[Bug libstdc++/36022] stl templates exported as weak symbols though visibility hidden is used

[bkoz at gcc dot gnu dot org]

------- Comment #4 from bkoz at gcc dot gnu dot org  2009-02-04 02:51 -------

This isn't a bug, but rather part of a deliberate linkage strategy. 

For C++, types that are to be used across shared libraries have to be visible.
See:

http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options

And in particular:

Note that `-fvisibility' does affect C++ vague linkage entities. This means
that, for instance, an 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." 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. Thus,
it is provided for information purposes only.

In the libstdc++ source files, anonymous namespaces are used for specific
entities that have both local/hidden linkage. This use of anonymous namespaces
is considered superior to attribute hidden as it uses ISO C++ and is thus more
portable than vendor extensions (pragmas or attributes).

However, in libstdc++ header files, attribute hidden has been difficult to use.
One might think that perhaps all the implementation details could be moved to
say std::__detail, and then marked with attribute hidden. Then, many of these
helper functions would be marked as hidden in your example below.

There are some pitfalls with this approach:

 1) all these implementation base types that are used by default derived
classes  would have to be default
 2) same with implementation details that use static locals
 3) same with virtual functions, etc etc.
 4) thus you end up with a pretty limited set of hidden things

In addition, this may exacerbate the dlopen + RTLD_LOCAL + weak symbol issue
for C++. 


-- 

bkoz at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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   GCC host triplet|                            |x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36022