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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: _Unwind_Backtrace crashes
Date: Thu, 27 Aug 2015 22:00:06 +0200
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* Andy Lutomirski <luto@amacapital.net> [2015-08-27 11:55:24 -0700]:
> This works on glibc.  It aborts on musl on i386 using the latest git version.
> 
> I suspect it's because whatever calls main isn't properly annotated,
> but I don't know how to debug this without rebuilding gcc, which is
> kind of a mess.
> 
> #define _GNU_SOURCE
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <signal.h>
> #include <unwind.h>
> #include <err.h>
> #include <string.h>
> 
> static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
>                int flags)
> {
>     struct sigaction sa;
>     memset(&sa, 0, sizeof(sa));
>     sa.sa_sigaction = handler;
>     sa.sa_flags = SA_SIGINFO | flags;
>     sigemptyset(&sa.sa_mask);
>     if (sigaction(sig, &sa, 0))
>         err(1, "sigaction");
> }
> 
> _Unwind_Reason_Code trace_fn(struct _Unwind_Context * ctx, void *opaque)
> {

you might want to add

	dprintf(1, "ip: %16p  cfa: %16p\n", (void*)_Unwind_GetIP(ctx), (void*)_Unwind_GetCFA(ctx));

here, so you see if trace_fn is called at all
(and s/printf/dprintf/g so you see what is printed before the crash)

>     return _URC_NO_REASON;
> }
> 
> static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
> {
>     printf("In signal handler.  Trying to unwind.\n");
>     _Unwind_Backtrace(trace_fn, 0);
> }
> 
> int main()
> {
>     printf("Unwind directly\n");
>     _Unwind_Backtrace(trace_fn, 0);
> 
>     printf("Unwind from signal handler\n");
>     sethandler(SIGUSR1, sigusr1, 0);
>     raise(SIGUSR1);
> 
>     printf("OK\n");
> }
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC