where will the system search for dynamic libraries?

I want to know where the system will search for dynamic libraries. Let’s take nxclient as an example. ldd /usr/NX/bin/nxclient gives me some output, for example

libpng12.so.0 => /usr/NX/lib/libpng12.so.0 (0x00007fcb4a16f000)

Now why did the system select /usr/NX/lib/libpng12.so.0 and not for example /usr/lib64/libpng12.so.0? Both exist. Where is this configured? /etc/ld.so.conf* does not contain /usr/NX and my $LD_LIBRARY_PATH is empty.

Answers:

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Method 1

It is possible that the library path might be hard coded in the binary.

From the RPATH Wikipedia Page

The dynamic linker of the GNU C Library and its derivative Embedded
GLIBC implement a rather complicated algorithm for searching for
shared libraries. The basic search order is:

1. The (colon-separated) paths in the DT_RPATH dynamic section attribute of the binary if present and DT_RUNPATH attribute does not
   exist.
2. The (colon-separated) paths in the environment variable LD_LIBRARY_PATH, unless the executable is a setuid/setgid binary, in
   which case it is ignored. LD_LIBRARY_PATH can be overridden by calling
   the dynamic linker with the option –library-path (e.g.
   /lib/ld-linux.so.2 –library-path $HOME/mylibs myprogram).
3. The (colon-separated) paths in the DT_RUNPATH dynamic section attribute of the binary if present.
4. Lookup based on the ldconfig cache file (often located at /etc/ld.so.cache) which contains a compiled list of candidate
   libraries previously found in the augmented library path (set by
   /etc/ld.so.conf). If, however, the binary was linked with the -z
   nodeflib linker option, libraries in the default library paths are
   skipped.
5. In the trusted default path /lib, and then /usr/lib. If the binary was linked with the -z nodeflib linker option, this step is
   skipped.

To see if a binary has an RPATH configured:

readelf -d filename Shows only data from the “dynamic” section

The “dynamic” section of the header is of interest because it contains
data used during the initial loading process, such as:

NEEDED: libraries needed by this module
RPATH: See “Loader search procedure” below
SONAME: If this module is a library, this item shows the “soname” of the library.

Source: The linux loader, and how it finds libraries: ld-linux and so on

To view all libraries in the ldconfig cache you can run

ldconfig -p

An example of RUNPATH set in a binary

$ readelf -d /opt/teamviewer9/tv_bin/TVGuiSlave.64 | grep -i RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: [${ORIGIN}/../lib]

Method 2

The system will search first within LD_LIBRARY_PATH and then in the paths specified in /etc/ld.so.conf.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0