Question

For VC, I can write a DEF file and use the ‘NONAME’ directive to leaving only the ordinal number in dll’s export table.

How could I do the same thing with gcc and ELF format shared library?

Or, is there something equivalent in ELF shared library like the ordinal number in a PE format DLL? If not, how could I hide the exported symbol’s name within a shared library?

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UPDATE: Some additional descriptions:

In Windows, you can export a function by only place a integer ID (the ordinal) with an empty name.

To show it, the normal layout for a dll’s export table looks like this: http://home.hiwaay.net/~georgech/WhitePapers/Exporting/HowTo22.gif.

the “NONAME” one looks like this: http://home.hiwaay.net/~georgech/WhitePapers/Exporting/HowTo23.gif.

Notice the functions name are “N/A” in the second picture. Here is a full explaination of it: hxxp://home.hiwaay.net/~georgech/WhitePapers/Exporting/Exp.htm.

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UPDATE: A lot of thanks for everyone who make me advice. Finally, I decide to keeping use static library on linux/posix platforms. But extract the small “special part” (which is using some features not suitable for static lib, e.g: TLS Slot, etc.) to a normal shared-library. Because the small normal shared-library only doing few things, and these work are totally insensitive, so there is no need to obscure/hide its APIs.

I think it’s the simplest way to solve my problem 😀

Answer 1

The previous answers regarding attribute ((visibility ("hidden"))) is good when you want to maintain the code long term, but if you only have a few symbols that you want visible and want a quick fix… On the symbols that you want to export use, add

__attribute__ ((visibility ("default"))) 

Then you can pass -fvisibility=hidden to the compiler

There is a thorough explanation here:

http://gcc.gnu.org/wiki/Visibility

Edit: An alternative would be to build a static library/archive (make .a archive with ar -cru mylib.a *.o) or combine the objects into a single object file according to this combine two GCC compiled .o object files into a third .o file

If you are asking "Why combine object files instead of just making a static library?" … because the linker will treat .o files differently than .a files (I don’t know why, just that it does), specifically it will allow you to link a .o file into a shared library or a binary even if all of the symbols are hidden (even the ones you are using) This has the added benefit of reducing startup times (one less DSO and a lot less symbols to look up) and binary size (the symbols typically make up ~20% of the size and stripping only takes care of about half of that – just the externally visible parts)

for binaries strip --strip-all -R .note -R .comment mybinary

for libraries strip --strip-unneeded -R .note -R .comment mylib.so

More on the benefits of static linking here: http://sta.li/faq but they don’t discuss licensing issues which are the main reason not to use a static library and since you are wanting to hide your API, that may be an issue

Now that we know have an object that is "symbol clean", it is possible to use our combined object to build a libpublic.so by linking private.o and public.c (which aliases/exports only what you want public) into a shared library.

This method lends itself well to finding the "extra code" that is unneeded in your public API as well. If you add -fdata-sections -ffunction-sections to your object builds, when you link with -Wl,--gc-sections,--print-gc-sections , it will eliminate unused sections and print an output of what was removed.

Edit 2 – or you could hide the whole API and alias only the functions you want to export

alias ("target")

The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));

defines f' to be a weak alias for __f’. In C++, the mangled name for the target must be used. It is an error if `__f’ is not defined in the same translation unit.

Not all target machines support this attribute.