c++ - Why does g++ look in LIBRARY_PATH/../lib64 and where is this documented?

Question

My LIBRARY_PATH environment variable has a custom directory in it: /cs/public/lib/pkg/opencv/lib.

But, when I use g++ --print-search-dirs, I get this instead:

libraries: =
/cs/public/lib/pkg/opencv/lib/x86_64-suse-linux/4.6/:
/cs/public/lib/pkg/opencv/lib/../lib64/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../../x86_64-suse-linux/lib/x86_64-suse-linux/4.6/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../../x86_64-suse-linux/lib/../lib64/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../x86_64-suse-linux/4.6/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../../lib64/:
/lib/x86_64-suse-linux/4.6/:
/lib/../lib64/:
/usr/lib/x86_64-suse-linux/4.6/:
/usr/lib/../lib64/:
/cs/public/lib/pkg/opencv/lib/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../../x86_64-suse-linux/lib/:
/usr/lib64/gcc/x86_64-suse-linux/4.6/../../../:
/lib/:
/usr/lib/

Why does g++ look in these alternatives and a whole bunch of other system locations before what I explicitly specify in the LIBRARY_PATH variable, and where is this documented?

I would understand if system defaults were searched before LIBRARY_PATH and LIBRARY_PATH/../lib64, etc, but g++ puts LIBRARY_PATH/../lib64, then system paths, then LIBRARY_PATH. Where is this ordering documented?

My g++ version is g++ (SUSE Linux) 4.6.2

My OS is openSUSE 12.1 (x86_64)

Answer 1

A similar question was asked here: g++ searches /lib/../lib/, then /lib/

These scary-looking search paths are determined at least in part when the compiler itself it built, for example during the configure phase. It's clear that it goes beyond environment variables because it's possible to have multiple copies of GCC installed and have each of them give different results for gcc --print-search-dirs. Also noting that g++ --print-search-dirs and gcc --print-search-dirs give different results points out that the g++ wrapper is also affecting the search path. Besides configure/build time differences, GCC is definitely aware of the path where its own executable is, and will search subdirectories of that path. A lot of this alchemy can be found in the GCC documentation:
http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Directory-Options.html#Directory-Options
http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Environment-Variables.html#Environment-Variables

As far as I know, the most forceful thing that you can without compiling your own copy of GCC is to specify your custom libraries using the -L option. The reason I say this is because -L is searched before e.g. LIBRARY_PATH (see the above link on environment variables). In order to make it more tolerable you could add an alias for g++ including the -L option in your .bashrc file.

If you want a definitive answer then downloading a copy of the GCC source code is one way. For example, in gcc.c the following highly suggestive comment appears:

/* Build a list of search directories from PATHS.
   PREFIX is a string to prepend to the list.
   If CHECK_DIR_P is true we ensure the directory exists.
   If DO_MULTI is true, multilib paths are output first, then
   non-multilib paths.
   This is used mostly by putenv_from_prefixes so we use `collect_obstack'.
   It is also used by the --print-search-dirs flag.  */

However the function that follows the comment is not very obvious.