Dynamic Loading: Undefined Symbol In Shared Static Library

Question

I have an executable that loads .so plugins.

The executable is linked with -rdynamic so that symbol callback can occur.

I have a static library that is included in the executable. This has a function called BLAH_hello() in the .a

The static library is not used in the executable. ie there are no calls to BLAH_hello() in the executable code.

However, the .so does have calls to BLAH_hello().

When I dlopen() the .so it complains about an undefined symbol to BLAH_hello()

If I include a dummy call to BLAH_hello() in the executable code, like BLAH_hello(NULL);. The symbol is included in the executable and when the .so is loaded it finds the symbol.

I'm sure I could also link the .so against the .a but multiple dynamically loaded .so's use the BLAH_hello call so it makes sense to have it in the executable. I'm also worried about symbol conflict if I link the library into each .so.

So what I'm wondering, is how to get the symbols of the .a into the executable even if they aren't actually used in the executable?

Linking the .so against the .a is the obvious and correct thing to do. — user2100815
– user2100815, Commented May 26, 2011 at 8:23
Thanks Neil. I have three .so plugins that will need to be linked against that .a I won't have symbol conflicts when i load all the .so's into my executable? — Matt Clarkson
– Matt Clarkson, Commented May 26, 2011 at 17:40
No, you won't, otherwise you would not be able to use operating system and other standard functions in .sos. — user2100815
– user2100815, Commented May 26, 2011 at 18:17

Community · Accepted Answer · 2020-06-20 09:12:55Z

4

When I dlopen() the .so it complains about an undefined symbol to BLAH_hello()

If I include a dummy call to BLAH_hello() in the executable code, like BLAH_hello(NULL);. The symbol is included in the executable and when the .so is loaded it finds the symbol.

That is completely normal and expected. It's just how linkers work with archive libraries.

If you must include BLAH_hello into the main executable, add -Wl,-u,BLAH_hello to the executable link line.

edited Jun 20, 2020 at 9:12

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answered Jul 24, 2011 at 16:56

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1 Comment

L. Langó Over a year ago

Is it possible to do the same from source code? (e.g.: __attribute__((something)))

madth3 · Accepted Answer · 2012-12-14 18:01:31Z

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@nbt:

Linking the .so against the .a is the obvious and correct thing to do.

This should not generate symbol conflicts when loading the .so into an executable.

edited Dec 14, 2012 at 18:01

madth3

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answered May 27, 2011 at 15:16

Matt Clarkson

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Comments

Nuclear · Accepted Answer · 2014-06-28 18:00:22Z

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As mentioned above, the linker discards .o files with no referenced symbols. This is problem, when the executable needs to dlopen() external shared libs. In such case the linker options '--whole-archive' and '--no-whole-archive' can be used, when linking the executable. All .a libs between these options will have all symbols included. It increases the executable size, but removes the need to link to executable's static libs.

answered Jun 28, 2014 at 18:00

Nuclear

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