Android _Unwind_Backtrace inside sigaction

Question

I am trying to catch signals such as SIGSEGV in my Android NDK app for debugging purpose. For that, I have set up a sigaction that is called.

I am now trying to get the

Answer 1

In my practice standard _Unwind_Backtrace failed to switch to pre-signal stack.

I've managed to get some before-signal stacks by calling internal libgcc __gnu_Unwind_Backtrace - it has an extra agrument being "current registry values" - so it operates on given stack, not on current stack.

//definitions copied from arm-specific libgcc 4.8 sources.
struct core_regs
{
  _uw r[16];
};

typedef struct
{
  _uw demand_save_flags;
  struct core_regs core;
} phase2_vrs;

extern "C"
_Unwind_Reason_Code
__gnu_Unwind_Backtrace(_Unwind_Trace_Fn trace, void * trace_argument,
               phase2_vrs * entry_vrs);

// Getting backtrace with those definitions
//istead of _Unwind_Backtrace(tracer, &state);
if (const ucontext_t* signal_context = last_sigaction_parameter)
{
      phase2_vrs pre_signal_state = {};
      pre_signal_state.core = *reinterpret_cast(&(signal_context->uc_mcontext.arm_r0));
      __gnu_Unwind_Backtrace(tracer, &state, &pre_signal_state);
}