Hiding instantiated templates in shared library created with g++ - c++

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

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:
g++ -c -fPIC -fvisibility=hidden test.cpp
g++ -shared -Wl,-soname,libtest.so.1 -o libtest.so.1.0 test.o
strip -x libtest.so.1.0
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

Answer 2

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 3

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 4

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 5

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)