.so search paths

Question

I am a Linux novice (coming from a Windows background). I'd like to understand the details of how shared objects (.so files) are loaded at runtime. According to http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html the file /etc/ld.so.conf configures the directories to search for .so files at runtime. However an experiment shown below seems to indicate that isn't the case. /usr/local/lib is in one of the .conf files in /etc/ld.so.conf.d, but when my .so is in /usr/local/lib it is not found at run-time. Conversely /usr/lib is not configured by /etc/ld.so.conf, but when my .so is in /usr/lib it is found at runtime.

What am I missing?

Thanks, Dave

davids@ds-ub64-7:/$ # Display the .so search path configured in /etc/ld.so.conf
davids@ds-ub64-7:/$ cat /etc/ld.so.conf
include /etc/ld.so.conf.d/*.conf
davids@ds-ub64-7:/$ cat /etc/ld.so.conf.d/*.conf
/usr/lib/mesa
/usr/lib32/mesa
/usr/lib32/alsa-lib
/usr/lib/alsa-lib
# libc default configuration
/usr/local/lib
# Multiarch support
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu

davids@ds-ub64-7:/$ # libsotest.so.1 is in /usr/local/lib
davids@ds-ub64-7:/$ ls -la /usr/local/lib/libsotest*
lrwxrwxrwx 1 root   root     14 2012-07-19 08:24 /usr/local/lib/libsotest.so -> libsotest.so.1
lrwxrwxrwx 1 root   root     18 2012-07-19 08:24 /usr/local/lib/libsotest.so.1 ->     libsotest.so.1.0.1
-rwxr-xr-x 1 davids davids 7952 2012-07-19 08:13 /usr/local/lib/libsotest.so.1.0.1

davids@ds-ub64-7:/$ # But when I run an executable that refrs to libsotest.so.1, the loader     doesn't find it.
davids@ds-ub64-7:/$ /projects/sotest/exe/sotestexe
/projects/sotest/exe/sotestexe: error while loading shared libraries: libsotest.so.1: cannot     open shared object file: No such file or directory

davids@ds-ub64-7:/$ # Configure loader to display the paths it's searching...it's     searching /usr/lib but not /usr/local/lib
davids@ds-ub64-7:/$ export LD_DEBUG=lib
davids@ds-ub64-7:/$ /projects/sotest/exe/sotestexe
warning: debug option `lib' unknown; try LD_DEBUG=help
/projects/sotest/exe/sotestexe: error while loading shared libraries: libsotest.so.1: cannot     open shared object file: No such file or directory
davids@ds-ub64-7:/$ export LD_DEBUG=libs
davids@ds-ub64-7:/$ /projects/sotest/exe/sotestexe
      6691: find library=libsotest.so.1 [0]; searching
      6691:  search cache=/etc/ld.so.cache
      6691:  search     path=/lib/tls/x86_64:/lib/tls:/lib/x86_64:/lib:/usr/lib/tls/x86_64:/usr/lib/tls:/usr/lib/x86_64:/usr/    lib:/lib/x86_64-linux-gnu/tls/x86_64:/lib/x86_64-linux-gnu/tls:/lib/x86_64-linux-    gnu/x86_64:/lib/x86_64-linux-gnu:/usr/lib/x86_64-linux-gnu/tls/x86_64:/usr/lib/x86_64-linux-    gnu/tls:/usr/lib/x86_64-linux-gnu/x86_64:/usr/lib/x86_64-linux-gnu        (system search path)
      6691:   trying file=/lib/tls/x86_64/libsotest.so.1
      6691:   trying file=/lib/tls/libsotest.so.1
      6691:   trying file=/lib/x86_64/libsotest.so.1
      6691:   trying file=/lib/libsotest.so.1
      6691:   trying file=/usr/lib/tls/x86_64/libsotest.so.1
      6691:   trying file=/usr/lib/tls/libsotest.so.1
      6691:   trying file=/usr/lib/x86_64/libsotest.so.1
      6691:   trying file=/usr/lib/libsotest.so.1
      6691:   trying file=/lib/x86_64-linux-gnu/tls/x86_64/libsotest.so.1
      6691:   trying file=/lib/x86_64-linux-gnu/tls/libsotest.so.1
      6691:   trying file=/lib/x86_64-linux-gnu/x86_64/libsotest.so.1
      6691:   trying file=/lib/x86_64-linux-gnu/libsotest.so.1
      6691:   trying file=/usr/lib/x86_64-linux-gnu/tls/x86_64/libsotest.so.1
      6691:   trying file=/usr/lib/x86_64-linux-gnu/tls/libsotest.so.1
      6691:   trying file=/usr/lib/x86_64-linux-gnu/x86_64/libsotest.so.1
      6691:   trying file=/usr/lib/x86_64-linux-gnu/libsotest.so.1
      6691: 
/projects/sotest/exe/sotestexe: error while loading shared libraries: libsotest.so.1: cannot     open shared object file: No such file or directory
davids@ds-ub64-7:/$

Answer 1

/usr/lib and /lib are hardcoded IIRC.

And adding a PATH in ld.conf.so is not enough, you have to regenerate the cache and the links, using ldconfig.

You can check which library are in the cache with ldconfig -p.