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Never mess with LD_LIBRARY_PATH to run your CUDA app again

If you are using CUDA on Linux then the following must be familiar:

~/.bashrc

...

export LD_LIBRARY_PATH=/usr/local/cuda-7.5/lib64

# or even smarter

export LD_LIBRARY_PATH=/usr/local/cuda-7.5/lib64:$LD_LIBRARY_PATH

This solves the problem with CUDA libraries search path and allows to actually run CUDA-powered programs. Every time you execute a program, the dynamic linker (GNU ld) reads the imported symbols table from the executable container (ELF) and has a hard time trying to load them from shared libraries available in the system (*.so). If it fails then the program will not start running. ELF carries the list of library names which should contain the imports, so that the loader does not end up with probing every library out there. You can view that dependency list using ldd command:

$ ldd /bin/ls
	linux-vdso.so.1 =>  (0x00007ffd27ffb000)
	libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f4823fa8000)
	libacl.so.1 => /lib/x86_64-linux-gnu/libacl.so.1 (0x00007f4823da0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f48239db000)
	libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f482379d000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f4823599000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f48241cb000)
	libattr.so.1 => /lib/x86_64-linux-gnu/libattr.so.1 (0x00007f4823394000)

Shared libraries are searched in several paths. Some of them are stored in ELF itself, some of them are not. In the latter case, common library search paths are scanned. They usually include “/lib”, “/usr/lib”, etc. The current list of library search paths can be printed with

$ ld --verbose | grep SEARCH_DIR | sed -E 's/("\); |"\);|)(SEARCH_DIR\("=?|$)/\n/g' | head -n -1 | tail -n +2
/usr/x86_64-linux-gnu/lib64
/usr/local/lib/x86_64-linux-gnu
/usr/local/lib64
/lib/x86_64-linux-gnu
/lib64
/usr/lib/x86_64-linux-gnu
/usr/lib64
/usr/local/lib
/lib
/usr/lib

GNU dynamic loader supports hacking into library loading and searching procedures. LD_PRELOAD, for example, forces ld to load the given libraries before normally loading any others. LD_LIBRARY_PATH extends the list of library search paths.

Sometimes, libraries are installed into some non-standard directories, as with CUDA. If the target binaries were not built with explicit mentioning those directories so that they crawl into ELF library search paths (-L option of gcc), then you are in trouble. Every time you attempt to run such program, the dependent libraries are not found and you fail. That is why usually people apply hacks with LD_LIBRARY_PATH then.

Is there any other solution? Yes.

An interesting question is, how ld knows about the list of common library search paths. It shouldn’t be hardcoded, right? Sure. This is when ldconfig comes out. It builds the special configuration file, usually /etc/ld.so.cache, which is picked up by the dynamic loader every time a program needs to start. ldconfig reads the paths from the configuration file, /etc/ld.so.conf, which usually includes files from /etc/ld.so.conf.d.

$ ls /etc/ld.so.conf.d
fakeroot-x86_64-linux-gnu.conf  libc.conf              x86_64-linux-gnu_EGL.conf  zz_i386-biarch-compat.conf
i386-linux-gnu_GL.conf          x86_64-linux-gnu.conf  x86_64-linux-gnu_GL.conf

Now the solution is clear: let’s create a new configuration file for ldconfig, say, /etc/ld.so.conf.d/cuda.conf, with the path from LD_LIBRARY_PATH:

/usr/local/cuda-7.5/targets/x86_64-linux/lib

Then run sudo ldconfig and voila!

Amazingly few people know about this, and even CUDA FAQ suggest augmenting LD_LIBRARY_PATH instead - bummer. Of course, CUDA users should not experience troubles at all and CUDA maintainers should definitely include /etc/ld.so.conf.d/cuda.conf into the distribution package. Alternatively, end-user CUDA applications should be compiled with -L /usr/local/cuda-7.5/lib64 since 99% folks do not change the default installation root (hey, Tensorflow!).

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