpenGL renderer string: GeForce FX 5600/AGP/SSE2
> OpenGL version string: 2.0.2 NVIDIA 87.76
> OpenGL extensions:
> direct rendering: Yes

Since you have the nvidia version of libGL.so.1 you get DRI.  Without it
you don't.

> so, if I understand you, if I want to make a program that use libGL 
> accelerated I have to compile it normally, with the libgl1-mesa-dev
> - then deinstall the libgl1-mesa-dev library
> - restart the nvidia-glx

Why would you deinstall the development package?  It has _nothing_ to do
with running the program.  Any program you compiled will use libGL.so.1
at runtime, which you showed is a link to the nvidia version of libGL.

> and run the program, that will use the libgl of nvidia instead of the 
> libgl.xmesa, no?

It will do so without those two steps.

It is as simple as:
compile program
run program

It will compile using the mesa headers, and run using the nvidia
library since they both implement the libGL.so version 1 ABI.

When you compile an opengl program, the linker is told to use libGL
(-lGL) which it goes looking for.  It find /usr/lib/libGL.so which is a
symlink to libGL.so.1.2.xlibmesa which is an implementation of libGL,
shared object, major ABI version 1.  So it writes into the binary that
it wants libGL.so.1 library loaded dynamically when the program is run.
It doesn't care who wrote libGL.so.1 as long as it works as the standard
says it should.  Since both mesa and nvidia implemented the libGL.so
with the correct ABI, either will work with the resulting program.  When
you run your program, the loader looks for /usr/lib/libGL.so.1 and finds
it is a link to libGL.so.1.0.8776 provided by nvidia, and uses that.
That particular implementation knows how to use an nvidia X server for
DRI and GLX and hence runs accelerated.  If instead you had a link to
the mesa version of libGL.so.1 then it would have used that and rendered
by software (or whatever mesa happens to support on your system).

So the .so is used by linker to find library (name and ABI version) to
compile for, while at runtime it uses that library name and ABI version
directly (so in this case libGL.so.1) to load the right library.

--
Len Sorensen