I am building a shared object on Ubuntu 16.04 which uses libgomp. My goal is to make this final object as portable as possible, by static linking anything not normally in a base distribution (using docker ubuntu or alpine images as a reference baseline). I've been able to do this with my other dependencies pretty easily, but I'm hung up on libgomp.
I can link just fine with the -fopenmp option, and get a dynamic link:
# ldd *.so
linux-vdso.so.1 => (0x00007fff01df4000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f9ba59db000)
libgomp.so.1 => /usr/lib/x86_64-linux-gnu/libgomp.so.1 (0x00007f9ba57b9000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9ba55a3000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9ba5386000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9ba4fbc000)
/lib64/ld-linux-x86-64.so.2 (0x00007f9ba6516000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f9ba4db8000)
But if I naively add -static before -fopenmp I get:
relocation R_X86_64_32 against `__TMC_END__' can not be used when making a shared object; recompile with -fPIC
Fair enough; with my other dependencies I've just built from source to enable PIC and any other options I needed. When I try to do the same with libgomp, though, I'm not having any luck. I checked out gcc 5.5 from http://gcc.gnu.org/svn/gcc, and tried building from the gcc/libgomp folder. There is a configure script already generated, but running it returns:
./config.status: line 1486: ./../../config-ml.in: No such file or directory
OK, apparently this has something to do with multilibrary support, which I don't believe I need. Running ./configure --help shows that there is an --enable-multilib option with no obvious default, but setting --enable-multilib=no or --disable-multilib still returns the same error. I've also tried running autoreconf -fiv to regenerate the configure script, but I get this error:
configure.ac:5: error: Please use exactly Autoconf 2.64 instead of 2.69.
If I explicitly install and use autoreconf2.64, I get this one:
configure.ac:65: error: Autoconf version 2.65 or higher is required
What am I missing?
What I was missing was the fact that libgomp is not buildable separate from the rest of gcc. It was just a matter of going up a level and running the whole build with -fPIC enabled:
export CFLAGS="-O3 -fPIC"
export CXXFLAGS="-O3 -fPIC"
./configure --disable-multilib --enable-languages=c,c++
make
make install
That gave me a copy of libgomp.a in /usr/local/lib64 ready for linking in to my shared object.
Follow up:
While this worked, at least in a test environment, after the comments above from Jim Cownie we decided to just disable OpenMP support from our library for now.