Symbol visibility and namespace

问题:

I'm experimenting with C++ symbol visibility on Linux and gcc. It seems that the preferred way is to use -fvisibility=hidden, and export used symbols one-by-one according to Visibility gcc wiki page (http://gcc.gnu.org/wiki/Visibility). My problem is that many libraries does not handle this well, they forget to explicitly export symbols, which is a serious problem. After several fixed bugs even some parts of boost may still be affected. Of course those bugs should be fixed, but till that I would like to use a "safe" way to hide as much as symbols as possible.

I came up with a solution: I place all the symbols in a namespace and I use symbol hide attribute on that and export the public interface, this way only my symbols can be affected.

The problem is that I got a warning message when I compile something against that library for every class that I haven't exported and I use in the application as class field.

namespace MyDSO __attribute__ ((visibility ("hidden"))) {
  struct Foo {
    void bar() __attribute__ ((visibility ("default"))) {}
  };
}

struct Bar {
  MyDSO::Foo foo;
};

int main() {}

The warning message can be reproduced in this small example, but of course the namespace should be in a library the other class in the application.

$ gcc-4.7.1 namespace.cpp -o namespace
namespace.cpp:7:8: warning: ‘Bar’ declared with greater visibility than the type of its field ‘Bar::foo’ [-Wattributes]

As I understand symbol visibility, hiding namespace should have quite similar effect to using -fvisibility=hidden, but I never got similar warnings using the latter. I see that when I pass -fvisibility=hidden to the application the class in the application will also be hidden, so I won't get a warning. But when I don't pass the option none of the symbols in the headers will seem hidden to the compiler, so I won't get a warning again.

What is the propose of this warning message? Is it a serious problem? In which situations can this cause any problem? How hiding namespace is different to fvisibility=hidden?

回答1:

Before I answer your specific question, I should mention for others reading that applying symbol visibility attributes per namespace is a GCC-specific feature. MSVC only supports dllexport on classes, functions and variables, and if you want your code to be portable you must match MSVC there. As my original GCC symbol visibility guide (the one you linked to on the GCC website) points out, MSVC's macro based dllexport machinery can be easily reused to achieve something similar on GCC, so porting to MSVC will get you symbol visibility handling "for free".

Regarding your specific problem, GCC is correct to warn you. If an external user tried to use public type Bar they almost certainly need to use everything inside Bar, including Bar::foo. For the exact same reason all private member functions, despite being private, need to be visible. A lot of people are surprised at this, reasoning that private member function symbols are by definition inaccessible to anyone, but they forget that just because the programmer doesn't have access doesn't mean that the compiler doesn't need access. In other words, private member functions are private to you, but not the compiler. If they appear in a header file, that generally means the compiler needs access, even in an anonymous namespace (which are only anonymous to programmers, not to compilers which tend to use a hash of the contents as the "real" namespace name).

Hiding a namespace has very different effects to -fvisibility=hidden. This is because GCC spews out many symbols above and beyond those for a specific type e.g. for vtables, for type_info etc. -fvisibility=hidden hides stuff you can't hide by any compiler instructed way, and it's stuff absolutely essential to loading two binaries into the same process with colliding symbols e.g. two shared objects built using different versions of Boost.

I appreciate your attempts to fix the problems caused by broken symbol visibility in ELF and the consequences on broken C++ binaries and much lost programmer productivity. However you can't fix them - they are faults in ELF itself, which was designed for C and not C++. If it's any consolation, I wrote an internal BlackBerry white paper a few months ago on this topic as ELF symbol visibility problems are just as much a problem for us in BB10 as they are for any large corporation with a significant C++ codebase. So, maybe you might see some solutions proposed for C++17, especially if Doug Gregor's C++ Modules implementation makes good progress.

回答2:

Your use of the visibility attributes seems backwards to me; I think you would have better results using -fvisibility=hidden and adding visibility "default" to the library declaration namespace, since the interface of the library presumably has default visibility or you couldn't use it from your application. If you don't want to modify the library headers, you could use #pragma GCC visibility push/pop around your #includes.

Also, as Niall says, marking individual member functions as default doesn't work, the whole Foo type needs to have default visibility if it's part of the interface of the library.