Hiding instantiated templates in shared library created with g++

Question

I have a file that contains the following:

#include <map>

class A {};

void doSomething() {
   std::map<int, A> m;
}

When compiled into a shared library with g++, the library contains dynamic symbols for all the methods of std::map<int, A>. Since A is private to this file, there is no possibility that std::map will be instantiated in any other shared library with the same parameters, so I'd like to make the template instantiation hidden (for some of the reasons described in this document).

I thought I should be able to do this by adding an explicit instantiation of the template class and marking it as hidden, like so:

#include <map>

class A {};
template class __attribute__((visibility ("hidden"))) std::map<int, A>;

void doSomething() {
   std::map<int, A> m;
}

However, this has no effect: the symbols are still all exported. I also tried surrounding the entire file with:

#pragma GCC visibility push(hidden)
...
#pragma GCC visibility pop

but this also has no effect on the visibility of the methods of std::map<int, A> (although it does hide doSomething). Similarly, compiling with -fvisibility=hidden has no effect on the visibility of the methods of std::map<int, A>.

The document I linked to above describes the use of export maps to restrict visibility, but that seems very tedious.

Is there a way to do what I want in g++ (other than using export maps)? If so, what is it? If not, is there a good reason why these symbols must always be exported, or is this just a omission in g++?

Answer 1

Disclaimer: I am not a GCC developer, and therefore this is a complete WAG (wild-ass guess):

My guess would be that GCC always exports template definitions, in order to allow the linker to remove duplicate copies of templates. If it were not exported and more than once source file used that template, the entire source for the std::map<k, v> class would be duplicated within the two files.

I think you're really paying more attention to this than it deserves. Exports are an implementation detail in C++. In C, it makes sense to not export internal functions, so that clients don't come to rely on them. But in C++, the exported functions never have to have anything to do with the source code. One version of GCC's std::map<k, v> might be completely different from another versions' and as a result the two binaries will not be link-compatible.

If you absolutely need portability, export a C interface and ignore the C++ specifics that get exported. Any client of your library trying to call such exports or do anything with them deserves to crash and burn for calling obvious internal implementation details.

EDIT: Made CW because I'm not 100% positive.

Answer 2

Maybe you could use objcopy with the --strip-symbol option?

The option is described in the objcopy man page

This might get tedious though...

Answer 3

In C++, if a template argument has limited visibility, this restriction is implicitly propagated to the template instantiation.

#include <map>

class __attribute__((visibility ("hidden"))) A {};

void doSomething() {
  std::map<int, A> m;
}

should do the job.

-- edit --
One more thing, `#pragma GCC visibility' affects only namespace-scope declarations. Class members and template specializations are not affected (Visibility pragmas)

Answer 4

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36022: The std::namespace is supposed to be exposed and is marked as such in the libstdc++ headers.

As for

#undef _GLIBCXX_VISIBILITY_ATTR

here is another quote:

If you were to hack in support for allowing namespace std to have hidden visibility, and run the testsuite with -fvisibility=hidden (see attached patch) you would notice the breakdown in testresults, with mass failures.

Answer 5

From GCC bug report #36022, which was marked INVALID, Benjamin Kosnik remarked:

[A]n exception class that will be thrown between DSOs must be explicitly marked with default visibility so that the `type_info' nodes will be unified between the DSOs. Thus, the rationale for libstdc++ having namespace std have visibility "default."

Also, looking through the libstdc++ source for std::map (mine is in /usr/include/c++/4.4.4/bits/stl_map.h), it appears that the way libstdc++ enforces default visibility is with the _GLIBCXX_BEGIN_NESTED_NAMESPACE macro that is used at the top of stl_map.h:

# define _GLIBCXX_VISIBILITY_ATTR(V) __attribute__ ((__visibility__ (#V)))
# define _GLIBCXX_BEGIN_NESTED_NAMESPACE(X, Y) _GLIBCXX_BEGIN_NAMESPACE(X)
# define _GLIBCXX_BEGIN_NAMESPACE(X) namespace X _GLIBCXX_VISIBILITY_ATTR(default) {

Therefore your STL implementation is explicitly overriding -fvisibility=hidden and #pragma GCC visibility push(hidden)/#pragma GCC visibility pop.

If you really wanted to force the std::map members to have hidden visibility then I think you could use something like:

// ensure that default visibility is used with any class that is used as an exception type
#include <memory>
#include <new>
#include <stdexcept>

// now include the definition of `std::map` using hidden visibility
#include <bits/c++config.h>
#undef _GLIBCXX_VISIBILITY_ATTR
#define _GLIBCXX_VISIBILITY_ATTR(V) __attribute__ ((__visibility__ ("hidden")))
#include <map>
#undef _GLIBCXX_VISIBILITY_ATTR
#define _GLIBCXX_VISIBILITY_ATTR(V) __attribute__ ((__visibility__ (#V))) // restore `_GLIBCXX_VISIBILITY_ATTR`

Then, the following series of commands will verify that the std::map<int, A> members can be stripped from a shared object:

1. g++ -c -fPIC -fvisibility=hidden test.cpp
2. g++ -shared -Wl,-soname,libtest.so.1 -o libtest.so.1.0 test.o
3. strip -x libtest.so.1.0
4. readelf -s libtest.so.1.0

Note that before step 3, readelf -s libtest.so.1.0 printed (for me):

Symbol table '.dynsym' contains 23 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     2: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND _ZdlPv@GLIBCXX_3.4 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT  UND __gxx_personality_v0@CXXABI_1.3 (3)
     5: 00000000     0 FUNC    GLOBAL DEFAULT  UND _Unwind_Resume@GCC_3.0 (4)
     6: 00000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.1.3 (5)
     7: 00000d02     5 FUNC    WEAK   DEFAULT   12 _ZNSt4pairIKi1AED1Ev
     8: 00000d6c    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEEC1ISt
     9: 00000d96    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    10: 000023bc     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    11: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    12: 00000d5e     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    13: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    14: 00000bac     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    15: 00000d08    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    16: 000007f4     0 FUNC    GLOBAL DEFAULT   10 _init
    17: 00000c4a    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    18: 00000df8     0 FUNC    GLOBAL DEFAULT   13 _fini
    19: 00000dba     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    20: 00000cde    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEED1Ev
    21: 00000d90     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    22: 00000ac6    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS

Symbol table '.symtab' contains 84 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 000000f4     0 SECTION LOCAL  DEFAULT    1 
     2: 00000118     0 SECTION LOCAL  DEFAULT    2 
     3: 000001c0     0 SECTION LOCAL  DEFAULT    3 
     4: 0000022c     0 SECTION LOCAL  DEFAULT    4 
     5: 0000039c     0 SECTION LOCAL  DEFAULT    5 
     6: 000006b6     0 SECTION LOCAL  DEFAULT    6 
     7: 000006e4     0 SECTION LOCAL  DEFAULT    7 
     8: 00000754     0 SECTION LOCAL  DEFAULT    8 
     9: 0000077c     0 SECTION LOCAL  DEFAULT    9 
    10: 000007f4     0 SECTION LOCAL  DEFAULT   10 
    11: 00000824     0 SECTION LOCAL  DEFAULT   11 
    12: 00000930     0 SECTION LOCAL  DEFAULT   12 
    13: 00000df8     0 SECTION LOCAL  DEFAULT   13 
    14: 00000e14     0 SECTION LOCAL  DEFAULT   14 
    15: 00000ef8     0 SECTION LOCAL  DEFAULT   15 
    16: 00001240     0 SECTION LOCAL  DEFAULT   16 
    17: 0000225c     0 SECTION LOCAL  DEFAULT   17 
    18: 00002264     0 SECTION LOCAL  DEFAULT   18 
    19: 0000226c     0 SECTION LOCAL  DEFAULT   19 
    20: 00002270     0 SECTION LOCAL  DEFAULT   20 
    21: 00002358     0 SECTION LOCAL  DEFAULT   21 
    22: 00002364     0 SECTION LOCAL  DEFAULT   22 
    23: 000023ac     0 SECTION LOCAL  DEFAULT   23 
    24: 000023b4     0 SECTION LOCAL  DEFAULT   24 
    25: 00000000     0 SECTION LOCAL  DEFAULT   25 
    26: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    27: 0000225c     0 OBJECT  LOCAL  DEFAULT   17 __CTOR_LIST__
    28: 00002264     0 OBJECT  LOCAL  DEFAULT   18 __DTOR_LIST__
    29: 0000226c     0 OBJECT  LOCAL  DEFAULT   19 __JCR_LIST__
    30: 00000930     0 FUNC    LOCAL  DEFAULT   12 __do_global_dtors_aux
    31: 000023b4     1 OBJECT  LOCAL  DEFAULT   24 completed.5942
    32: 000023b8     4 OBJECT  LOCAL  DEFAULT   24 dtor_idx.5944
    33: 000009b0     0 FUNC    LOCAL  DEFAULT   12 frame_dummy
    34: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    35: 00002260     0 OBJECT  LOCAL  DEFAULT   17 __CTOR_END__
    36: 0000123c     0 OBJECT  LOCAL  DEFAULT   15 __FRAME_END__
    37: 0000226c     0 OBJECT  LOCAL  DEFAULT   19 __JCR_END__
    38: 00000dc0     0 FUNC    LOCAL  DEFAULT   12 __do_global_ctors_aux
    39: 00000000     0 FILE    LOCAL  DEFAULT  ABS test.cpp
    40: 00000d64     8 FUNC    LOCAL  HIDDEN   12 _ZNKSt8_Rb_treeIiSt4pairI
    41: 000023b0     4 OBJECT  LOCAL  HIDDEN   23 DW.ref.__gxx_personality_
    42: 00000b40    11 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    43: 00000bc8   129 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    44: 00000bb1    11 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    45: 00000b4c    96 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    46: 00000ca0    62 FUNC    LOCAL  HIDDEN   12 _ZNKSt8_Rb_treeIiSt4pairI
    47: 00000ab2    19 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    48: 00002364     0 OBJECT  LOCAL  HIDDEN  ABS _GLOBAL_OFFSET_TABLE_
    49: 00000a56    92 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    50: 000009ec    30 FUNC    LOCAL  HIDDEN   12 _Z11doSomethingv
    51: 00000c6e    49 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    52: 00000a32    35 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    53: 000023ac     0 OBJECT  LOCAL  HIDDEN   23 __dso_handle
    54: 00000a0a    19 FUNC    LOCAL  HIDDEN   12 _ZNSt3mapIi1ASt4lessIiESa
    55: 00002268     0 OBJECT  LOCAL  HIDDEN   18 __DTOR_END__
    56: 00000bbc    11 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    57: 00000a1e    19 FUNC    LOCAL  HIDDEN   12 _ZNSt3mapIi1ASt4lessIiESa
    58: 00000d2c    50 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    59: 00000aea    85 FUNC    LOCAL  HIDDEN   12 _ZNSt8_Rb_treeIiSt4pairIK
    60: 000009e7     0 FUNC    LOCAL  HIDDEN   12 __i686.get_pc_thunk.bx
    61: 00002270     0 OBJECT  LOCAL  HIDDEN  ABS _DYNAMIC
    62: 00000d02     5 FUNC    WEAK   DEFAULT   12 _ZNSt4pairIKi1AED1Ev
    63: 00000c4a    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    64: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    65: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    66: 00000000     0 FUNC    GLOBAL DEFAULT  UND _ZdlPv@@GLIBCXX_3.4
    67: 00000df8     0 FUNC    GLOBAL DEFAULT   13 _fini
    68: 00000d6c    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEEC1ISt
    69: 00000dba     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    70: 00000cde    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEED1Ev
    71: 00000d5e     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    72: 00000d90     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    73: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    74: 00000d96    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    75: 00000bac     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    76: 000023bc     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    77: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    78: 00000ac6    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    79: 00000000     0 FUNC    GLOBAL DEFAULT  UND __gxx_personality_v0@@CXX
    80: 00000000     0 FUNC    GLOBAL DEFAULT  UND _Unwind_Resume@@GCC_3.0
    81: 00000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@@GLIBC_2.1
    82: 00000d08    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    83: 000007f4     0 FUNC    GLOBAL DEFAULT   10 _init

And afterward:

Symbol table '.dynsym' contains 23 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     2: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND _ZdlPv@GLIBCXX_3.4 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT  UND __gxx_personality_v0@CXXABI_1.3 (3)
     5: 00000000     0 FUNC    GLOBAL DEFAULT  UND _Unwind_Resume@GCC_3.0 (4)
     6: 00000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.1.3 (5)
     7: 00000d02     5 FUNC    WEAK   DEFAULT   12 _ZNSt4pairIKi1AED1Ev
     8: 00000d6c    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEEC1ISt
     9: 00000d96    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    10: 000023bc     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    11: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    12: 00000d5e     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    13: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    14: 00000bac     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    15: 00000d08    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    16: 000007f4     0 FUNC    GLOBAL DEFAULT   10 _init
    17: 00000c4a    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    18: 00000df8     0 FUNC    GLOBAL DEFAULT   13 _fini
    19: 00000dba     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    20: 00000cde    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEED1Ev
    21: 00000d90     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    22: 00000ac6    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS

Symbol table '.symtab' contains 51 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 000000f4     0 SECTION LOCAL  DEFAULT    1 
     2: 00000118     0 SECTION LOCAL  DEFAULT    2 
     3: 000001c0     0 SECTION LOCAL  DEFAULT    3 
     4: 0000022c     0 SECTION LOCAL  DEFAULT    4 
     5: 0000039c     0 SECTION LOCAL  DEFAULT    5 
     6: 000006b6     0 SECTION LOCAL  DEFAULT    6 
     7: 000006e4     0 SECTION LOCAL  DEFAULT    7 
     8: 00000754     0 SECTION LOCAL  DEFAULT    8 
     9: 0000077c     0 SECTION LOCAL  DEFAULT    9 
    10: 000007f4     0 SECTION LOCAL  DEFAULT   10 
    11: 00000824     0 SECTION LOCAL  DEFAULT   11 
    12: 00000930     0 SECTION LOCAL  DEFAULT   12 
    13: 00000df8     0 SECTION LOCAL  DEFAULT   13 
    14: 00000e14     0 SECTION LOCAL  DEFAULT   14 
    15: 00000ef8     0 SECTION LOCAL  DEFAULT   15 
    16: 00001240     0 SECTION LOCAL  DEFAULT   16 
    17: 0000225c     0 SECTION LOCAL  DEFAULT   17 
    18: 00002264     0 SECTION LOCAL  DEFAULT   18 
    19: 0000226c     0 SECTION LOCAL  DEFAULT   19 
    20: 00002270     0 SECTION LOCAL  DEFAULT   20 
    21: 00002358     0 SECTION LOCAL  DEFAULT   21 
    22: 00002364     0 SECTION LOCAL  DEFAULT   22 
    23: 000023ac     0 SECTION LOCAL  DEFAULT   23 
    24: 000023b4     0 SECTION LOCAL  DEFAULT   24 
    25: 00000000     0 SECTION LOCAL  DEFAULT   25 
    26: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    27: 00000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    28: 00000000     0 FILE    LOCAL  DEFAULT  ABS test.cpp
    29: 00000d02     5 FUNC    WEAK   DEFAULT   12 _ZNSt4pairIKi1AED1Ev
    30: 00000c4a    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    31: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    32: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
    33: 00000000     0 FUNC    GLOBAL DEFAULT  UND _ZdlPv@@GLIBCXX_3.4
    34: 00000df8     0 FUNC    GLOBAL DEFAULT   13 _fini
    35: 00000d6c    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEEC1ISt
    36: 00000dba     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    37: 00000cde    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt4pairIKi1AEED1Ev
    38: 00000d5e     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    39: 00000d90     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    40: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    41: 00000d96    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    42: 00000bac     5 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    43: 000023bc     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    44: 000023b4     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    45: 00000ac6    35 FUNC    WEAK   DEFAULT   12 _ZNSaISt13_Rb_tree_nodeIS
    46: 00000000     0 FUNC    GLOBAL DEFAULT  UND __gxx_personality_v0@@CXX
    47: 00000000     0 FUNC    GLOBAL DEFAULT  UND _Unwind_Resume@@GCC_3.0
    48: 00000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@@GLIBC_2.1
    49: 00000d08    35 FUNC    WEAK   DEFAULT   12 _ZN9__gnu_cxx13new_alloca
    50: 000007f4     0 FUNC    GLOBAL DEFAULT   10 _init

See also:

• Visibility - GCC Wiki
• Controlling Symbol Visibility