Stripping symbols from shared library seems not to work

Question

I am trying to strip non-public symbols from my shared library using the strip command, but when I afterwards apply "nm -C -D" I still see all symbols.

I've alread checked this and other posts: Stripping linux shared libraries as well as the gcc Wiki: https://gcc.gnu.org/wiki/Visibility But with my sample code the proposed solution seems not to work.

I declare two simple classes, one meant to remain visible, the other to be removed. For the public one I declare __attribute__ ((visibility ("default"))) in the source.

When I compile and link with -fvisibility=hidden and subsequently run "strip --strip-all --discard-all" on the shared library, both classes are still visible in the T(ext) section.

This is my header file:

#pragma once

#if __GNUC__ >= 4
    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
#else
    #define DLL_PUBLIC
#endif

class DLL_PUBLIC PublicClass
{
public:
  PublicClass() = default;
  ~PublicClass() = default;

  void doSomethingPublic();
};

class PrivateClass
{
public:
  PrivateClass() = default;
  ~PrivateClass() = default;

  void doSomethingPrivate();
};

and this the source:

#include <iostream>
#include "test.hpp"

void PublicClass::doSomethingPublic() { std::cout << "public call" << std::endl; } 
void PrivateClass::doSomethingPrivate() { std::cout << "private call" << std::endl; }

I compile an object file and link the shared library with gcc 7.4.0 on Ubuntu 18.04:

$ /usr/local/bin/c++ -fvisibility=hidden -O3 -DNDEBUG -fPIC -std=gnu++14 -c test.cpp -o test.cpp.o
$ /usr/local/bin/c++ -fvisibility=hidden -fPIC -DNDEBUG -O3 -shared -Wl,-soname,libtest.so -o libtest.so test.cpp.o

Applying the strip command and displaying the symbol table:

$ strip --strip-all --discard-all --verbose libtest.so
copy from `libtest.so' [elf64-x86-64] to `stSNqAeg' [elf64-x86-64]
$ nm -C -D libtest.so
0000000000201058 B __bss_start
                 U __cxa_atexit
                 w __cxa_finalize
0000000000201058 D _edata
0000000000201060 B _end
0000000000000b84 T _fini
                 w __gmon_start__
00000000000008a8 T _init
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
0000000000000a60 T PublicClass::doSomethingPublic()
0000000000000af0 T PrivateClass::doSomethingPrivate()
                 U std::ctype<char>::_M_widen_init() const
0000000000000b80 W std::ctype<char>::do_widen(char) const
                 U std::ostream::put(char)
                 U std::ostream::flush()
                 U std::ios_base::Init::Init()
                 U std::ios_base::Init::~Init()
                 U std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)
                 U std::__throw_bad_cast()
                 U std::cout

The strip command actually rewrites the shared library (date and size is changed) but I still see the private symbols. I would expect the PrivateClass::doSomethingPrivate method to be stripped from the symbol table.

I'd be grateful for any kind of hint.

Does this reproduce with stock g++ instead of /usr/local/bin/c++? — yugr

yugr yugr · Accepted Answer · 2019-10-24T06:29:06

You need to compile with -fvisibility=hidden:

$ g++ -fvisibility=hidden -shared -fPIC tmp.cc
$ readelf -D -sW a.out | grep doSomething
    9  16: 000000000000092c    58 FUNC    GLOBAL DEFAULT  10 _ZN11PublicClass17doSomethingPublicEv

Stripping symbols from shared library seems not to work

1 Answers