Shared libraries in linux

June 9, 2016

Programs on a system need to use libraries of code. It makes sense to share those libraries because several programs might utilize the same library. That way it doesn’t require as much RAM and disk space to run 10 programs that it would if they didn’t share libraries.

ldd

The above command allows me to find out what libraries that a particular program depends on. Such as:

$ ldd /bin/ls
 linux-vdso.so.1 =&gt; (0x00007ffe290c4000)
 libselinux.so.1 =&gt; /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f19c3966000)
 libacl.so.1 =&gt; /lib/x86_64-linux-gnu/libacl.so.1 (0x00007f19c375d000)
 libc.so.6 =&gt; /lib/x86_64-linux-gnu/libc.so.6 (0x00007f19c3392000)
 libpcre.so.3 =&gt; /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f19c3125000)
 libdl.so.2 =&gt; /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f19c2f21000)
 /lib64/ld-linux-x86-64.so.2 (0x0000557b7392b000)
 libattr.so.1 =&gt; /lib/x86_64-linux-gnu/libattr.so.1 (0x00007f19c2d1b000)
 libpthread.so.0 =&gt; /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f19c2afd000)

Alakazam, you have a list of libraries in use by the ‘ls’ command.

Some key items in /etc/:

ld.so.conf « tells system where to look for library files (points to ld.so.conf.d/)
ld.so.conf.d/ « a folder full of conf files specifying locations of various libraries in available packages
ld.so.cache « Makes it so the system doesn’t have to read the ld.so.conf file, then following the bread crumbs. The ‘ldconfig’ command generates this cache file. It’s binary.

This environment variable allows you to expand the system’s reach to other libraries you may want to put in other places:

LD_LIBRARY_PATH

I tested on ubuntu and centos though and it’s empty by default, so it must be used only as a way to add something to what’s already in ld.so.cache

libraries