Re: Exception handling on AIX5.3 with gcc 3.4.6

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Exception handling on AIX5.3 with gcc 3.4.6
\ Shlom Marom (20 Mar 2008)
. \ Brian Dessent (20 Mar 2008)
. . \ Shlom Marom (21 Mar 2008)
. . \ Ian Lance Taylor (26 Mar 2008)
. \ David Edelsohn (20 Mar 2008)
. \ Brian Dessent (22 Mar 2008)
. . \ Shlom Marom (23 Mar 2008)

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Subject:	Re: Exception handling on AIX5.3 with gcc 3.4.6
Group:	Gcc-help
From:	Ian Lance Taylor
Date:	26 Mar 2008

Brian Dessent <brian> 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