[SOLVED] __attribute__ ((constructor)) function does not execute OR where's an example source

micropanther · 03-16-2011, 09:31 AM

My setup is Ubuntu 10.04, eclipse 3.5.2

I have created the source for a shared object and a test program to test it. The program executes dlopen() and I get back a valid library handle. I can use that handle and dlsym() to get the address of my C exported functions just as I expect.

I recently added a change to have the so instantiate an object at load time by using a __attribute__ ((constructor)) function named so_init(). I also added a main() function (one wasn't required before). Adding a bunch of printf()s and using the debugger in eclipse I watch the code run and the so_init() and main() functions are never called.

As far as I can tell, eclipse has -shared, -fPIC and -Wl setup and specified correctly.

Am I missing a compile/link flag? Is there some magic I need in the so_main.cpp to get this working.

FWIW, I have this written as dual source code. The same code works quite nicely in Windows which calls dllMain() as expected.

Wes

ta0kira · 03-19-2011, 01:33 AM

Can you post a short piece of code that can reproduce the problem? A makefile would also be helpful.
Kevin Barry

micropanther · 03-21-2011, 08:00 AM

Here is the So part. I actually have two so's but their setups are identical.

EDIT: to add .h items

Code:

soMain.h
    void __attribute__ ((constructor)) so_init(void);   // Linux entry point
    void __attribute__ ((destructor)) so_fini(void);    // Linux exit point

Code:

soMain.cpp
     int   main( void )
      {
         printf( "MAIN WAS RUN/n" );
      }

      void so_init()
      {
    	 printf( "SO_INIT WAS RUN/n" );
      }

      void so_fini()
      {
    	 printf( "SO_FINI WAS RUN/n" );
      }

Here is the call from test.cpp:

Code:

dlerror();       // Call dlerror() to clear any previous error code
      hLib = dlopen( m_libName.c_str(), RTLD_NOW | RTLD_GLOBAL );

      m_ProcAddr_LibObject = (PROCADDR_LIBOBJECT)GetFuncAddress( "GetLibraryObjectPtr" );

As for the makefile. This is Eclipse so it/they are splattered all over the place. Tell me what you need to see or that I need to search for. We'll end up using pastebin.com since LinuxQuestions won't accept the extensions associated with make.

Wes

micropanther · 03-23-2011, 09:23 AM

Interesting new detail.

I actually have two SO's I'm building. They started out nearly identical. Discover.so is the one that doesn't find so_init, etc. Edge.so works just fine. Nearly as I can tell their makefiles are "identical", but they are in Eclipse and who knows what Eclipse is hiding.

Wes

micropanther · 04-13-2011, 09:45 AM

It turns out that the behavior I was seeing is caused by my libraries being statically linked rather than dynamically (at runtime). Because the libs were statically oaded before the main program, and both had an so_init(), the first one found was executed twice, once per lib load, the second one just being the wrong so_init.

No I have to figure out how to get dynamic linking to work!

Wes