[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Libunwind-devel] unwinding from signal handler (ARM)

From: Sven Neumann
Subject: [Libunwind-devel] unwinding from signal handler (ARM)
Date: Fri, 19 Aug 2011 14:39:51 +0200


we are using libunwind for a while now to obtain backtraces for crash
reports. This on an ARM with EABI. Right now we are using a somewhat
outdated git snapshot (99e60be), but I'm currently trying to update to
1.0-rc1. I don't have much success though.

With 99e60be we had the problem that we could not unwind from a signal
handler. But that is exactly what we need to do since we want to get a
backtrace from the signal handler that is called when the application
crashes (usually signal 11). We found a workaround though, instead of
calling unw_getcontext() to get an initial unw_context_t pointer to pass
to unw_init_local(), we use the pointer that we got from the signal
handler. Here's some pseudo code for this:

    struct sigaction new_action;

    new_action.sa_sigaction = logCrash;
    sigemptyset (&new_action.sa_mask);
    new_action.sa_flags = SA_SIGINFO;

    sigaction(SIGSEGV, &new_action, NULL);
    sigaction(SIGFPE, &new_action, NULL);

and in logCrash we do something along the lines of:

static void
logCrash(int sig, siginfo_t *info, void *ctx)
  unw_init_local(&cursor, ctx);

    char        name[128];
    unw_word_t  valp;
    unw_word_t  offset;
    if (unw_get_reg(&cursor, UNW_REG_IP, &valp) != 0)

    /* log function name etc. here */

    ret = unw_step (&cursor);
  while (ret > 0);

This worked reasonably well giving us a nice stack trace most of the

Now I've updated libunwind to 1.0-rc1 and this trick doesn't seem to
work any longer. The code appears to crash in libunwind if used as
above. I had a look at the ARM implementation and found that there is
now code that's supposed to unw_step a signal handler. So I tried
without our hack and just use unw_getcontext(). Now it doesn't crash any
longer, but it also doesn't unwind over the signal handler. All I get is
a stack trace like this:

0x4016cde0 logUnwind() from /usr/lib/
0x40205654 ?? from /usr/lib/

Any ideas what we could do?


reply via email to

[Prev in Thread] Current Thread [Next in Thread]