Re: Exception handling on AIX5.3 with gcc 3.4.6

[Ian Lance Taylor <iant@google.com>]

Brian Dessent <brian@dessent.net> writes:

> Shlom Marom wrote:
>
>> If I compiled the program with shared lib-gcc and with the pthread
>> libstdc++ - everything works fine and also the base exception is being
>> caught as it should.
>> ...
>> Shared linking: gcc -o tester_shared main.cpp -pthread -I.
>> -shared-libgcc -L/usr/lib -ldl -lpthread -lstdc++
>> Static linking: gcc -o tester_static main.cpp -L/usr/lib/threads
>> -pthread -I. -static-libgcc -L/usr/lib -ldl -lpthread `gcc
>
> A shared libgcc is pretty much required for proper exception handling, I
> just don't think you're going to be able to get around that.  This
> should hold true for pretty much every target so the fact that it works
> with static libgcc on Linux is by accident, or perhaps luck.

Brian's answer is in general correct.  However, there is a case where
it works.  You can use -static-libgcc when throwing exceptions across
shared library boundaries if:

1) gcc is configured to use unwind-dw2-glibc-fde.c instead of
   unwind-dw2-glibc.c (this happens on GNU/Linux systems and some
   others--look for uses of t-linux or t-libunwind-elf in
   gcc/config.gcc).

2) gcc is configured to pass --eh-frame-hdr to the linker (this
   happens automatically if the linker available when configuring gcc
   supports this option).

3) The executable and all shared libraries were in fact built by a gcc
   which passes --eh-frame-hdr to the linker.

4) Your C library has a working copy of dl_iterate_phdr (this is true
   for recent versions of glibc on GNU/Linux).

If all those conditions are true, then unwind information does not
need to be registered at program startup.  Instead, the unwind library
will find it at runtime using dl_iterate_phdr to look for
PT_GNU_EH_FRAME segments created by the linker.  And in that case
-static-libgcc will work.

Ian