[Tutor] Set LD_LIBRARY_PATH and equivalents platform-independently

[Albert-Jan Roskam]

> On Thu, Jan 17, 2013 at 10:33 AM, Albert-Jan Roskam <fomcl at yahoo.com> 
> wrote:
>> 
>>  The goal is to load the C libraries (dll, so, dylib, etc) that my program
>>  needs.
>> 
>>  Anyway, I looked up your two suggestions about library_dirs and
>>  runtime_library_dirs. What is meant by "at link time"?
> 
> library_dirs adds search paths for the linker (e.g. ld) for finding
> shared libraries (.so) and static libraries (.a archives of
> relocatable object files).
> 
> Linking to a shared library is handled by the runtime loader/linker
> (e.g. /lib/ld-linux.so.2). For Linux, ld.so searches the
> system-defined directories (/lib, /usr/lib, and those set by
> /etc/ld.so.conf), which are cached in /etc/ld.so.cache (created by
> ldconfig). This is where distutils runtime_library_dirs comes into
> play. For ELF it configures an embedded RPATH, which the loader
> searches before the system directories. You can also convert the RPATH
> to a RUNPATH, which can be overridden by LD_LIBRARY_PATH.

Thank you! I am getting a whole lot wiser wrt Linux. I checked 'man ld' and 'man ldconfig'.
Especially the ld command is pretty extensive. The Rpath/Runpath solution seems nice in that
no wrapper is needed (contrary to when one uses LD_LIBRARY_PATH). But is Windows the only
exception in the way that libraries are dealt with? Or do DLLs also have a dynamic area?

 
>>  Does this mean that the ELF header of the library itself is modified
> 
> readelf -d shows the .dynamic section (including strings from
> .dynstr), which you can use to verify the RPATH/RUNPATH. chrpath lets
> you change (but not add) an RPATH, up to its existing length. It also
> lets you convert an RPATH to a RUNPATH. patchELF can add or extend an
> RPATH/RUNPATH.
> 
> http://nixos.org/patchelf.html
> 
>>  The libraries I am using are copyrighted (one can freely use them, but
>>  no reverse engineering, disentangling, etc). I am not sure whether
>>  adding an rpath will be copyright infringement. Logically, I'd say no,
>>  but I doubt whether logic applies in legal stuff.
> 
> It should be OK for internal administration. You'd have to ask a legal
> expert about distribution.

Yes, that would indeed be interesting to know. Unfortuntately, unlike developers, these folks are hard to get in touch with.
Maybe theyŕe also a rare species. I think I'ĺl have to follow conservative approach here and not patch the ELF.

> If you stick with LD_LIBRARY_PATH, etc, keep in mind it has to be set
> before the process starts, typically in a wrapper script. On Windows,
> however, you can modify PATH at runtime.

So if, at runtime, I do something evil like del os.environ['PATH'] in Windows this actually deletes/propagates to my path?? Unlike other OS where os.environ is 'just' a dict that contains the paths, that once loaded, cannot be modified (well you can modify them, but the OS won know about the changes).