I'm developing a small OpenMP tool library that will allow me to utilise OpenMP callbacks. This library is written entirely in C. I can compile and link it into my user code. To compile the library I use the following commands in a Makefile:
CC = clang
CFLAGS =-O3 -fopenmp
LDLIBS =-lomp
libompt.so: counter.c ompt.c
$(CC) $(CFLAGS) -shared -fPIC $^ -o $@
To compile my little example statically or dynamically I have these targets in the Makefile:
test_daxpy_sta: counter.c ompt.c daxpy.c test_daxpy.c
$(CC) $(CFLAGS) $(LDLIBS) $^ -o $@
test_daxpy_dyn: daxpy.c test_daxpy.c libompt.so
$(CC) $(CFLAGS) -L. -lompt $(LDLIBS) $^ -o $@
All works nice and well. Now I would like to use a C++ object and its methods inside OpenMP on_ompt_callback_thread_begin() and on_ompt_callback_thread_end() callbacks. To achieve that I need to compile my little OpenMP tool library with a C++ compiler. The only change to the commands above is:
CC = clang++
Compiled with clang++ the OpenMP tool no longer works. It does not register callbacks and does not do any "profiling" work behind the scenes. I do not know why. What is a correct way to enable an OpenMP tool with callbacks in a C++ code?
I compiled my library and static and dynamic executables with clang and clang++. Then I used ldd on my library and the executables. The only difference is that test_daxpy_sta compiled with clang++ uses libgcc_s.so.1 => /usr/lib/libgcc_s.so.1. Is this right? Shouldn't clang++ rely on an LLVM equivalent of libgcc_s? I found clang++ option --rtlib, but I don't know, what is LLVM's counterpart of GNU's libgcc_s.
The OpenMP runtime loads a tool by searching for a globally-visible function with the name ompt_start_tool [1]. If you are compiling your tool with a C++ compiler, make sure this function is declared extern "C" so the function's name is not mangled and can be found by the OpenMP runtime [2].
You can check whether the ompt_start_tool function has been mangled using the nm command to see the symbols defined in your compiled library:
nm libompt.so | grep ompt_start_tool
As an additional tip for diagnosing the loading of OpenMP tools, you can use the OMP_TOOL_VERBOSE_INIT environment variable to diagnose problems with detecting and loading tools at runtime [3]. This shold help you verify whether the runtime is able to find and load your tool at all.
[1] https://www.openmp.org/spec-html/5.0/openmpsu177.html