信号处理程序内部的回溯

Question

I'm trying to following the code from this post to have signal handlers print a backtrace on errors such as floating point and segmentation faults. I'm using seg fault signals as a starting point. Here is the code:

#include <cstdlib>      //for exit()
#include <signal.h>     //signal handling   
#include <execinfo.h>   //backtrace, backtrace_symbols and backtrace_fd
#include <iostream>
#include <string.h>
#include <stdio.h>

#define TRACE_MSG fprintf(stderr, "TRACE at: %s() [%s:%d]\n", \
        __FUNCTION__, __FILE__, __LINE__)

void show_stackframe()
{
    void *trace[1024];
    char **messages = (char **) NULL;
    int i, trace_size = 0;
    TRACE_MSG;
    trace_size = backtrace(trace, 1024);    // segfault here???
    // More code here to print backtrace, but not needed at the moment..
    TRACE_MSG;
}

void sigSegvHandler( int signum, siginfo_t* info, void* arg )
{
    TRACE_MSG;
    show_stackframe();
    return;
}

double func_b()
{
    show_stackframe();  // Show that backtrace works without being 
                    // called inside sighandler.

    TRACE_MSG;
    int int_a[5];
    int_a[0] = 4;
    int_a[11] = 10;         // cause a segfault on purpose to see
                    // how the signal handling performs.

    return 1.1;
}

int main()
{
    // Examine and change the seg fault signal
    struct sigaction segvAction;   // File: /usr/include/bits/sigaction.h

   // Initialize segvAction struct to all zeros for initialiation
   memset( &segvAction,  0, sizeof( segvAction ) );

   segvAction.sa_sigaction = sigSegvHandler;
   segvAction.sa_flags = SA_SIGINFO;    //Invoke signal catching function with 3 arguments instead of 1

   // Set the action for the SIGSEGV signal
   sigaction( SIGSEGV, &segvAction,  NULL );

   func_b();    // Produce a SIGSEGV error
}

I am compiling using:

 g++ -rdynamic testprogram.cpp -o testprogram

I receive the following output from the program:

TRACE at: show_stackframe() [stackoverflow.cpp:15]
TRACE at: show_stackframe() [stackoverflow.cpp:17]
TRACE at: func_b() [stackoverflow.cpp:33]
TRACE at: sigSegvHandler() [stackoverflow.cpp:22]
TRACE at: show_stackframe() [stackoverflow.cpp:15]
Segmentation fault

My question is why does show_stackframe() cause a segmentation fault inside of sigaction but works fine when not inside of the sigaction handler? I obviously seem to be setting up the signal handler/action incorrect but I haven't been able to find it all day. GDB doesn't seem to be any help in this case.

Answer 1

As stated here , the backtrace function is AS-Unsafe, which means it is unsafe to call from an asynchronous signal handler. Doing so invokes undefined behavior.