Linker warning "direct access to global weak symbol"

[herzbube]

Since upgrading from Xcode 3 to Xcode 4, and the accompanying compiler upgrade,
the build produces the following linker warning:

ld: warning: direct access in __ZN5boost6system12system_errorD2Ev to global
weak symbol __ZTVN5boost6system12system_errorE means the weak symbol cannot be
overridden at runtime. This was likely caused by different translation units
being compiled with different visibility settings.

Note: This issue was moved here from the old bugtracker.

[herzbube]

The mangled names mentioned in the warning can be "decrypted" using c++filt:

__ZN5boost6system12system_errorD2Ev
boost::system::system_error::~system_error()

and

__ZTVN5boost6system12system_errorE
vtable for boost::system::system_error

[herzbube]

It is pretty clear that there must be some sort of discrepancy between the 3rdparty software build (via command line) and the Little Go application build (via Xcode).

A first comparison of the command line options used to compile a given .cpp file in both environments did not provide any useful clues. Specifically, the options "-fvisibility=hidden" and "-fvisibility-inlines-hidden" are present in both builds.

Note that in the Xcode build the compiler options of an .mm file need to be examined, because .mm files contain C++ code while .m files contain only Objective-C code. For instance, "-fvisibility-inlines-hidden" is missing when .m files are compiled.

[herzbube]

More information:

• The symbols mentioned in the linker warning are NOT part of libboost_system. Check this using the command

nm
~/Documents/dev/littlego/3rdparty/install/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/lib/libboost_system.a

• The 2nd symbol (__ZTVN5boost6system12system_errorE, or "vtable for boost::system::system_error") is part of Fuego, e.g. go/GoBook
• It is not yet clear how the symbol can appear in Fuego, since system_error is not used directly anywhere in Fuego. Probably some sort of transitive dependency.
• Looking at boost/system/system_error.hpp, one can see that the class system_error is marked using the preprocessor macro BOOST_SYMBOL_VISIBLE. To my knowledge, all class members inherit their visibility settings from the parent class, and I would hope that this is also true for the destructor and for the vtable (although I didn't realize that a vtable is a symbol). So it is still unclear what the actual problem is...
• Recompiling the entire 3rdparty stuff did not fix the problem.

[herzbube]

Next steps:

1. Reproduce the problem using a small test project on the command line. This will help getting rid of the complexity of Xcode.
2. Analyze and understand the problem.

[herzbube]

Fixed the following 3rd party build issues:

• The simulator build now uses llvm-gcc/llvm-g++ just like the iPhoneOS build (previously used gcc/g++)
• Both builds now use IPHONEOS_DEPLOYMENT_TARGET=4.3 (previously used 4.2)

Despite these fixes, the linker warning is still printed in all builds, EXCEPT the simulator build for one of the release/distribute configurations. Maybe a clue can be found by comparing the simulator SDK build settings of the Debug and Release configurations...

[herzbube]

The linker warning disappeared after the Fuego build process was outsourced to the fuego-on-ios project (which includes building Boost and Fuego with clang).