Changes introduced in gcc 4.5 with respect to linking?

Question

I have a project that produces a shared library which is linked against another, also shared, library.

When I compile and link it with gcc 4.4, everything works:

1. no compile-time warning or error,
2. no linking time warning or error and
3. ldd libmyproject.so correctly reports the dependency with the other shared library.

When I compile and link it with gcc 4.5, on the other hand (with the exact same flags), I have the following symptoms:

1. no compile-time warning or error,
2. no linking time warning or error but
3. the library is not correctly linked against the other shared lib: this manifest itself when I run ldd and don't see the connection, and also when I try to use it: while it works with gcc 4.4, it crashes at run-time with gcc 4.5 with a "symbol not found" error (of course from the other lib).

I looked at the release notes and my intuition is that it has something to do with the new link-time optimization, but I could not understand them in enough details.

Did anyone encounter a similar situation and/or has any advice to offer?

(Note that results with 4.6 are in appearance identical to 4.5).

Answer 1

To summarize Mat's answer from GCC 4.5 vs 4.4 linking with dependencies and the discussion in the comments, you need to link with:

--copy-dt-needed-entries and --no-as-needed

Answer 2

You can debug your dynamically linked application with LD_DEBUG environment variable. This is an option of ld-linux.so.2; ldd is a script to set such option too. All options are described at http://linux.die.net/man/8/ld-linux man page.

How to use LD_DEBUG (in bash; the easiest way):

 $ LD_DEBUG=all ./your_program

This will turn on debugging of ld-linux.so.2 - the run-time dynamic linker. It will print a lot of debugging to stdout or stderr and you will be able to

• 1) compare output of "LD_DEBUG=all ./your_program_4.4" and "LD_DEBUG=all ./your_program_4.5"
• 2) see the last symbols trying to be resolved and locate buggy symbol.

Also, you should give us more information:

• 0) What is your OS and cpu type? (show us output of uname -a) What is the version of libc? (run in bash for a in /lib*/libc.so.*;do echo $a; $a; done)
• 1) What are compiling flags of your library itself?
• 2) What is exact error when you try to run the application?
• 3) Last lines from output of LD_DEBUG can contain valuable information

UPDATE: Good and exact answer is here: GCC 4.5 vs 4.4 linking with dependencies (by Mat)