Where does ffi.load look for dynamic libraries by default in TeXLive?

Question

Failing to compile an amazing piece of code by Henri Menke, he pointed me out in comments that ffi was maybe not able to find the right dynamic library on my system.

Here is a MCE to work with (must be compiled with lualatex --shell-escape).

\documentclass{standalone}
\usepackage{luacode}
\begin{luacode*}
  local ffi = require("ffi")
  local gsl = ffi.load("gsl")
\end{luacode*}
\begin{document}
Test
\end{document}

I'm running an up-to-date vanilla TeXLive 2019 on archlinux. The library (gsl) is installed in /usr/lib/ by my package manager:

christophe@plutonium ~ % ls -1 /usr/lib/libgsl*
/usr/lib/libgslcblas.so
/usr/lib/libgslcblas.so.0
/usr/lib/libgslcblas.so.0.0.0
/usr/lib/libgsl.so
/usr/lib/libgsl.so.25
/usr/lib/libgsl.so.25.0.0

Header files are in /usr/include/gsl/ (see file list).

For now, I'm getting this error:

[\directlua]:2: could not load library gsl
stack traceback:
    [C]: in function 'FFISUPPORTED'
    [\directlua]:2: in main chunk.
\luacode@dbg@exec ...code@maybe@printdbg {#1} #1 }

l.6 \end{luacode*}

My questions are:

1. Where does ffi.load looks by default for the library (here gsl).
2. Is there a way to configure this default path globally in TeXLive?
3. Why does an explicit path like `ffi.load("/usr/lib/gsl") does not work either?

By the way with a slightly different error message:

[\directlua]:2: could not load library /usr/lib/gsl
stack traceback:
    [C]: in function 'ffi.load'
    [\directlua]:2: in main chunk.
\luacode@dbg@exec ...code@maybe@printdbg {#1} #1 }

l.6 \end{luacode*}

---

Update after Henri's comment.

Here is a complete view of the issue.

christophe@plutonium /tmp % cat t.tex 
\documentclass{standalone}
\usepackage{luacode}
\begin{luacode*}
  local ffi = require("ffi")
  local gsl = ffi.load("/usr/lib/libgsl.so")
\end{luacode*}
\begin{document}
Test
\end{document}
% Local Variables:
% coding: utf-8-unix
% TeX-engine: luatex
% End:
christophe@plutonium /tmp % luatex --version
This is LuaTeX, Version 1.10.0 (TeX Live 2019)

Execute  'luatex --credits'  for credits and version details.

There is NO warranty. Redistribution of this software is covered by
the terms of the GNU General Public License, version 2 or (at your option)
any later version. For more information about these matters, see the file
named COPYING and the LuaTeX source.

LuaTeX is Copyright 2019 Taco Hoekwater and the LuaTeX Team.

christophe@plutonium /tmp % luatex --credits
This is LuaTeX, Version 1.10.0 (TeX Live 2019)

The LuaTeX team is Hans Hagen, Hartmut Henkel, Taco Hoekwater, Luigi Scarso.

LuaTeX merges and builds upon (parts of) the code from these projects:

tex       : Donald Knuth
etex      : Peter Breitenlohner, Phil Taylor and friends
omega     : John Plaice and Yannis Haralambous
aleph     : Giuseppe Bilotta
pdftex    : Han The Thanh and friends
kpathsea  : Karl Berry, Olaf Weber and others
lua       : Roberto Ierusalimschy, Waldemar Celes and Luiz Henrique de Figueiredo
metapost  : John Hobby, Taco Hoekwater, Luigi Scarso, Hans Hagen and friends
pplib     : Paweł Jackowski
fontforge : George Williams (partial)
luajit    : Mike Pall (used in LuajitTeX)

Compiled with libpng 1.6.36; using 1.6.36
Compiled with lua version 5.3.5
Compiled with mplib version 2.00
Compiled with zlib 1.2.11; using 1.2.11

Development id: 7127

christophe@plutonium /tmp % ls /usr/lib/*gsl*
/usr/lib/libgslcblas.so  /usr/lib/libgslcblas.so.0  /usr/lib/libgslcblas.so.0.0.0  /usr/lib/libgsl.so  /usr/lib/libgsl.so.25  /usr/lib/libgsl.so.25.0.0
christophe@plutonium /tmp % lualatex --shell-escape t.tex
This is LuaTeX, Version 1.10.0 (TeX Live 2019) 
 system commands enabled.
(./t.tex
LaTeX2e <2019-10-01> patch level 2
tput: unknown terminal "xterm-termite"

luaotfload | main : initialization completed in 0.138 seconds
(/usr/local/texlive/2019/texmf-dist/tex/latex/standalone/standalone.cls
Document Class: standalone 2018/03/26 v1.3a Class to compile TeX sub-files stan
dalone
(/usr/local/texlive/2019/texmf-dist/tex/latex/tools/shellesc.sty)
(/usr/local/texlive/2019/texmf-dist/tex/generic/iftex/ifluatex.sty
(/usr/local/texlive/2019/texmf-dist/tex/generic/iftex/iftex.sty))
(/usr/local/texlive/2019/texmf-dist/tex/latex/xkeyval/xkeyval.sty
(/usr/local/texlive/2019/texmf-dist/tex/generic/xkeyval/xkeyval.tex
(/usr/local/texlive/2019/texmf-dist/tex/generic/xkeyval/xkvutils.tex
(/usr/local/texlive/2019/texmf-dist/tex/generic/xkeyval/keyval.tex))))
(/usr/local/texlive/2019/texmf-dist/tex/latex/standalone/standalone.cfg)
(/usr/local/texlive/2019/texmf-dist/tex/latex/base/article.cls
Document Class: article 2019/10/25 v1.4k Standard LaTeX document class
(/usr/local/texlive/2019/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2019/texmf-dist/tex/generic/luatex85/luatex85.sty)
(/usr/local/texlive/2019/texmf-dist/tex/generic/oberdiek/pdftexcmds.sty
(/usr/local/texlive/2019/texmf-dist/tex/generic/oberdiek/infwarerr.sty)
(/usr/local/texlive/2019/texmf-dist/tex/generic/oberdiek/ltxcmds.sty)
(/usr/local/texlive/2019/texmf-dist/tex/generic/iftex/ifpdf.sty)))
(/usr/local/texlive/2019/texmf-dist/tex/lualatex/luacode/luacode.sty
(/usr/local/texlive/2019/texmf-dist/tex/luatex/luatexbase/luatexbase.sty
(/usr/local/texlive/2019/texmf-dist/tex/luatex/ctablestack/ctablestack.sty)))[\
directlua]:2: could not load library /usr/lib/libgsl.so
stack traceback:
    [C]: in function 'ffi.load'
    [\directlua]:2: in main chunk.
\luacode@dbg@exec ...code@maybe@printdbg {#1} #1 }

l.6 \end{luacode*}


? 

---

Update after second comment.

christophe@plutonium /tmp % LD_DEBUG=libs lualatex --shell-escape --interaction=batchmode t.tex   
      2773: find library=libdl.so.2 [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:   trying file=/usr/lib/libdl.so.2
      2773: 
      2773: find library=libm.so.6 [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:   trying file=/usr/lib/libm.so.6
      2773: 
      2773: find library=libc.so.6 [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:   trying file=/usr/lib/libc.so.6
      2773: 
      2773: 
      2773: calling init: /usr/lib/libc.so.6
      2773: 
      2773: 
      2773: calling init: /usr/lib/libm.so.6
      2773: 
      2773: 
      2773: calling init: /usr/lib/libdl.so.2
      2773: 
      2773: 
      2773: initialize program: lualatex
      2773: 
      2773: 
      2773: transferring control: lualatex
      2773: 
      2773: find library=libc.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:  search path=/usr/lib/tls/haswell/x86_64:/usr/lib/tls/haswell:/usr/lib/tls/x86_64:/usr/lib/tls:/usr/lib/haswell/x86_64:/usr/lib/haswell:/usr/lib/x86_64:/usr/lib        (system search path)
      2773:   trying file=/usr/lib/tls/haswell/x86_64/libc.so
      2773:   trying file=/usr/lib/tls/haswell/libc.so
      2773:   trying file=/usr/lib/tls/x86_64/libc.so
      2773:   trying file=/usr/lib/tls/libc.so
      2773:   trying file=/usr/lib/haswell/x86_64/libc.so
      2773:   trying file=/usr/lib/haswell/libc.so
      2773:   trying file=/usr/lib/x86_64/libc.so
      2773:   trying file=/usr/lib/libc.so
      2773: find library=libgcc.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:  search path=/usr/lib       (system search path)
      2773:   trying file=/usr/lib/libgcc.so
      2773: 
      2773: find library=libm.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:  search path=/usr/lib       (system search path)
      2773:   trying file=/usr/lib/libm.so
      2773: find library=libdl.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:   trying file=/usr/lib/libdl.so
      2773: 
This is LuaTeX, Version 1.10.0 (TeX Live 2019) 
 system commands enabled.
      2775: find library=libreadline.so.8 [0]; searching
      2775:  search cache=/etc/ld.so.cache
      2775:   trying file=/usr/lib/libreadline.so.8
      2775: 
      2775: find library=libdl.so.2 [0]; searching
      2775:  search cache=/etc/ld.so.cache
      2775:   trying file=/usr/lib/libdl.so.2
      2775: 
      2775: find library=libc.so.6 [0]; searching
      2775:  search cache=/etc/ld.so.cache
      2775:   trying file=/usr/lib/libc.so.6
      2775: 
      2775: find library=libncursesw.so.6 [0]; searching
      2775:  search cache=/etc/ld.so.cache
      2775:   trying file=/usr/lib/libncursesw.so.6
      2775: 
      2775: 
      2775: calling init: /usr/lib/libc.so.6
      2775: 
      2775: 
      2775: calling init: /usr/lib/libncursesw.so.6
      2775: 
      2775: 
      2775: calling init: /usr/lib/libdl.so.2
      2775: 
      2775: 
      2775: calling init: /usr/lib/libreadline.so.8
      2775: 
      2775: 
      2775: initialize program: sh
      2775: 
      2775: 
      2775: transferring control: sh
      2775: 
      2775: find library=libtinfow.so.6 [0]; searching
      2775:  search path=/home/christophe/anaconda3/bin/../lib/tls/haswell/x86_64:/home/christophe/anaconda3/bin/../lib/tls/haswell:/home/christophe/anaconda3/bin/../lib/tls/x86_64:/home/christophe/anaconda3/bin/../lib/tls:/home/christophe/anaconda3/bin/../lib/haswell/x86_64:/home/christophe/anaconda3/bin/../lib/haswell:/home/christophe/anaconda3/bin/../lib/x86_64:/home/christophe/anaconda3/bin/../lib        (RPATH from file tput)
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/tls/haswell/x86_64/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/tls/haswell/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/tls/x86_64/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/tls/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/haswell/x86_64/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/haswell/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/x86_64/libtinfow.so.6
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/libtinfow.so.6
      2775: 
      2775: find library=libc.so.6 [0]; searching
      2775:  search path=/home/christophe/anaconda3/bin/../lib      (RPATH from file tput)
      2775:   trying file=/home/christophe/anaconda3/bin/../lib/libc.so.6
      2775:  search cache=/etc/ld.so.cache
      2775:   trying file=/usr/lib/libc.so.6
      2775: 
      2775: 
      2775: calling init: /usr/lib/libc.so.6
      2775: 
      2775: 
      2775: calling init: /home/christophe/anaconda3/bin/../lib/libtinfow.so.6
      2775: 
      2775: 
      2775: initialize program: tput
      2775: 
      2775: 
      2775: transferring control: tput
      2775: 
tput: unknown terminal "xterm-termite"
      2775: 
      2775: calling fini: tput [0]
      2775: 
      2775: 
      2775: calling fini: /home/christophe/anaconda3/bin/../lib/libtinfow.so.6 [0]
      2775: 

luaotfload | main : initialization completed in 0.134 seconds
      2773: find library=gsl [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:  search path=/usr/lib       (system search path)
      2773:   trying file=/usr/lib/gsl
      2773: 
      2773: find library=gsl.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:  search path=/usr/lib       (system search path)
      2773:   trying file=/usr/lib/gsl.so
      2773: 
      2773: find library=libgsl.so [0]; searching
      2773:  search cache=/etc/ld.so.cache
      2773:   trying file=/usr/lib/libgsl.so
      2773: 
      2773: /usr/lib/libgsl.so: error: symbol lookup error: undefined symbol: cblas_ctrmv (fatal)
      2773: 
      2773: calling fini: lualatex [0]
      2773: 
      2773: 
      2773: calling fini: /usr/lib/libdl.so.2 [0]
      2773: 
      2773: 
      2773: calling fini: /usr/lib/libm.so.6 [0]

Answer 1

The default paths are just your regular system default library paths. Technically the name is passed (at least on UNIX-like systems like Linux) to the system function dlopen. This function is responsible for searching the library and it's behaviour is documented in it's man page DLOPEN(3). (On Windows the function is called LoadLibraryA)

But you should never have to worry about it, just put the libraries where all your normal libraries are and you will be fine. You can also pass a path, but then you need to use the right filename: There is no file named /usr/lib/gsl, the library is in /usr/lib/libgsl.so. (This is automatically inserted if you omit the path)

But in your case, the problem isn't that the library isn't found, the problem is that the library can't be loaded alone:

On your system, the GSL always has to be loaded together with a CBLAS implementation, for example gslcblas which is shipped with GSL. (See the documentation for details on linking GSL, especially on how to load another BLAS implementation to get better performance)

As Henri Menke mentioned, this problem is not necessary on all platforms, especially Debian links the library in a way which ensures that this is not necessary. But even to Debian users I would recommend to always load a CBLAS implementation manually: This makes your document more portable and ensures that it can also be compiled on other systems.

So your code works if you add the additional library:

\documentclass{standalone}
\usepackage{luacode}
\begin{luacode*}
  local ffi = require("ffi")
  ffi.load("gslcblas", true) -- Make the symbols available globally
  local gsl = ffi.load"gsl"
\end{luacode*}
\begin{document}
Test
\end{document}