Find default library paths location in Ubuntu

Question

I'm on an Ubuntu 22.04 system and I'm working with C language and libraries.

I know (from different books, included "The Linux Programming Interface" by Kerrisk) that this algorithm is used when looking for runtime shared libraries locations (I just omitted the preloaded libraries for the sake of simplicity):

When RUNPATH field is specified (i.e. DT_RUNPATH is non-empty)

1. LD_LIBRARY_PATH
2. runpath (DT_RUNPATH field)
3. ld.so.cache
4. default library paths (/lib and /usr/lib)

In the absence of RUNPATH (i.e. DT_RUNPATH is empty string)

1. RPATH
2. LD_LIBRARY_PATH
3. ld.so.cache
4. default library paths (/lib and /usr/lib)

The problem is that most books say that generally speaking the default library paths are /lib and /usr/lib but they don't mention how I can get the exact default library paths for my system.

1. What command can I use on my Ubuntu 22.04 system to get the exact default library path?

Answer 1

You can use ld, which can show system library paths and also your user's defined library paths.

ld --verbose | grep SEARCH_DIR | tr -s ' ;' \\012

If ld does not exists in your system you may install it by

sudo apt install binutils

Note: It might be a bit heavy however you will get exact results, but in your case you must be already installed it.

Answer 2

Ubuntu uses glibc, and glibc supports some debugging options which can be of help here. From man ld.so:

LD_DEBUG (since glibc 2.1)
      Output verbose debugging information about operation of the  dynamic  linker.   The
      content  of  this variable is one of more of the following categories, separated by
      colons, commas, or (if the value is quoted) spaces:

      help        Specifying help in  the  value  of  this  variable  does  not  run  the
                  specified  program,  and displays a help message about which categories
                  can be specified in this environment variable.

      all         Print all debugging information  (except  statistics  and  unused;  see
                  below).
      bindings    Display information about which definition each symbol is bound to.
      files       Display progress for input file.
      libs        Display library search paths.
      reloc       Display relocation processing.
      scopes      Display scope information.
      statistics  Display relocation statistics.
      symbols     Display search paths for each symbol look-up.
      unused      Determine unused DSOs.
      versions    Display version dependencies.

And in addition, you can suppress the cache using:

--inhibit-cache
      Do not use /etc/ld.so.cache.

With that, all you need to do is ensure that LD_LIBRARY_PATH isn't set, and find a binary that doesn't mess with RPATH or RUNPATH. For example:

env -u LD_LIBRARY_PATH LD_DEBUG=libs /lib64/ld-linux-x86-64.so.2 --inhibit-cache /bin/true # on amd64
env -u LD_LIBRARY_PATH LD_DEBUG=libs /lib/ld-linux-armhf.so.3 --inhibit-cache /bin/true # on armhf

Answer 3

Assuming you're on amd64, you can execute /lib64/ld-linux-x86-64.so.2 --help, the bottom sections will give you what you're looking for:

Shared library search path:
  (libraries located via /etc/ld.so.cache)
  /lib/x86_64-linux-gnu (system search path)
  /usr/lib/x86_64-linux-gnu (system search path)
  /lib (system search path)
  /usr/lib (system search path)

Subdirectories of glibc-hwcaps directories, in priority order:
  x86-64-v4 (supported, searched)
  x86-64-v3 (supported, searched)
  x86-64-v2 (supported, searched)

You can use similar approaches for other arches, the specific name and location of the linker will likely be different though.