How can I make is so that a function in a shared library can't be called without declaring it static?

Question

The headline may not be entirely clear.

I'm using two source files (one in C, one in Haskell) to compile a shared library (.so). I want some of the functions in one file to be callable from functions in the other file. However, I don't want them to be callable from the program that loads the library.

If I declare them static, the other file's code can't see them.

Is there a way to accomplish this? Does it require configuring the linker or altering the.so file after the fact?

Answer 1

You can/should do this with a linker script when creating the .so

But, first, look at info gcc and the -fvisibility= section [included below].

You can also use __attribute__((visibility("..."))) on each function.

For example, if most of the functions are internal (and only a few are part of the public API), you could use a default visibility of hidden and add explicit attributes to the public functions.

---

From info gcc :

-fvisibility=[default|internal|hidden|protected]

Set the default ELF image symbol visibility to the specified option--all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

Despite the nomenclature, default always means public; ie, available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden . The default if -fvisibility isn't specified is default , ie, make every symbol public.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/ )--however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.

For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop . Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; ie, declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads--making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers.

extern declarations are not affected by -fvisibility , so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs.

An overview of these techniques, their benefits and how to use them is at https://gcc.gnu.org/wiki/Visibility