Thank you for the report, let me take a look at this within couple of days.

Actually I think nm is imprecise here. readelf shows that symbol is indeed hidden:

$ readelf --dyn-syms -W libmkl_rt.so.2 | grep '\<zupmtr_64_\>'
 45616: 00000000006acbb0   176 FUNC    GLOBAL DEFAULT   13 zupmtr_64_
$ ~/src/SymbolHider/bin/sym-hider libmkl_rt.so.2 zupmtr_64_
$ readelf --dyn-syms -W libmkl_rt.so.2 | grep '\<zupmtr_64_\>'
 45616: 00000000006acbb0   176 FUNC    GLOBAL HIDDEN    13 zupmtr_64_

Great, it's exactly what I need!

I plan to use it in the cross-compiler BinaryBuilder.jl.
Is it working if the shared library libmkl_rt.so and sym-hider are compiled on different platforms?

I will compile SymbolHider on the host (x86_64-linux-musl) and use it to hide the symbols of the shared library of
the following platforms:

• x86_64-apple-darwin
• x86_64-linux-gnu
• x86_64-w64-mingw32

I suppose that the support of 32-bit ELFs is needed for the platforms:

• i686-linux-gnu
• i686-w64-mingw32

But it's already great if I can use it for the 3 main 64-bits platforms.

I plan to use it in the cross-compiler BinaryBuilder.jl. Is it working if the shared library libmkl_rt.so and sym-hider are compiled on different platforms?

It should although TBH I haven't tested it. I'll be around for bugs if any.

x86_64-apple-darwin
x86_64-w64-mingw32

These might be a problem - unfortunately they use different object file formats (Mach-O and PE) whereas SymbolHider only supports ELFs at the moment (I should make it clear in README). I can't say right now whether it's possible to modify symbol table for linked executables on these platforms.

I suppose that the support of 32-bit ELFs is needed for the platforms:
* i686-linux-gnu
* i686-w64-mingw32

This can be added if needed.