How to apply -fvisibility option to symbols in static libraries?

Question

I have a shared library project that is built from 4 static libraries (.a) and one object (.o) file. I am trying to add the -fvisibility=hidden option to restrict symbols in the output to only those that I mark in the source with an __attribute__.

I've added the -fvisibility=hidden option to the compile options for the .so project (which covers the .o file) and for the .a projects.

The symbols in the object file are removed as expected from the final .so. However the symbols from the .a projects are all still in the final .so file. Adding the -fvisibility=hidden option to the .so link command has no effect.

What am I doing wrong?

My purpose here is to remove from the .so all symbols except the interface functions to the library.

EDIT: I actually used a version map to solve this for now. However it requires continued maintenance of the version script as external symbols change. Accepted answer has a better idea.

Answer 1

Basically, visibility is handled during linking, and the linker doesn't seem impose it on static archives. A related question (though not a duplicate) was asked on SO here.

What I would advise you to do is to replace your linking stage: gcc -shared -o mylib.so foo.o libbar.a into a two stages process where you get back the object files:

• ar x libbar.a (possibly into a suitable, empty directory)
• gcc -fvisibility=hidden -shared -o mylib.so foo.o tempdir/*.o

Answer 2

Simply pass -Wl,--exclude-libs,ALL to gcc

This will tell the linker to transform all the symbols in the static libraries to hidden.

--exclude-libs also accepts a list of archives (i.e. static library names) for finer granularity on which libraries to hide symbols from.

Note: this will only work in systems using GNU binutils (e.g. Linux) or with a linker supporting --exclude-libs (e.g. it won't work with OSX's ld64)

Answer 3

This is an answer to the problem for OS X.

The Mac ld doesn't support --exclude-libs, but it does support -exported_symbol sym and it does apply this to object files in static libraries. And when you're filtering to a public API, the whitelist is small enough to spell it out.

I ended up with the following in my Makefile to generate a -Wl,-exported_symbol,_api_func_1 flag for each exported symbol:

SYMBOLS   = api_func_1 api_func_2 api_func_3 api_func_4
SYMBOLS   += api_func_5 # add more as necessary
COMMA     = ,
LDFLAGS   += $(addprefix -Wl$(COMMA)-exported_symbol$(COMMA)_,$(SYMBOLS))

# ...

libmyapi.so: # ...
    $(CC) -shared -o $@ ... $(LDFLAGS)

Then you can if-gate between this version of the flags and the GNU ld version after detecting which linker the system has.