Ld_Library_Path Doesn't Seem to Work

LD_LIBRARY_PATH doesn't seem to work

Try setting LIBRARY_PATH, instead of LD_LIBRARY_PATH.

From the gcc man page:

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries the
directories thus specified when searching for special linker files, if
it can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses
these directories when searching for ordinary libraries for the -l
option (but directories specified with -L come first).

Why changing LD_LIBRARY_PATH has no effect in Ubuntu?

If you look at the output of your ls -al

These are soft links that you have. Your softlink libQt5MultimediaWidgets.so.5 points to libQt5MultimediaWidgets.so.5.9.2 in the same directory and the file is not there at all. So you need to either set the correct softlink path or have the file in same directory

LD_LIBRARY_PATH doesn't take effect for LibXML.so

tl;dr

ExtUtils::MakeMaker hardcodes paths of the shared libraries in the binaries by means of RPATH entry in .dynamic section. But you can preload the needed library,

$ LD_PRELOAD=path/to/custom/libxml2/.libs/libxml2.so.2 ldd ./blib/arch/auto/XML/LibXML/LibXML.so

or make the loader ignore RPATH entry,

$ LD_LIBRARY_PATH=path/to/custom/libxml2/.libs /usr/lib/ld-linux-x86-64.so.2 --inhibit-rpath /abs/path/to/LibXML.so /abs/path/to/perl ./1.pl

or convert RPATH entry into RUNPATH one,

$ chrpath --convert blib/arch/auto/XML/LibXML/LibXML.so

or delete RPATH entry,

$ chrpath --delete blib/arch/auto/XML/LibXML/LibXML.so

or specify XMLPREFIX variable,

$ perl Makefile.PL XMLPREFIX=path/to/custom/libxml2/build

Under the hood

ExtUtils::Liblist::ext() takes a list of libraries to be linked with, like -lxml2 -lz -llzma -licui18n -licuuc -licudata -lm -ldl, and turns them it into four or five variables, that make their way into the generated Makefile. One of those is LD_RUN_PATH. It contains all the paths where the libraries where found.

$is_dyna is true if the library is dynamic (not ends with .a). $is_perl is true if Perl was not built with it, and %ld_run_path_seen is to not duplicate values in LD_RUN_PATH.

The part that calculates LD_RUN_PATH, that generates part of the Makefile with the variable, and the part that passes it to the linker.

Consequences

LD_RUN_PATH and in its turn RPATH entry in the binary make the loader search for libraries (at runtime) in the directories where they where found (at compile time).

-rpath=dir
Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All -rpath arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The -rpath option is also used when locating shared objects which are needed by shared objects explicitly included in the link; see the description of the -rpath-link option. If -rpath is not used when linking an ELF executable, the contents of the environment variable "LD_RUN_PATH" will be used if it is defined.

https://jlk.fjfi.cvut.cz/arch/manpages/man/core/binutils/ld.1.en

If a shared object dependency does not contain a slash, then it is searched for in the following order:

o Using the directories specified in the DT_RPATH dynamic section attribute of the binary if present and DT_RUNPATH attribute does not exist. Use of DT_RPATH is deprecated.

o Using the environment variable LD_LIBRARY_PATH, unless the executable is being run in secure-execution mode (see below), in which case this variable is ignored.

https://jlk.fjfi.cvut.cz/arch/manpages/man/core/man-pages/ld.so.8.en

$ perl Makefile.PL
$ make

Result,

$ readelf --dynamic ./blib/arch/auto/XML/LibXML/LibXML.so | grep RPATH
 0x000000000000000f (RPATH)              Library rpath: [/usr/lib]

$ objdump -x ./blib/arch/auto/XML/LibXML/LibXML.so | egrep RPATH
  RPATH                /usr/lib

$ LD_LIBRARY_PATH=path/to/custom/libxml2/.libs ldd ./blib/arch/auto/XML/LibXML/LibXML.so | grep libxml2
    libxml2.so.2 => /usr/lib/libxml2.so.2 (0x00007f6cfabb2000)

--dynamic - display contents of the .dynamic section, -x - display all headers.

Solution

One way to remedy this is to preload needed instance of libxml2,

$ LD_PRELOAD=path/to/custom/libxml2/.libs/libxml2.so.2 ldd ./blib/arch/auto/XML/LibXML/LibXML.so | grep libxml2
    path/to/custom/libxml2/build/lib/libxml2.so.2 (0x00007fe183aeb000)

Another is to tell Makefile.PL search for libxml2 in the desired location (at compile time).

$ perl Makefile.PL XMLPREFIX=path/to/custom/libxml2/build
$ make

Result,

$ readelf --dynamic ./blib/arch/auto/XML/LibXML/LibXML.so | grep RPATH
 0x000000000000000f (RPATH)              Library rpath: [path/to/custom/libxml2/build/lib:/usr/lib]

$ objdump -x ./blib/arch/auto/XML/LibXML/LibXML.so | egrep RPATH
  RPATH                path/to/custom/libxml2/build/lib:/usr/lib

$ ldd ./blib/arch/auto/XML/LibXML/LibXML.so | grep libxml2
    libxml2.so.2 => path/to/custom/libxml2/build/lib/libxml2.so.2 (0x00007fe183aeb000)

On a side note

Should you care to debug XML::LibXML, you can run make with OPTIMIZE variable (add -B to make make rebuild everything in case the library has already been built),

$ make OPTIMIZE='-g3 -O0'

There's also -O2, and no -g in LDDLFLAGS, but not sure if that matters much.

Alternatively, WriteMakeFile takes OPTIMIZE parameter, so you can add a line here,

'OPTIMIZE' => '-g3 -O0'

Or you can add the variable here, and pass it to Makefile.PL,

$ perl Makefile.PL OPTIMIZE='-g3 -O0'

LD_LIBRARY_PATH is ignored by GCC

I was wrong! thanks to Alexandre C and David Schwartz!

LD_LIBRARY_PATH is only for loader(runtime).

LIBRARY_PATH is what I need according to the docs:

The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries the
directories thus specified when searching for special linker files, if
it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses
these directories when searching for ordinary libraries for the -l
option (but directories specified with -L come first).

LD_LIBRARY_PATH not working while LD_PRELOAD works fine

You need to give paths in LD_LIBRARY_PATH variable:

LD_LIBRARY_PATH=$PWD ./program args

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