https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67365
--- Comment #3 from Ian Lance Taylor <ian at airs dot com> --- The missing address is part of the signal handling code. It's the code that returns to normal execution after the signal handler completes, by calling the rt_sigreturn system call. The backtrace code routinely decrements the return address by 1, so that it can report the file/line of the function call rather than the line that follows the call. In this case, decrementing by 1 gives it an address 1 byte before _restore_rt. It can't find any file/line information for that one byte, so you get a ???. This code is very processor-specific. GCC's internal unwind library knows that we are looking at a signal handler return, but it doesn't expose the information in any way that I can see (the _Unwind_IsSignalFrame function returns true for the function in which the signal occurred, which is the frame above the frame we are talking about). I think the best fix is going to be to add a new flag to _Unwind_Context: SIGRETURN_BIT or something, to indicate that the current context is a signal handler frame. Then we can add an _Unwind_IsSigreturn function so that the unwind code can check that bit. Then libbacktrace can return something like <signal handler> as gdb does. (gdb currently uses processor-specific code for this.)
