How do debuggers manage to break on any throw? - c++

In GDB and other debuggers it's possible to ask the debugger (using catch throw) to stop anytime an exception is thrown before the process passes said exception to the respective exception handler.
By what mechanism is this possible? Is there an OS signal that can be used to hook in? Is there a function pointer to monkey patch to allow this? Does it single step to make this happen?
Is this mechanism otherwise available outside the debugger?

GDB sets a breakpoint on the library function which does the stack unwinding (__cxa_throw() for x86_64) to implement catch throw. It will use the same mechanism to set this breakpoint it uses to set any other type of code breakpoint.
By what mechanism is this possible? Is there an OS signal that can be used to hook in? Is there a function pointer to monkey patch to allow this? Does it single step to make this happen?
None of these. It is just a normal breakpoint on __cxa_throw(). GDB uses knowledge of the implementation of the C++ runtime, with all the disadvantages that brings. C++ exceptions are below the radar of the operating system, so the operating system would not know about them. The confusion stems from the fact that certain signals (e.g. segfaults) are called exceptions on Windows and can be handled in a very similar way to C++ exceptions on Windows when debugging. But this is making OS-signals (e.g. segfault) behaving like C++ exceptions, not the other way round.
See also ftp://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_30.html

Related

Keep running the program after SIGABRT c++ signal

I use a third library in my c++ program which under certain circumstances emits SIGABRT signal. I know that trying to free non-initialized pointer or something like this can be the cause of this signal. Nevertheless I want to keep running my program after this signal is emitted, to show a message and allow the user to change the settings, in order to cope with this signal.
(I use QT for developing.)
How can I do that?
I use a third library in my c++ program which under certain circumstances emits SIGABRT signal
If you have the source code of that library, you need to correct the bug (and the bug could be in your code).
BTW, probably SIGABRT happens because abort(3) gets indirectly called (perhaps because you violated some conventions or invariants of that library, which might use assert(3) - and indirectly call abort). I guess that in caffe the various CHECK* macros could indirectly call abort. I leave you to investigate that.
If you don't have the source code or don't have the capacity or time to fix that bug in that third party library, you should give up using that library and use something else.
In many cases, you should trust external libraries more than your own code. Probably, you are abusing or misusing that library. Read carefully its documentation and be sure that your own code calling it is using that library correctly and respects its invariants and conventions. Probably the bug is in your own code, at some other place.
I want to keep running my program
This is impossible (or very unreliable, so unreasonable). I guess that your program has some undefined behavior. Be very scared, and work hard to avoid UB.
You need to improve your debugging skills. Learn better how to use the gdb debugger, valgrind, GCC sanitizers (e.g. instrumentation options like -fsanitize=address, -fsanitize=undefined and others), etc...
You reasonably should not try to handle SIGABRT even if in principle you might (but then read carefully signal(7), signal-safety(7) and hints about handling Unix signals in Qt). I strongly recommend to avoid even trying catching SIGABRT.
Unfortunately, you can't.
SIGABRT signal is itself sent right after abort()
Ref:
https://stackoverflow.com/a/3413215/9332965
You can handle SIGABRT, but you probably shouldn't.
The "can" is straightforward - just trap it in the usual way, using signal(). You don't want to return from this signal handler - you probably got here from abort() - possibly originally from assert() - and that function will exit after raising the signal. You could however longjmp() back to a state you set up earlier.
The "shouldn't" is because once SIGABRT has been raised, your data structures (including those of Qt and any other libraries) are likely in an inconsistent state and actually using any of your program's state is likely to be unpredictable at best. Apart from exiting immediately, there's not much you can do other than exec() a replacement program to take over in a sane initial state.
If you just want to show a friendly message, then you perhaps could exec() a small program to do that (or just use xmessage), but beware of exiting this with a success status where you would have had an indication of the SIGABRT otherwise.
Unfortunately there isn't much you can do to prevent SIGABRT from terminating your program. Not without modifying some code that was hopefully written by you.
You would either need to change code to not throw an abort, or you would have to spawn a new process that runs the code instead of the current process. I do not suggest you use a child process to solve this problem. It's most likely caused by misuse of an api or computer resources, such as low memory.

How to end a C++ program after error?

I am refactoring an old code, and one of the things I'd like to address is the way that errors are handled. I'm well aware of exceptions and how they work, but I'm not entirely sure they're the best solution for the situations I'm trying to handle.
In this code, if things don't validate, there's really no reason or advantage to unwind the stack. We're done. There's no point in trying to save the ship, because it's a non-interactive code that runs in parallel through the Sun Grid Engine. The user can't intervene. What's more, these validation failures don't really represent exceptional circumstances. They're expected.
So how do I best deal with this? One thing I'm not sure I want is an exit point in every class method that can fail. That seems unmaintainable. Am I wrong? Is it acceptable practice to just call exit() or abort() at the failure point in codes like this? Or should I throw an exception all the way back to some generic catch statement in main? What's the advantage?
Throwing an exception to be caught in main and then exiting means your RAII resource objects get cleaned up. On most systems this isn't needed for a lot of resource types. The OS will clean up memory, file handles, etc. (though I've used a system where failing to free memory meant it remained allocated until system restart, so leaking on program exit wasn't a good idea.)
But there are other resource types that you may want to release cleanly such as network or database connections, or a mechanical device you're driving and need to shut down safely. If an application uses a lot of such things then you may prefer to throw an exception to unwind the stack back to main, and then exit.
So the appropriate method of exiting depends on the application. If an application knows it's safe then calling _Exit(), abort(), exit(), or quickexit() may be perfectly reasonable. (Library code shouldn't call these, since obviously the library has no idea whether its safe for every application that will ever use the library.) If there is some critical clean up that must be performed before an application exits but you know it's limited, then the application can register that clean up code via atexit() or at_quick_exit().
So basically decide what you need cleaned up, document it, implement it, and try to make sure it's tested.
It is acceptable to terminate the program if it cannot handle the error gracefully. There are few things you can do:
Call abort() if you need a core dump.
Call exit() if you want to give a chance to run to those routines registered with atexit() (that is most likely to call destructors for global C++ objects).
Call _exit() to terminate a process immediately.
There is nothing wrong with using those functions as long as you understand what you are doing, know your other choices, and choose that path willingly. After all, that's why those functions exist. So if you don't think it makes any sense to try to handle the error or do anything else when it happens - go ahead. What I would probably do is try to log some informative message (say, to syslog), and call _exit. If logging fails - call abort to get a core along the termination.
I'd suggest to call global function
void stopProgram() {
exit(1);
}
Later you can change it's behavior, so it is maintainable.
As you pointed out, having an exit or abort thrown around throughout your code is not maintainable ... additionally, there may be a mechanism in the future that could allow you to recover from an error, or handle an error in a more graceful manner than simply exiting, and if you've already hard-coded this functionality in, then it would be very hard to undo.
Throwing an exception that is caught in main() is your best-bet at this point that will also give you flexibility in the future should you run the code under a different scenario that will allow you to recover from errors, or handle them differently. Additionally, throwing exceptions could help should you decide to add more debugging support, etc., as it will give you spots to implement logging features and record the program state from isolated and maintainable points in the software before you decide let the program exit.

Function guaranteed to be called in C++ during abrupt termination or exit

What function in C++ is guaranteed to be called during abrupt termination or exit which can perform the clean up activity ..
Depending on what you mean by "abrupt termination" there are several different options:
Global destructors will be called upon normal termination (return from main, or call to exit()).
atexit() registers a function to be called on normal termination.
std::set_terminate registers a function that will be called when an exception is thrown but not caught, or when "exception handling has to be terminated for some other reason".
sigaction() registers functions to be called when your program receives signals, many of which will normally abruptly terminate your program. Signal handlers may be called when the program is in an internally-inconsistent state, and therefore are extremely limited in what they can do. For instance, they cannot do anything that might allocate memory. Also, this API is not available on Windows; there are equivalents but I am not familiar with them.
Note that all operating systems in common use provide at least one way to abruptly terminate your program that cannot be intercepted from your code. For instance, Unix has signal 9 (SIGKILL) which you can't register a handler for. This is a feature, not a bug. You, the user, need a way to make a process go away even if it has done everything in its power to make itself indestructible. Furthermore, no code can protect your process when the user's pet rabbit gnaws through the power cord on the computer. Because of this, it might be a better use of your time to design your program to recover from crashes cleanly, rather than trying to clean up when a crash happens. See this article on "crash-only design" for more about that.
Read about atexit here. However it will not be called in all cases (for example, calling abort will not trigger the function you registered with atexit).
You can implement your own signal handler, then all the signals will pass there and you can do whatever for each of them.
You are looking for set_terminate().
http://www.cplusplus.com/reference/std/exception/set_terminate/
There are other similar function in the same header, that are usable for complementary scenarios.
int main()
{
try
{
// all your code here
}
catch(...)
{
// cleanup
}
return 0;
}
What environment you're working in? by abrupt do you mean Ctrl+C or kill -9 signal?
On unix/linux you can mask some signals and provide handlers, but as far as I am aware, you cannot mask all signal (9 is an example of a signal that can't be masked, and it'll kill your process abruptly)
Some even lower level overriding on OS operation could be available, but I'm not familiar with that.
I am not an expert and I just know some few things about C++, but I know you can create handles in Unix and C in order to detect a concrete signal and then, execute a function and later, terminate the program by "exit(n)" for example.
You can do it using signal or sigaction, but the problem is that you only can use this method for any signal except SIGKILL or SIGSTOP.
Are you familiar with signal handling? I would recommend that you study that first and then come back with questions regarding it. It looks like a couple people have already alluded to it, but here is a good resource to check:
http://www.gnu.org/s/hello/manual/libc/Signal-Handling.html
Writing your own signal handlers will allow you to determine what you want to do when a particular signal is caught. As stated, there are some that can't be overridden, and for good reason. You don't want to let someone override kill -9 simply because a program that's impossible to kill could be created. However, a straight kill signal or something such as ctrl-c, ctrl-d, etc, can be caught and handled in the way of your choosing.
There is no function that captures all scenarios and works on all platfroms. If you need something for Windows you will have to handle SEH(Structured exception handling) as well. You will have to define and set handlers for various scenarios(SEH, C++ Exceptions, SIGABRT, Terminate etc.) that execute common cleanup code and. Check zack's response here for handling SIGABRT signals.
For SEH you can add a SE converter to handle SE excpetions and convert them to C++ exceptions, look at _set_se_translator for more information about how to handle SEH exceptions.
You can refer to this documentation for set_terminate handler and this is a good reference for set_unexpected.
You will have to write your own handler that will be called for every scenario.
In the end I would reccomend using some existing libraries for this purpose, I like crashrprt.

DUMP in unhandled C++ exception

In MSVC, how can I make any unhandled C++ exception (std::runtime_error, for instance) crash my release-compiled program so that it generates a dump with the full stack from the exception throw location?
I have installed NTSD in the AeDebug registry and can generate good dumps for things like memory access violation, so the matter here comes down to crashing the program correctly, I suppose.
Thanks in advance.
I finally cracked it down.
Use the set_terminate() function to register a handler for every thread
In you main function(), make it impossible for external DLLs (event Windows') to successfully call SetUnhandledExceptionFilter(). A great article on how to do that here: http://www.debuginfo.com/articles/debugfilters.html#overwrite .
As for the handle itself, it is quite straightforward:
void Terminate()
{
OutputDebugStringA("Terminate\r\n");
RaiseException(0xE0000010, EXCEPTION_NONCONTINUABLE, 0, 0);
}
Calling RaiseException() like the above example is enough to make the process crash and produce my mush desired dump.
Just so you know, the problem I was having was:
The IPHelper Windows API loads dynamically another Windows DLL
This DLL uses Windows own version of the C runtime (MSVCRT instead of MSVCRT90)
The new C++ runtime calls SetUnhandledExceptionFilter() on startup to catch C++ exceptions. Since the latest filter for C++ exceptions is the one who gets to call the handle set by set_terminate(), my handle wasn't called.
SetUnhandledExceptionFilter and DebugBreak should probably do the job.
Edit: oops, rereading, you want to deal with uncaught C++ exceptions. That would be a bit trickier to do well -- by the time you (normally) get wind of a C++ exception, it's already unwound the stack back to the level of the handler. You don't have any real way to know an exception has been thrown until a catch is invoked, but by then the stack has been unwound.
Look into using the Windows Debugger.
Windbg – wraps KD and NTSD with a decent UI.
Also check out ADPlus that comes with the Windows Debugger.
Here is a good place to start learning how to use it.

Is it possible for a process to catch an unhandled exception of another process on windows?

Is it possible for a process to catch an unhandled exception of another process on the system?
If possible, under which circumstances is it possible? Is it for instance possible if the second process is not started by the first?
I am mainly looking for an answer regarding native c++.
Native (AKA standard) C++ has no real concept of multiple processes, and has no means of catching exceptions thrown across process boundaries. And no means of throwing exceptions across such boundaries, come to that.
Windows Exceptions: Structured Exception Handling (SEH) is per thread. Another thread in the process might be able to manipulate the stack of the target thread to insert its own handler, but that is going to be hard to get right (especially with the lack of consistent calling convention on x86). Another process could inject a dll & thread into a process to do this. This will be hard to get right, especially without close coupling to the details of the target process (what functions are called and how).
On second thoughts debuggers can do this, so the Win32 debugger APIs must have this capability. A process can debug other processes in the same session (with lower or equal integrity level), or if the user has the "debug process" privileged any process.
Yes. Matt Pietrek explains how. Scroll down to the "VectoredExceptionHandling is a clean, easily extensible way to see all exceptions" part. There's example code as well.