nm symbol both “U” and “T”, what does that mean?

问题

I am having an undefined symbol error at runtime, and when I look for the symbol in the relevant library, I get the result:

nm -C -D /home/farmer/anaconda3/envs/general/lib/python3.6/site-packages/pyscannerbit/libScannerBitCAPI.so | grep empty_
                 U YAML::detail::node_data::empty_scalar[abi:cxx11]
00000000002b5860 T YAML::detail::node_data::empty_scalar[abi:cxx11]()

But how is that possible? The symbol is both undefined, and also in the library? What? Or are these actually different symbols? When mangled the names are indeed slightly different:

nm -D /home/farmer/anaconda3/envs/general/lib/python3.6/site-packages/pyscannerbit/libScannerBitCAPI.so | grep empty_
                 U _ZN4YAML6detail9node_data12empty_scalarB5cxx11E
00000000002b5860 T _ZN4YAML6detail9node_data12empty_scalarB5cxx11Ev

Does this make sense?

回答1:

There are two variants of yaml-cpp:

https://github.com/jbeder/yaml-cpp

https://github.com/jbeder/yaml-cpp.new-api

In the first one the symbol in question is declared as member static const std::string& empty_scalar();. In the second one it is declared as member static std::string empty_scalar;.

The two symbol names you see match to these two different declarations. This should not be allowed by the compiler if it sees empty_scalar declared inconsistently like this.

I think you linked object files which were compiled with different versions of the header file which declared the symbol. The linker would then consider the two symbols different because of their different names. The object file you used did contain the definition for the old-api variant, but some code is using the new one.

来源：https://stackoverflow.com/questions/53345773/nm-symbol-both-u-and-t-what-does-that-mean