Signal handlers and _Unwind_IsSignalFrame

Signal handlers and _Unwind_IsSignalFrame

[Martin Bouzek]

Hi,

I have a problem. I want to handle signals like SIGSEGV in non-main
threads of C++ program and terminate thread by pthread_exit call. Of
course I need all destructors of objects on stack to be called (eg.
mutex unlocking).

When compiling with -fnon-call-exceptios everything almost works but:

In context is saved address of failing instruction, not adress of
instruction after the failing one. Thus decrementing of this address in
__gxx_personality_v0 sometimes leads to calling of terminate(), because
no suitable entry in call-site table (?is it the right name?) is found
in .gcc_except_table. 

It is clearly supposed to be handled by SIGNAL_FRAME_BIT in flagss in
struct _Unwind_Context. But this bit is not set in my context. That is
why __gxx_personality misses proper handler by one byte, and calls
terminate instead. If I put one simple pointer dereference before my
failing instruction, decremented address is now inside the range
described in call-site table and everything works fine.

I am not sure I understand well how the SIGNAL_FRAME_BIT flag is
supposed to be set. Shall there be a CIE with "S" augmentation
in .eh_frame for signal handler? Threre is not such one in my program.
How can I make gcc to create one? Or am I completely wrong?

Just for completenes I have tried with gcc 4.1.0 and 4.2.1 on x86_64
(Suse 10.1 and 10.3).

Thanks in advance

Re: Signal handlers and _Unwind_IsSignalFrame

[Ian Lance Taylor]

Martin Bouzek <martin.bouzek@radas-atc.cz> writes:

> I have a problem. I want to handle signals like SIGSEGV in non-main
> threads of C++ program and terminate thread by pthread_exit call. Of
> course I need all destructors of objects on stack to be called (eg.
> mutex unlocking).
>
> When compiling with -fnon-call-exceptios everything almost works but:
>
> In context is saved address of failing instruction, not adress of
> instruction after the failing one. Thus decrementing of this address in
> __gxx_personality_v0 sometimes leads to calling of terminate(), because
> no suitable entry in call-site table (?is it the right name?) is found
> in .gcc_except_table. 
>
> It is clearly supposed to be handled by SIGNAL_FRAME_BIT in flagss in
> struct _Unwind_Context. But this bit is not set in my context. That is
> why __gxx_personality misses proper handler by one byte, and calls
> terminate instead. If I put one simple pointer dereference before my
> failing instruction, decremented address is now inside the range
> described in call-site table and everything works fine.
>
> I am not sure I understand well how the SIGNAL_FRAME_BIT flag is
> supposed to be set. Shall there be a CIE with "S" augmentation
> in .eh_frame for signal handler? Threre is not such one in my program.
> How can I make gcc to create one? Or am I completely wrong?

In ordinary use the signal_frame and SIGNAL_FRAME_BIT flags is set by
machine dependent code which recognizes a symbol frame.  In the gcc
source code for x86_64 GNU/Linux this is in the file
gcc/config/i386/linux-unwind.h.  Perhaps that code is not working for
you, but I have no idea why not.

Note that you can have this sort of problem when using some versions
of libunwind instead of the unwind code which comes with gcc.  You
would know if you are doing that, though; you need to configure gcc
with --with-system-libunwind.

Ian