I am getting segmentation fault despite having a signal handler for SIGSEGV. The first time data is written to protected memory, I can see that signal handler is called, but before the signal handler exits, segmentation fault appears.
What can be causing it? Isn't my SIGSEGV handler supposed to catch all the segmentation faults?
I could see where if the segmentation fault is related to the stack pointer accessing memory where it's not allowed by the OS, then you won't be able to make any calls with the current stack pointer for your process ... that includes calls to signal handlers. In other words the compiler-created prologue for your signal handler function has to setup an activation record on the stack ... if the stack pointer itself is invalid, then that won't be possible. One way this could happen is by overflowing a memory array that then writes-over the activation record for the currently executing function.
You can define another area of memory to be used as a stack for your signal handlers though sigaltstack(), and then setting the SA_ONSTACK option in sigaction() for the signal. This might be something you might want to try.
Finally, you could also run into issues if you're using non-async-safe functions or somehow are accessing pointers or memory that is outside the memory segment allotted to your process by the OS in your signal handler.
If your signal handler in turn provokes another signal that of course is not caught by your signal handler as you would then have a kind of infinite loop.
Related
I want to create an Units test framwork, but to provide a good reporting I need to catch SIGABRT, SIGSEGV and probably others signals to prevent my process from being killed (And so, to be able to continue the tests processing)...
But I don't know how to do this and so, I need information:
SIGABRT is a thread direct signal ?
What happens if I only use the main thread to catch the SIGABRT (or SIGSEGV) signal? Could the thread that called abort return from its call (I hope not) ?
If you have any useful documents, links or tutorial, I'm interested. It's for a C code using pthreads.
Thanks for your help
I need to catch SIGABRT, SIGSEGV and probably others signals to prevent my process from being killed
This is an exercise in futility. After SIGABRT or SIGSEGV is raised, you (in general) have no idea about the state of the process -- it may have corrupted heap, stack, global data internal to your test framework, global data internal to the C runtime system, etc. etc. Continuing such process is exceedingly likely to continue crashing at random (correct) places in the code.
The only sane way to handle this in a test framework is to fork and have the parent process handle child error exits, report them and continue running additional tests.
SIGABRT is a thread direct signal ?
There is no such thing as "direct signal". SIGABRT may be sent to the process from outside, or it can be raised inside the process.
What happens if I only use the main thread to catch the SIGABRT (or SIGSEGV) signal?
SIGSEGV and SIGABRT (when not sent from outside) is sent to the thread which caused the invalid memory operation (or raised it).
In addition, there is no way to "only use main thread" -- sigaction is global across all threads (though you can set a thread-specific signal mask).
I have written a phone book and my person input like "name-number-type" and if the user give "_" or any other character that is not "-" as seperator the program will crash by seg. fault because i used strtok() to split input. Also if the user gives number that is not proper input like 1232414eree224, my program will crash again. So i just think that if i handle the seg. fault signal in os level may be i can fix this bug. I know that i can write a controller function but i am searching for a new way to deal with like that error. Is there any possible way to do this?
So i just think that if i handle the seg. fault signal in os level may
be i can fix this bug.
It does not seem like a good idea at all, a SIGSEGV handler is not intended to ignore or try to fix the error, they are used to consistently exit your process, e.g. remove the used heap so that there are no memory leaks, to write in a log reporting the error ...
In addition, in the POSIX standard:
The behavior of a process is undefined after it returns normally from
a signal-catching function for a [XSI] SIGBUS, SIGFPE, SIGILL, or
SIGSEGV signal that was not generated by kill(), [RTS] sigqueue(), or
raise().
I have a test program for part of a C library where I handle signals. There is a lot of code in this test program - so much that I feel that posting it here would only make it more difficult to answer my question.
Basically what's happening is that I am forcing the system to send me a SIGSEGV inside a background thread. I handle that signal, and then the test is over. The main thread will be waiting for this and part of the signal handler is to restart the main thread. When the main thread restarts I restore the default signal handler by signal(SIGSEGV, SIG_DFL) and then I get another SIGSEGV.
I understand that I have a lot of code and this could very well be my fault, but is there a common explanation of why a SIGSEGV would fire twice? It is not happening when I cause other signals (though I haven't tried all of them).
To clarify I am on OS X, writing in C, compiling with gcc.
If you have any problem inside the signal handler, for example, memory overwrite, trying to access beyond the allocated area, then a new signal will be raised. Check the following page for details.
http://www.gnu.org/software/libc/manual/html_node/Signals-in-Handler.html
You should also use reentrant functions inside the signal handlers, For example localtime_r instead of localtime.
Moreover it is recommended to use sigaction() instead of signal().
More details here: What is the difference between sigaction and signal?
I have an application which I use to catch any segmentation fault or ctrl-c.
Using the below code, I am able to catch the segmentation fault but the handler is being called again and again. How can I stop them.
For your information, I don't want to exit my application. I just can take care to free all the corrupted buffers.
Is it possible?
void SignalInit(void )
{
struct sigaction sigIntHandler;
sigIntHandler.sa_handler = mysighandler;
sigemptyset(&sigIntHandler.sa_mask);
sigIntHandler.sa_flags = 0;
sigaction(SIGINT, &sigIntHandler, NULL);
sigaction(SIGSEGV, &sigIntHandler, NULL);
}
and handler goes like this.
void mysighandler()
{
MyfreeBuffers(); /*related to my applciation*/
}
Here for Segmentation fault signal, handler is being called multiple times and as obvious MyfreeBuffers() gives me errors for freeing already freed memory. I just want to free only once but still dont want to exit application.
Please help.
The default action for things like SIGSEGV is to terminate your process but as you've installed a handler for it, it'll call your handler overriding the default behavior. But the problem is segfaulting instruction may be retried after your handler finishes and if you haven't taken measures to fix the first seg fault, the retried instruction will again fault and it goes on and on.
So first spot the instruction that resulted in SIGSEGV and try to fix it (you can call something like backtrace() in the handler and see for yourself what went wrong)
Also, the POSIX standard says that,
The behavior of a process is undefined after it returns normally from
a signal-catching function for a [XSI] SIGBUS, SIGFPE, SIGILL, or
SIGSEGV signal that was not generated by kill(), [RTS] sigqueue(),
or raise().
So, the ideal thing to do is to fix your segfault in the first place. Handler for segfault is not meant to bypass the underlying error condition
So the best suggestion would be- Don't catch the SIGSEGV. Let it dump core. Analyze the core. Fix the invalid memory reference and there you go!
I do not agree at all with the statement "Don't catch the SIGSEGV".
That's a pretty good pratice to deal with unexpected conditions. And that's much cleaner to cope with NULL pointers (as given by malloc failures) with signal mechanism associated to setjmp/longjmp, than to distribute error condition management all along your code.
Note however that if you use ''sigaction'' on SEGV, you must not forget to say SA_NODEFER in sa_flags - or find another way to deal with the fact SEGV will trigger your handler just once.
#include <setjmp.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
static void do_segv()
{
int *segv;
segv = 0; /* malloc(a_huge_amount); */
*segv = 1;
}
sigjmp_buf point;
static void handler(int sig, siginfo_t *dont_care, void *dont_care_either)
{
longjmp(point, 1);
}
int main()
{
struct sigaction sa;
memset(&sa, 0, sizeof(sigaction));
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_NODEFER;
sa.sa_sigaction = handler;
sigaction(SIGSEGV, &sa, NULL); /* ignore whether it works or not */
if (setjmp(point) == 0)
do_segv();
else
fprintf(stderr, "rather unexpected error\n");
return 0;
}
If the SIGSEGV fires again, the obvious conclusion is that the call to MyfreeBuffers(); has not fixed the underlying problem (and if that function really does only free() some allocated memory, I'm not sure why you would think it would).
Roughly, a SIGSEGV fires when an attempt is made to access an inaccessible memory address. If you are not going to exit the application, you need to either make that memory address accessible, or change the execution path with longjmp().
You shouldn't try to continue after SIG_SEGV. It basically means that the environment of your application is corrupted in some way. It could be that you have just dereferenced a null pointer, or it could be that some bug has caused your program to corrupt its stack or the heap or some pointer variable, you just don't know. The only safe thing to do is terminate the program.
It's perfectly legitimate to handle control-C. Lots of applications do it, but you have to be really careful exactly what you do in your signal handler. You can't call any function that's not re-entrant. So that means if your MyFreeBuffers() calls the stdlib free() function, you are probably screwed. If the user hits control-C while the program is in the middle of malloc() or free() and thus half way through manipulating the data structures they use to track heap allocations, you will almost certainly corrupt the heap if you call malloc() or free() in the signal handler.
About the only safe thing you can do in a signal handler is set a flag to say you caught the signal. Your app can then poll the flag at intervals to decide if it needs to perform some action.
Well you could set a state variable and only free memory if its not set. The signal handler will be called everytime, you can't control that AFAIK.
I can see at case for recovering from a SIG_SEGV, if your handling events in a loop and one of these events causes a Segmentation Violation then you would only want to skip over this event, continue processing the remaining events. In my eyes SIG_SEGV is similar to the NullPointerException in Java. Yes the state will be inconsistent and unknown after either of these, however in some cases you would like to handle the situation and carry on. For instance in Algo trading you would pause the execution of an order and allow a trader to manually take over, with out crashing the entire system and ruining all other orders.
Looks like at least under Linux using the trick with -fnon-call-exceptions option can be the solution. It will give an ability to convert the signal to general C++ exception and handle it by general way.
Look the linux3/gcc46: "-fnon-call-exceptions", which signals are trapping instructions? for example.
What happens when two threads of the same process running on different logical cpu hit a seg fault?
Default action is for the process to exit. If you handle the segfault, I suppose you could try to arrange for just the thread where it happened to terminate. However, since the only things which cause a segfault to occur naturally (as opposed to raise or kill) stem from undefined behavior, the program is in an indeterminate state and you can't rely on being able to recover anything.
Normal handling of a Segmentation Fault involves the termination of the process. That means that both of them are terminated.
I think the default action on all major OSes is to terminate the process. However, you could conceivably install (e.g using signal) an alternate handler that only terminated the thread. Of course, once you have a segmentation fault, behavior typically becomes undefined, and attempting to continue is risky.
Signals generated due to illegal execution are handled synchronously by the kernel. So even if both the threads generate seg fault at the same time, only one gets thru'.
The segfault handler is called for the process. If you don't do anything special, the OS will kill the process and reclaim its resources.