Shared Library Linking Problems

From: Danny Diaz (ddiaz_at_gehennom.dot.net)
Date: 08/27/03 

Date: Wed, 27 Aug 2003 21:39:47 +0000 (UTC)

I'm wondering if some behavior I am experiencing is "expected" or not.
What I am attempting to do is create some shared libraries with 2 things
in mind; 1) I want to share a global variable from the app with a
library, and 2) I want to share some global functions from those
libraries. For the global variable, I label the variable in my
dll's as "extern". For the functions, I am well aware that all
global functions within shared libraries are exported by default.

Ok, so, all is good until I get around to linking the shared library. In
the first case, I use the -Bsymbolic option in my linking step. With
the -Bsymbolic, I find that the application and the shared library have
seperate copies of the global variable; what gets set in one does not
get set in the other. Also, functions behave as I would like them to
behave: if LibA has a function called FOO, and libB has a function
called FOO, libA will call his own FOO and libB will call his own FOO
and the application will call whichever one was linked first if a call
to FOO was made.

The global variable issue killed me, so I used -Bgroup instead of
-Bsymbolic. Now the global variables are exactly in sync; there aren't
multiple copies of the variables any more. However, functions went all
to hell. Now, no matter if LibA has a FOO function or not, LibA
will call LibB's foo if it was linked first. This is very bad, you
cannot guarantee any more that a function will call another function in
the same library FIRST before looking at the global symbol table.

So, My question is this: Is the behavior I have explained "expected" or
is it a bug? And secondly, does anyone know of a way to get the global
variable behavior of -Bgroup and the exported symbol functionality of
-Bsymbolic?

I have included some source code to help you understand my problem
better. Save the code snippets to the named files, and execute one of
the scripts to compile and run with the appropriate link option. I'd
appreciate all of your input!

NOTE: This works in gcc 3.2.2, I have not tried to get it to work in,
say, gcc 2.9.x.

------ FILE: dllTestA.c ------
#include <stdio.h>
#include "dllTestA.h"

int ibsta = 0;

int foo( void ) {
   printf( "in libA::foo\n" );
   return 0;
}
int mylibA(int ibstaValue) {
   printf("in mylibA\n");
   printf("Setting ibsta to %d, ibsta addr: %p\n", ibstaValue, &ibsta);
   ibsta = ibstaValue;
   printf("Calling Foo... ");
   foo();
   return 0;
}
------ END OF FILE -----------

------ FILE: dllTestB.c ------
#include <stdio.h>
#include "dllTestB.h"

int ibsta = 0;

int foo( void ) {
   printf( "in libB::foo\n" );
   return 0;
}
int mylibB(int ibstaValue) {
   printf("in mylibB\n");
   printf("Setting ibsta to %d, ibsta addr: %p\n", ibstaValue, &ibsta);
   ibsta = ibstaValue;
   printf("Calling Foo... ");
   foo();
   return 0;
}
------ END OF FILE -----------

------ FILE: dllTestA.h ------
#ifndef ___libA_h___
#define ___libA_h___

extern int ibsta;
int mylibA(int inibsta);

#endif
------ END OF FILE -----------

------ FILE: dllTestB.h ------
#ifndef ___libB_h___
#define ___libB_h___

extern int ibsta;
int mylibB(int inibsta);

#endif
------ END OF FILE -----------

------ FILE: main.c ------
#include <stdio.h>
#include "dllTestA.h"
#include "dllTestB.h"

int main()
{
   mylibA(0x130);
   mylibB(0x130);
   printf("in main\n");
   printf("ibsta: %d, addr: %p\n", ibsta, &ibsta);
   printf("Calling Foo from Main... ");
   foo();
   return 0;
}
------ END OF FILE -----------

------ FILE: run-Bgroup.sh ------
#!/bin/sh -v
export LD_LIBRARY_PATH=.
gcc -c -odllTestA.o -fPIC -g -O0 dllTestA.c
gcc -c -odllTestB.o -fPIC -g -O0 dllTestB.c
gcc -Wl,-Bgroup -shared -Wl,-soname,libdllTestAu.so.1 -o libdllTestAu.so.1.0.0 dllTestA.o
gcc -Wl,-Bgroup -shared -Wl,-soname,libdllTestBu.so.1 -o libdllTestBu.so.1.0.0 dllTestB.o
gcc -c -omain.o -g -O0 main.c
gcc -omain main.o libdllTestBu.so.1.0.0 libdllTestAu.so.1.0.0
ln -sf libdllTestAu.so.1.0.0 libdllTestAu.so.1
ln -sf libdllTestBu.so.1.0.0 libdllTestBu.so.1
./main
# *** NOTE: with -Bgroup in the link line, the main, libdllTestAu.so.1
# *** and libdllTestBu.so.1 has one instance of the libsta
# *** variable, as intended, but there is only one instance of
# *** foo().
------ END OF FILE -----------

------ FILE: run-Bsymbolic.sh ------
#!/bin/sh -v
export LD_LIBRARY_PATH=.
gcc -c -odllTestA.o -fPIC -g -O0 dllTestA.c
gcc -c -odllTestB.o -fPIC -g -O0 dllTestB.c
gcc -Wl,-Bsymbolic -shared -Wl,-soname,libdllTestAu.so.1 -o libdllTestAu.so.1.0.0 dllTestA.o
gcc -Wl,-Bsymbolic -shared -Wl,-soname,libdllTestBu.so.1 -o libdllTestBu.so.1.0.0 dllTestB.o
gcc -c -omain.o -g -O0 main.c
gcc -omain main.o libdllTestBu.so.1.0.0 libdllTestAu.so.1.0.0
ln -sf libdllTestAu.so.1.0.0 libdllTestAu.so.1
ln -sf libdllTestBu.so.1.0.0 libdllTestBu.so.1
./main
# *** NOTE: with -Bsymbolic in the link line, the main, libdllTestAu.so.1
# *** and libdllTestBu.so.1 have multiple instances of the libsta
# *** variable, but there are multiple instances of foo() - and
# *** they are called correctly within their libraries.
------ END OF FILE -----------

Thanks!
-Danny 

-- 
Danny Diaz         | "Don't you try to outweird me, 
texasdiaz          |  I get stranger things than you
@austin.rr.com     |  free in my breakfast cereal." - BZ
Confidence is what you had before you understood the situation.

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