Exception handling is not working in GCC compile shared object

Question

Hello,
I am facing very strange issue on Solaris x86_64 platform with C++ code compiled usging gcc.3.4.3.
I have compiled shared object that load into web server process space while initialization. Whenever any exception generate in code base, it is not being caught by exception handler. Even though exception handlers are there. Same code is working fine since long time but on Solaris x86, Sparc arch, Linux platform
With Dbx, I am getting following stack trace.
Stack trace is
dbx: internal error: reference through NULL pointer at line 973 in file symbol.cc
[1] 0x11335(0x1, 0x1, 0x474e5543432b2b00, 0x59cb60, 0xfffffd7fffdff2b0, 0x11335), at 0x11335
---- hidden frames, use 'where -h' to see them all ----
=>[4] __cxa_throw(obj = (nil), tinfo = (nil), dest = (nil), , line 75 in "eh_throw.cc"
[5] OBWebGate_Authent(r = 0xfffffd7fff3fb300), line 86 in "apache.cpp"
[6] ap_run_post_config(0x0, 0x0, 0x0, 0x0, 0x0, 0x0), at 0x444624
[7] main(0x0, 0x0, 0x0, 0x0, 0x0, 0x0), at 0x42c39a
I am using following link options.
Compile option is
/usr/sfw/bin/g++ -c -I/scratch/ashishas/view_storage/build/coreid1014/palantir/apache22/solaris-x86_64/include -m64 -fPIC -D_REENTRANT -Wall -g -o apache.o apache.cpp
Link option is
/usr/sfw/bin/g++ -shared -m64 -o apache.so apache.o -lsocket -lnsl -ldl -lpthread -lthread
At line 86, we are just throwing simple exception which have catch handlers in place. Also we do have catch(...) handler as well.
Surpursing things are..same issue didn't observe if we make it as executable.
Issue only comes if this is shared object loaded on webserver. If this is plain shared object, opened by anyother exe, it works fine.
Can someone help me out. This is completly blocking issue for us. Using Solaris Sun Studio compiler is no option as of now.

Answer 1

shared object that load into web server process space
... same issue didn't observe if we make it as executable.When you "inject" your shared object into some other process a well-being of your exception handling depends on that other process.
Mechanics of x64 stack traversing (unwind) performed when you throw the exception is quite complicated,
particularly involving a "nearly-standartized" Unwind interface (say, Unwind_RaiseException).
When we are talking about g++ on Solaris there are two implementations of unwind interface, one in libc and one in libgcc_s.so.
When you g++-compile the executable you get it directly linked with libgcc_s.so and Unwind stuff resolves into libgccs.
When g++-compiled shared object is loaded into non-g++-compiled executable's process _Unwind calls are most likely already resolved into Solaris libc.
Thats why you might see the difference.
Now, what exactly causes this difference can vary, I can only speculate.
All that would not be a problem if _Unwind interface was completely standartized and properly implemented.
However there are two issues currently:
* gcc (libstdc++ in particular) happens to use additional non-standard _Unwind calls which are not present in Solaris libc
naturally, implementation details of Unwind implementation in libc differs to that of libgccs, so when all the standard _Unwind
routines are resolved into Solaris version and one non-standard _Unwind routine is resolved into gcc version you get a problem
(most likely that is what happens with you)
* libc Unwind sometimes is unable to decipher the code generated by gcc.
However that is likely to happen with modern gcc (say, 4.4+) and not that likely with 3.4.3
Btw, you can check your call frame to see where _Unwind calls come from:
where -h -lIf you indeed stomped on "mixed _Unwind" problem then the only chance for you is to play with linker
so it binds Unwind stuff from your library directly into libgccs.
Not tried it myself though.
regards,
__Fedor.