Is there a .def file equivalent on Linux for controlling exported function names in a shared library?

Question

I am building a shared library on Ubuntu 9.10. I want to export only a subset of my functions from the library. On the Windows platform, this would be done using a module definition (.def) file which would contain a list of the external and internal names of the functions exported from the library.

I have the following questions:

1. How can I restrict the exported functions of a shared library to those I want (i.e. a .def file equivalent)

2. Using .def files as an example, you can give a function an external name that is different from its internal name (useful for prevent name collisions and also redecorating mangled names etc)

3. On windows I can use the EXPORT command (IIRC) to check the list of exported functions and addresses, what is the equivalent way to do this on Linux?

Answer 1

The most common way to only make certain symbols visible in a shared object on linux is to pass the -fvisibility=hidden to gcc and then decorate the symbols that you want to be visible with __attribute__((visibility("default"))).

If your looking for an export file like solution you might want to look at the linker option --retain-symbols-file=FILENAME which may do what you are looking for.

I don't know an easy way of exporting a function with a different name from its function name, but it is probably possible with an elf editor. Edit: I think you can use a linker script (have a look at the man page for ld) to assign values to symbols in the link step, hence giving an alternative name to a given function. Note, I haven't ever actually tried this.

To view the visible symbols in a shared object you can use the readelf command. readelf -Ds if I remember correctly.

Answer 2

How can I restrict the exported functions of a shared library to those I want (i.e. a .def file equivalent)

Perhaps you're looking for GNU Export Maps or Symbol Versioning

g++ -shared spaceship.cpp -o libspaceship.so.1 -Wl,-soname=libspaceship.so.1 -Wl, --version-script=spaceship.expmap

Answer 3

gcc also supports the VC syntax of __declspec(dllexport). See this.

Answer 4

Another option is to use the strip command with this way:

strip --keep-symbol=symbol_to_export1 --keep-symbol=symbol_to_export2 ... \
     libtotrip.so -o libout.so