How to catch segmentation fault in Linux?

On Linux we can have these as exceptions, too.

Normally, when your program performs a segmentation fault, it is sent a SIGSEGV signal. You can set up your own handler for this signal and mitigate the consequences. Of course you should really be sure that you can recover from the situation. In your case, I think, you should debug your code instead.

Back to the topic. I recently encountered a library (short manual) that transforms such signals to exceptions, so you can write code like this:

try
{
    *(int*) 0 = 0;
}
catch (std::exception& e)
{
    std::cerr << "Exception caught : " << e.what() << std::endl;
}

Didn't check it, though. Works on my x86-64 Gentoo box. It has a platform-specific backend (borrowed from gcc's java implementation), so it can work on many platforms. It just supports x86 and x86-64 out of the box, but you can get backends from libjava, which resides in gcc sources.


C++ solution found here (http://www.cplusplus.com/forum/unices/16430/)

#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void ouch(int sig)
{
    printf("OUCH! - I got signal %d\n", sig);
}
int main()
{
    struct sigaction act;
    act.sa_handler = ouch;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    sigaction(SIGINT, &act, 0);
    while(1) {
        printf("Hello World!\n");
        sleep(1);
    }
}

Here's an example of how to do it in C.

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void segfault_sigaction(int signal, siginfo_t *si, void *arg)
{
    printf("Caught segfault at address %p\n", si->si_addr);
    exit(0);
}

int main(void)
{
    int *foo = NULL;
    struct sigaction sa;

    memset(&sa, 0, sizeof(struct sigaction));
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = segfault_sigaction;
    sa.sa_flags   = SA_SIGINFO;

    sigaction(SIGSEGV, &sa, NULL);

    /* Cause a seg fault */
    *foo = 1;

    return 0;
}

For portability, one should probably use std::signal from the standard C++ library, but there is a lot of restriction on what a signal handler can do. Unfortunately, it is not possible to catch a SIGSEGV from within a C++ program without introducing undefined behavior because the specification says:

  1. it is undefined behavior to call any library function from within the handler other than a very narrow subset of the standard library functions (abort, exit, some atomic functions, reinstall current signal handler, memcpy, memmove, type traits, std::move, std::forward, and some more).
  2. it is undefined behavior if handler use a throw expression.
  3. it is undefined behavior if the handler returns when handling SIGFPE, SIGILL, SIGSEGV

This proves that it is impossible to catch SIGSEGV from within a program using strictly standard and portable C++. SIGSEGV is still caught by the operating system and is normally reported to the parent process when a wait family function is called.

You will probably run into the same kind of trouble using POSIX signal because there is a clause that says in 2.4.3 Signal Actions:

The behavior of a process is undefined after it returns normally from a signal-catching function for a SIGBUS, SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(), sigqueue(), or raise().

A word about the longjumps. Assuming we are using POSIX signals, using longjump to simulate stack unwinding won't help:

Although longjmp() is an async-signal-safe function, if it is invoked from a signal handler which interrupted a non-async-signal-safe function or equivalent (such as the processing equivalent to exit() performed after a return from the initial call to main()), the behavior of any subsequent call to a non-async-signal-safe function or equivalent is undefined.

This means that the continuation invoked by the call to longjump cannot reliably call usually useful library function such as printf, malloc or exit or return from main without inducing undefined behavior. As such, the continuation can only do a restricted operations and may only exit through some abnormal termination mechanism.

To put things short, catching a SIGSEGV and resuming execution of the program in a portable is probably infeasible without introducing undefined behavior. Even if you are working on a Windows platform for which you have access to Structured exception handling, it is worth mentioning that MSDN suggest to never attempt to handle hardware exceptions: Hardware Exceptions.

At last but not least, whether any SIGSEGV would be raised when dereferencing a null valued pointer (or invalid valued pointer) is not a requirement from the standard. Because indirection through a null valued pointer or any invalid valued pointer is an undefined behaviour, which means the compiler assumes your code will never attempt such a thing at runtime, the compiler is free to make code transformation that would elide such undefined behavior. For example, from cppreference,

int foo(int* p) {
    int x = *p;
    if(!p)
        return x; // Either undefined behavior above or this branch is never taken
    else
        return 0;
}
 
int main() {
    int* p = nullptr;
    std::cout << foo(p);
}

Here the true path of the if could be completely elided by the compiler as an optimization; only the else part could be kept. Said otherwise, the compiler infers foo() will never receive a null valued pointer at runtime since it would lead to an undefined behaviour. Invoking it with a null valued pointer, you may observe the value 0 printed to standard output and no crash, you may observe a crash with SIGSEG, in fact you could observe anything since no sensible requirements are imposed on programs that are not free of undefined behaviors.

Tags:

C++

Try Catch

Segmentation Fault

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