Coming back to life after Segmentation Violation

Question

Is it possible to restore the normal execution flow of a C program, after the Segmentation Fault error?

struct A {
    int x;
};
A* a = 0;

a->x = 123; // this is where segmentation violation occurs

// after handling the error I want to get back here:
printf("normal execution");
// the rest of my source code....

I want a mechanism similar to NullPointerException that is present in Java, C# etc.

Note: Please, don't tell me that there is an exception handling mechanism in C++ because I know that, dont' tell me I should check every pointer before assignment etc.

What I really want to achieve is to get back to normal execution flow as in the example above. I know some actions can be undertaken using POSIX signals. How should it look like? Other ideas?

Answer 1

You can catch segmentation faults using a signal handler, and decide to continue the excecution of the program (at your own risks).

The signal name is SIGSEGV.

You will have to use the sigaction() function, from the signal.h header.

Basically, it works the following way:

struct sigaction sa1;
struct sigaction sa2;

sa1.sa_handler = your_handler_func;
sa1.sa_flags   = 0;
sigemptyset( &sa1.sa_mask );

sigaction( SIGSEGV, &sa1, &sa2 );

Here's the prototype of the handler function:

void your_handler_func( int id );

As you can see, you don't need to return. The program's execution will continue, unless you decide to stop it by yourself from the handler.

Answer 2

Call this, and when a segfault will occur, your code will execute segv_handler and then continue back to where it was.

void segv_handler(int)
{
  // Do what you want here
}

signal(SIGSEGV, segv_handler);

Answer 3

In POSIX, your process will get sent SIGSEGV when you do that. The default handler just crashes your program. You can add your own handler using the signal() call. You can implement whatever behaviour you like by handling the signal yourself.

Answer 4

"All things are permissible, but not all are beneficial" - typically a segfault is game over for a good reason... A better idea than picking up where it was would be to keep your data persisted (database, or at least a file system) and enable it to pick up where it left off that way. This will give you much better data reliability all around.

Answer 5

This glib manual gives you a clear picture of how to write signal handlers.

A signal handler is just a function that you compile together with the rest
of the program. Instead of directly invoking the function, you use signal 
or sigaction to tell the operating system to call it when a signal arrives.
This is known as establishing the handler.

In your case you will have to wait for the SIGSEGV indicating a segmentation fault. The list of other signals can be found here.

Signal handlers are broadly classified into tow categories

1. You can have the handler function note that the signal arrived by tweaking some global data structures, and then return normally.
2. You can have the handler function terminate the program or transfer control to a point where it can recover from the situation that caused the signal.

SIGSEGV comes under program error signals

Answer 6

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <signal.h>
#include <stdlib.h>
#include <ucontext.h>

void safe_func(void)
{
    puts("Safe now ?");
    exit(0); //can't return to main, it's where the segfault occured.
}

void
handler (int cause, siginfo_t * info, void *uap)
{
  //For test. Never ever call stdio functions in a signal handler otherwise*/
  printf ("SIGSEGV raised at address %p\n", info->si_addr);
  ucontext_t *context = uap;
  /*On my particular system, compiled with gcc -O2, the offending instruction
  generated for "*f = 16;" is 6 bytes. Lets try to set the instruction
  pointer to the next instruction (general register 14 is EIP, on linux x86) */
  context->uc_mcontext.gregs[14] += 6; 
  //alternativly, try to jump to a "safe place"
  //context->uc_mcontext.gregs[14] = (unsigned int)safe_func;
}

int
main (int argc, char *argv[])
{
  struct sigaction sa;
  sa.sa_sigaction = handler;
  int *f = NULL;
  sigemptyset (&sa.sa_mask);
  sa.sa_flags = SA_SIGINFO;
  if (sigaction (SIGSEGV, &sa, 0)) {
      perror ("sigaction");
      exit(1);
  }
  //cause a segfault
  *f = 16; 
  puts("Still Alive");
  return 0;
}

$ ./a.out
SIGSEGV raised at address (nil)
Still Alive

I would beat someone with a bat if I saw something like this in production code though, it's an ugly, for-fun hack. You'll have no idea if the segfault have corrupted some of your data, you'll have no sane way of recovering and know that everything is Ok now, there's no portable way of doing this. The only mildly sane thing you could do is try to log an error (use write() directly, not any of the stdio functions - they're not signal safe) and perhaps restart the program. For those cases you're much better off writing a superwisor process that monitors a child process exit, logs it and starts a new child process.

Answer 7

See R.'s comment to MacMade answer.

Expanding on what he said, (after handling SIGSEV, or, for that case, SIGFPE, the CPU+OS can return you to the offending insn) here is a test I have for division by zero handling:

#include <stdio.h>
#include <limits.h>
#include <string.h>
#include <signal.h>
#include <setjmp.h>

static jmp_buf  context;

static void sig_handler(int signo)
{
    /* XXX: don't do this, not reentrant */
    printf("Got SIGFPE\n");

    /* avoid infinite loop */
    longjmp(context, 1);
}

int main()
{
    int a;
    struct sigaction sa;

    memset(&sa, 0, sizeof(struct sigaction));
    sa.sa_handler = sig_handler;
    sa.sa_flags = SA_RESTART;
    sigaction(SIGFPE, &sa, NULL);

    if (setjmp(context)) {
            /* If this one was on setjmp's block,
             * it would need to be volatile, to
             * make sure the compiler reloads it.
             */
            sigset_t ss;

            /* Make sure to unblock SIGFPE, according to POSIX it
             * gets blocked when calling its signal handler.
             * sigsetjmp()/siglongjmp would make this unnecessary.
             */
            sigemptyset(&ss);
            sigaddset(&ss, SIGFPE);
            sigprocmask(SIG_UNBLOCK, &ss, NULL);

            goto skip;
    }

    a = 10 / 0;
skip:
    printf("Exiting\n");

    return 0;
}

Answer 8

No, it's not possible, in any logical sense, to restore normal execution following a segmentation fault. Your program just tried to dereference a null pointer. How are you going to carry on as normal if something your program expects to be there isn't? It's a programming bug, the only safe thing to do is to exit.

Consider some of the possible causes of a segmentation fault:

• you forgot to assign a legitimate value to a pointer
• a pointer has been overwritten possibly because you are accessing heap memory you have freed
• a bug has corrupted the heap
• a bug has corrupted the stack
• a malicious third party is attempting a buffer overflow exploit
• malloc returned null because you have run out of memory

Only in the first case is there any kind of reasonable expectation that you might be able to carry on

If you have a pointer that you want to dereference but it might legitimately be null, you must test it before attempting the dereference. I know you don't want me to tell you that, but it's the right answer, so tough.

Edit: here's an example to show why you definitely do not want to carry on with the next instruction after dereferencing a null pointer:

void foobarMyProcess(struct SomeStruct* structPtr)
{
    char* aBuffer = structPtr->aBigBufferWithLotsOfSpace; // if structPtr is NULL, will SIGSEGV
    //
    // if you SIGSEGV and come back to here, at this point aBuffer contains whatever garbage was in memory at the point
    // where the stack frame was created
    //
    strcpy(aBuffer, "Some longish string");  // You've just written the string to some random location in your address space
                                             // good luck with that!

}

Answer 9

You can use the SetUnhandledExceptionFilter() function (in windows), but even to be able to skip the "illegal" instruction you will need to be able to decode some assembler opcodes. And, as glowcoder said, even if it would "comment out" in runtime the instructions that generates segfaults, what will be left from the original program logic (if it may be called so)? Everything is possible, but it doesn't mean that it has to be done.

Answer 10

There is no meaningful way to recover from a SIGSEGV unless you know EXACTLY what caused it, and there's no way to do that in standard C. It may be possible (conceivably) in an instrumented environment, like a C-VM (?). The same is true for all program error signals; if you try to block/ignore them, or establish handlers that return normally, your program will probably break horribly when they happen unless perhaps they're generated by raise or kill.

Just do yourself a favour and take error cases into account.