How to make a standalone dynamic library (instead of an executable)?

[stepelu]

Description

Following the documentation at https://github.com/stan-dev/math#installation I have been able to successfully compile the test program.
The issue is that in my use case I would like to call some stan::math functions from another language relying on a dynamic library with C ABI.
To make things more concrete suppose that the test_fun.cpp file looks like this:

#include <stan/math.hpp>

extern "C"
double test_fun(double x)
{
  return stan::math::normal_log(x, 2, 3);
}

Basically I would need something like: make -f path/to/stan-math/make/standalone-dyn-lib test_fun that compiles a dynamic library instead of an executable.
As I am not familiar with makefiles, I have been relying on inspecting the output of make --debug=v -f path/to/stan-math/make/standalone test_fun to figure out the clang++ (I am on OSX right now) command, and appending the -fPIC -shared flags.
This seems to work but I am getting a segmentation fault when the software using this dynamic library terminates, so I am not too confident about whether this approach is appropriate.
Would what I am asking for be easily achievable by modifying the standalone makefile?
Is the flag combination responsible for the segmentation fault I am getting?
For completeness the whole clang++ command is:

$ clang++ -std=c++1y -Wno-unknown-warning-option -Wno-tautological-compare -Wno-sign-compare -D_REENTRANT       -I /Users/sp/prj/math/lib/tbb_2019_U8/include -O3  -I /Users/sp/prj/math/ -I /Users/sp/prj/math/lib/eigen_3.3.3 -I /Users/sp/prj/math/lib/boost_1.72.0 -I /Users/sp/prj/math/lib/sundials_4.1.0/include    -DBOOST_DISABLE_ASSERTS            -Wl,-L,"/Users/sp/prj/math/lib/tbb" -Wl,-rpath,"/Users/sp/prj/math/lib/tbb"  test_fun.cpp          /Users/sp/prj/math/lib/sundials_4.1.0/lib/libsundials_nvecserial.a /Users/sp/prj/math/lib/sundials_4.1.0/lib/libsundials_cvodes.a /Users/sp/prj/math/lib/sundials_4.1.0/lib/libsundials_idas.a /Users/sp/prj/math/lib/sundials_4.1.0/lib/libsundials_kinsol.a  /Users/sp/prj/math/lib/tbb/libtbb.dylib /Users/sp/prj/math/lib/tbb/libtbbmalloc.dylib /Users/sp/prj/math/lib/tbb/libtbbmalloc_proxy.dylib -fPIC -shared -o test_fun.dylib

Thank you in advance for any help!

Current Version:

HEAD as of this issue submission time.

[IvanYashchuk]

Hello @stepelu, what you need can be done with a few additional lines in the original standalone makefile. Here is the modified file https://gist.github.com/IvanYashchuk/d2e878cd5a7402ecaaf2b7e6b5f59bcd

Run it with make -f standalone-shared-lib.make test_fun.so and it will produce the shared library that you can later link to other programs.
I think OSX supports both -shared that produces .so files and -dynamiclib flags that produce .dylib. If you want .dylib extension try adding to the makefile

%.dylib: %.o
	$(CXX) -dynamiclib $< -o $@