Someone in a different question suggested using catch(...) to capture all otherwise unhandled - unexpected/unforseen exceptions by surrounding the whole main() with the try{}catch(...){} block.
It sounds like an interesting idea that could save a lot of time debugging the program and leave at least a hint of what happened.
The essence of the question is what information can be recovered that way (other than whatever debug globals I leave behind), and how to recover it (how to access and recognize whatever catch was called with)
Also, what caveats are connected with it. In particular:
will it play nice with threads that sprout later?
will it not break handling segfaults (captured elsewhere as signal)
will it not affect other try...catch blocks inevitably nested inside, that are there to handle expected exceptions?
Yes it is a good idea.
If you let an exception escape main it is implementation defined weather the stack is unwound before the application is shut down. So in my opinion it is essential you catch all exceptions in main.
The question then becomes what to do with them.
Some OS (See MS and SE) provide some extra debugging facilities so it is useful to just re-throw the exception after you catch it (because the stack has been unwound now anyway).
int main()
{
try
{
/// All real code
}
// I see little point in catching other exceptions at this point
// (apart from better logging maybe). If the exception could have been caught
// and fixed you should have done it before here.
catch(std::exception const& e)
{
// Log e.what() Slightly better error message than ...
throw;
}
catch(...) // Catch all exceptions. Force the stack to unwind correctly.
{
// You may want to log something it seems polite.
throw; // Re-throw the exception so OS gives you a debug opportunity.
}
}
will it play nice with threads that sprout later?
It should have no affect on threads. Usually you have to manually join any child threads to make sure that they have exited. The exact details of what happens to child threads when main exits is not well defined (so read your documentation) but usually all child threads will die instantly (a nasty and horrible death that does not involve unwinding their stacks).
If you are talking about exceptions in child threads. Again this is not well defined (so read your documentation) but if a thread exits via an exception (ie the function used to start the thread exits because of an exception and not a return) then this usually causes the application to terminate (same affect as above). So it is always best to stop ALL exceptions from exiting a thread.
will it not break handling segfaults (captured elsewhere as signal)
Signals are not affected by the exception handling mechanism.
But because signal handlers may place an odd structure on the stack (for their own return handling back to normal code) it is not a good idea to throw an exception from within a signal handler as this may cause unexpected results (and is definitely not portable).
will it not affect other try...catch blocks inevitably nested inside, that are there to handle expected exceptions?
Should have no effect on other handlers.
As far as I remember, catch(...) on Win32 catches also SEH exceptions, and you do not want to do that. If you get a SEH exception it's because something very scary happened (mainly access violations), so you can't trust your environment anymore. Almost everything you could do may fail with another SEH exception, so it's not even worth trying. Moreover, some SEH exceptions are intended to be caught by the system; more on this here.
So, my advice is to use a base exception class (e.g. std::exception) for all your exceptions, and catch just that type in the "catchall"; your code cannot be prepared to deal with other kind of exceptions, since they are unknown by definition.
A global try catch block is useful for production systems, in order to avoid displaying a nasty message to the user. During development I believe that are best avoided.
Regarding your questions:
I believe that a global catch block won't catch exceptions in another thread. Each thread has its own stack space.
I am not sure about this.
Nested try...catch blocks aren't affected and will execute as usual. An exception propagates up the stack, until it finds a try block.
You could try a solution I use if you're making a .net application. That captures all unhandled exceptions. I generally only enable the code (with #ifndef DEBUG) for production code when I'm not using the debugger.
It's worth pointing out as kgiannakakis mentions that you can't capture exceptions in other threads, but you can use the same try-catch scheme in those threads and post the exceptions back to the main thread where you can re-throw them to get a full stack track of what went wrong.
and how to recover it (how to access
and recognize whatever catch was
called with)
If you mean how to recover the type of exception that was thrown, you can chain catch blocks for specific types (proceeding from more specific to more general) before falling back to catch (...):
try {
...
} catch (const SomeCustomException& e) {
...
} catch (const std::bad_alloc& e) {
...
} catch (const std::runtime_error& e) {
// Show some diagnosic for generic runtime errors...
} catch (const std::exception& e) {
// Show some diagnosic for any other unhandled std::exceptions...
} catch (...) {
// Fallback for unknown errors.
// Possibly rethrow or omit this if you think the OS can do something with it.
}
Note that if you find yourself doing this in multiple places and want to consolidate code (maybe multiple main functions for separate programs), you can write a function:
void MyExceptionHandler() {
try {
throw; // Rethrow the last exception.
} catch (const SomeCustomException& e) {
...
}
...
}
int main(int argc, char** argv) {
try {
...
} catch (...) {
MyExceptionHandler();
}
}
A catch-all will not be terribly useful since there is no type/object information that you can query. However, if you can make sure all exceptions raised by your application are derived from a single base object, you can use a catch block for the base exception. But then that wouldn't be a catch-all.
Related
In the application I am writing, I make usage of exceptions for most of my error handling. I've not defined my own exception classes just yet, I merely did the following:
namespace Mage {
typedef std::exception Exception;
}
This way I won't have to change all of my code when I define my own type later which should use the same interface.
That said, any exception crashes my application. Taking the above definition in mind, why would this crash?
void Mage::Root::initialize(Mage::String& p_log) {
// initialize GLFW and GLEW.
if (!glfwInit()) {
throw new Mage::Exception("failed to initialize OpenGL");
return;
} else m_GLFWInitialized = true;
Whether I remove or keep 'new', it would still crash. Am I missing something? I've looked up tutorials but those don't get me any wiser.
I also catch the error right here:
try {
MAGE_ROOT.initialize(Mage::String("Mage.log"));
} catch (Mage::Exception& e) {
std::cerr << e.what() << std::endl;
}
The crash I'm getting is:
Debug Error!
Program: ...sual Studio 2010\Project\Mage3D\Binaries\Debug\Test.exe
R6010
- abort() has been called
(Press Retry to debug application)
The problem is that you are not catching your exception.
I wasn't aware you -have- to catch the exception (from the comments)
Yes, you have to. If you do not catch a thrown exception, std::terminate() will be called. This is the intended behavior: exceptions exist to prevent programmers from forgetting about error handling.
This said, I suggest:
throwing by value;
catching by reference
For instance:
void foo()
{
// ...
throw std::logic_error("Error!");
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^
// Throw by value (std::logic_error derives from std::exception)
// ...
}
void bar()
{
try
{
// ...
foo();
// ...
}
catch (std::exception& e)
^^^^^^^^^^^^^^^
// Catch by reference
{
std::cout << e.what(); // For instance...
}
}
UPDATE:
With regards to the piece of code you posted, you are throwing a pointer and catching by reference. The handler won't match. And since there is no other matching handler, std::terminate() will be called.
Instead, you should throw your exception by value:
throw Mage::Exception("failed to initialize OpenGL");
And if the code you posted is indeed the one you are using, you will see that control is transferred to your handler.
Based on the error message, you're using Visual Studio (2010) for your project. Unless you wrap your throw in a try/catch block, it will "sail through the roof" and be "handled" by the C++ runtime, which means calling abort(). You probably want something like this higher in the call stack:
try
{
SomeFunctionThatUltimatelyThrows();
}
catch(Exception & e)
{
// .. handle error - log, resume, exit, whatever
}
Note also Scott Meyers advice to always catch exceptions by reference. The "exception": If you're using MFC CExceptions though, you want to catch by pointer and call the Delete method for self-destructing heap-based exceptions.
Based on your edit, you may have a mismatch between throwing "by pointer" and catching "by reference". If you've resolved that and are still not getting your catch block to execute, you could try debugging the abort() call by using the CRT SetAbortHandler to install your own abort function. This could simply chain into the existing one, but would give an opportunity to set a breakpoint and examine the call stack to see what is going wrong.
C++ try-catch-throw logic for dummies. Note that this does NOT cover RAII / stack-based allocation / destruction.
When you throw an exception, an exception is said to be "propagating". It propagates up the call stack until it finds the first handler that can handle it (so it's caught) or until it's reached the root of your call stack.
If it is caught, execution continues from the point the exception is caught. The exception is destructed at the end of the catch block.
If it finds the root, it calls std::unhandled_exception, which usually calls std::terminate, which usually calls abort(). In short, everything's dropped ASAP.
If you throw an exception while an exception is currently propagating, you would have two propagating at a time. Java and C# have cutesy ways of dealing with this, but this should never happen in the first place - there's no exception handler that is logically going to handle combinations of exceptions. Do not throw exceptions while one is currently propagating. This rule isn't too hard to hold even if you don't use std::uncaught_exception() which you shouldn't.
While unwinding the stack / propagating the exception all objects found on the stack are destructed. These destructors should never throw an exception - after all, when destroying an object "fails", what else are you going to do after the destructor to fix it?
Always throw by value, catch by reference. If you throw & catch by pointer, you will most likely leak something, which is impossible by reference. If you catch by value, you will slice off your derived exception types. Catch by reference.
At the root of your software, include a catch-all - catch(...). This doesn't allow you to find out what exactly you've caught, but at least you can crash safely. Do this also when the called code may throw "something" that you don't know.
I read code in a large project, that includs a lot of code like:
try
{
}
catch(...)
{
}
Literally, in the parenthesis after "catch", there is "..." in it. Not something like "exception e".
This makes me a little worry.
Is this practice good or safe?
thanks.
No, this is a terrible practice.
If you catch(...), you have no idea what you've caught. It catches all C++ exceptions (and on some platforms with some settings, it catches other exceptions too, like structured exceptions in Visual C++).
If you have no idea what exception was thrown, you have no idea what the state of the system is: how do you know whether it is safe for the program to continue running?
The only two ways that it is absolutely safe to exit a catch(...) block is to terminate the program or to rethrow the exception (using throw;). The latter is occasionally useful if you need to perform some cleanup when an exception is thrown but can't rely on a destructor.
catch(...) catches all exceptions.
Normally, you would not want to do this. You have no idea what it is you just caught, and if you leave the catch block you have just silently ignored some kind of error. This could lead to very bad things happening later on. Since you have no idea what error just happened, you have no way to recover from it, and thus the only reasonable thing to do is to either allow the exception to continue (re-throw it) or abort the program's execution (call abort() or exit()).
However, if you have some cleanup you need to perform, it may be reasonable to catch all exceptions, perform the cleanup, then re-throw the exception:
try {
// ...
} catch (...) {
abortTransaction();
throw;
}
That said, it's usually better to use so-called RAII classes to automate this cleanup:
DBTransaction txn = db.StartTransaction();
// do stuff that may throw; if it throws, txn will be destroyed,
// and its destructor can abort the transaction
// As such, an explicit try { } catch(...) { } isn't needed
It catches all exceptions. See this code:
try {
throw CSomeOtherException();
}
catch(...) { // Handle all exceptions
// Respond (perhaps only partially) to exception
throw; // Pass exception to some other handler
}
From MSDN
You do not need to declare this parameter; in many cases it may be
sufficient to notify the handler that a particular type of exception
has occurred. However, if you do not declare an exception object in
the exception-declaration, you will not have access to that object in
the catch handler clause.
A throw-expression with no operand re-throws the exception currently
being handled. Such an expression should appear only in a catch
handler or in a function called from within a catch handler. The
re-thrown exception object is the original exception object (not a
copy). For example:
Hope this helps!
This practice is safe only if you have proper handlers written for exceptions. Avoiding it is better in my opinion.
I'm writing a C++ application. I realized that one of my worker threads may terminate unexpectedly. The (VS 2005) debug log says:
The thread 'Win32 Thread' (0x5d98) has
exited with code -858993460
(0xcccccccc).
I surrounded all the worker thread code with a try/catch block. So, if the reason was an exception, I would catch it. But I can't:
try{
...
Connection* conn = connectionPool->getConnection(); // unexpected exit occurs here
...
} catch(exception& e) {
...
}
I have ten threads running concurrently, and only one of them gets crashed after some time, while the others continue running (and getting new [OCCI] connections).
Is there an exception type that is not caught by "exception"? Or what do I not know about threads/exceptions?
Thanks.
Not all errors raise C++ exceptions that can be caught with the C++ try...catch mechanism. For example, division by zero will not raise a C++ exception, but will cause undefined behaviour which may well make your application exit.
However, your code may be throwing things that are not derived from std::exception , so you may want to rewrite:
try{
Connection* conn = connectionPool->getConnection(); // unexpected exit occurs here
}
catch(exception& e) {
// handle things derived from std::exception
}
catch ( ... ) {
// handle things that are not so derived
}
which will deal with things like throw "eeek!";
Also, but nothing to do with the problem, you should normally catch exceptins via a const reference.
Is there an exception type that is not caught by "exception"?
Yes, SEH exceptions. To catch them you need to either use __try/__except MSVC extension (see Structured Exception Handling), or write a global SEH/VEH handler (see SetUnhandledExceptionFilter and AddVectoredExceptionHandler).
The easiest way to find the problem would be to run your application under a debugger and enable breaking on Win32 Exceptions. Whenever a Win32 Exception is encountered, the application would break into the debugger and you can find out whats going wrong.
If you are not debugging and want to catch a win32 structured exception, you have to use _set_se_translator api to set a translator function. The registered function will be called whenever there is a Win32 exception and you get a chance to convert it to a C++ exception of your choice.
catch(exception& e) catches C++ exceptions derived from std::exception, and nothing else.
It doesn't catch C++ exceptions that are not derived from that class (If I do throw 42, it won't be caught for example), and it doesn't catch exceptions or errors at the system level.
Windows uses Structured Exception Handling (SEH) to signal errors, and those are not caught with a plain C++ catch statement. This might include errors like division by zero, as well as access violations or pretty much anything else that might go wrong at the OS or hardware level.
The docs have a nice explanation of how to catch SEH exceptions.
I found an important clue. When you close a handle with CloseHandle function, the thread exits with code 0xCCCCCCCC. With the help of this clue, I realized that in a very rare situation, I close my thread's handle even though the thread's working. Why does it exit exactly while getting connection? That also has an explanation, but it's related to the structure of the code, which may be difficult to explain here.
Thank you all, who brainstormed with me on the exceptions issue :$.
Each time I have seen the catch all statement:
try
{
// some code
}
catch (...)
{
}
it has always been an abuse.
The arguments against using cache all clauses are obvious. It will catch anything including OS generated exceptions such as access violations.
Since the exception handler can't know what it's dealing with, in most cases the exceptions will manifest as obscure log messages or some incoherent message box.
So catch(...) seems inherently evil.
But it is still implemented in C++ and other languages (Java, C#) implements similar mechanisms. So is there some cases when its usage is justified?
(1) It's not true that the statement will catch OS exceptions. Your use of the term "Access Violation" betrays a Windows background; it was true for older MSVC++ versions.
(2) Regardsless, the catch-all behavior is useful for threads with specific purposes. Catching the failure allows the thread to report it failed. Without it, the other parts of the program need to deal with the possibility of a thread just disappearing. It also allows you to log which thread failed, and the arguments used to start the thread.
The case where it's justified in general is when you log the exception (or do something similar) or do some cleanup, and then immediately rethrow.
In C++ in particular, logging in a catch(...) block is pretty pointless since you don't have any way to obtain the exception, and cleanup is pointless because you should be using RAII for that. Using it in destructors seems to be about the only legitimate case.
the arguments against using cache all clauses are obvious , it will catch anything including OS generated exceptions such as access violation. since the exception handler can't know what its dealing with, in most cases the exceptions will manifest as obscure log message or some incoherent message box.
And if those same exceptions aren't caught you get... an incoherent message box.
catch(...) lets me at least present my own message box (and invoke custom logging, save a crash dump, etc.).
I think there are also reasonable uses of catch(...) in destructors. Destructors can't throw--well, I mean, they can throw, but if a destructor throws during stack unwinding due to an in-progress exception the program terminates, so they should not ever allow exceptions to escape. It is in general better to allow the first exception to continue to be unwound than to terminate the program.
Another situation is in a worker thread that can run arbitrary functions; generally you don't want an unceremonious crash if the task throws an exception. A catch(...) in the worker thread provides the opportunity for semi-orderly clean-up and shutdown.
In addition to what other posters have already said, I'd like to mention one nice point from the C++ Standard:
If no matching handler is found in a
program, the function std::terminate()
is called; whether or not the stack is
unwound before this call to
std::terminate() is
implementation-deļ¬ned.
(15.3/9)
This means that main() and every thread function must be wrapped in a catch-all handler; otherwise, one can't even be sure that destructors for automatic objects will be called if an uncaught exception is thrown.
try {...} catch (...) is needed around body of callback function which is called from code
that doesn't understand C++ exceptions (usually C library).
Otherwise, if some C++ library you use throws an exception that doesn't derive from
std::exception, it will probably cause calling code to crash or corrupt its internal state.
Instead you should catch this exception and either finish program immediately or
return some error code (meaning "we are doomed and I don't know why", but it's still better
then letting C++ exception to pass through).
Around thread procedure. Mostly because of the same reason as 1.
And because otherwise thread failure would pass unnoticed.
catch(...) has been useful for me in two circumstances, both of which are unjustified (I can't even remember the second)
The first is my overall application safety. While throwing exceptions that don't derive from std::exception is a No-No, I have one just in case in my main() function:
int execute(void); // real program lies here
int main(void)
{
try
{
return execute();
}
catch(const std::exception& e)
{
// or similar
std::cerr << "Unhandled exception: " << e.what() << std::endl;
return EXIT_FAILURE;
}
catch(...)
{
std::cerr << "Unknown exception!" << std::endl;
return EXIT_FAILURE;
}
}
Now, it's only there "just in case", and it's not really justified. There should be no reason to ever enter that catch clause, as that would mean somebody has done a Bad Thing. Observe how useless the statement really is; "Something bad happened, no clue what!" It's only a step above just crashing in the first place.
The second use might be in destructors or some other function that needs to do manual management before letting the exception propagate. That's not really a justification either, as things should clean themselves up safely with RAII. But I may have used it once or twice for some reason I can't recall, and I can't see a reason to ever do so again.
catch (...) allows you to write code in which you can legitimately claim a guarantee that your code will not crash even when you are not in long term complete control of the submodules your code depends on. Your claim is tantamount to claiming that this semantic cannot be used except as a means of abuse. Maybe so, but military specifications may differ from you on this issue.
catch(...) is necessary in the absence of the finally clause as found in other languages:
try {
...
} catch(...) {
cleanup...
throw;
}
The alternative - making stack objects to 'own' everything - is often much more code and less readable and maintainable. The platform API is often C, and does not come with it conveniently bundled.
It is also useful around plugin code that you do not control or simply do not trust from a stability perspective. It won't stop them crashing, but it might keep things a little saner.
Finally, there are times when you really do not care about the outcome of something.
I found some code in a project which looks like that :
int main(int argc, char *argv[])
{
// some stuff
try {
theApp.Run();
} catch (std::exception& exc) {
cerr << exc.what() << std::endl;
exit(EXIT_FAILURE);
}
return (EXIT_SUCCESS);
}
I don't understand why the exceptions are being catched. If they weren't, the application would simply exit and the exception would be printed.
Do you see any good reason to catch exceptions here ?
EDIT : I agree that it is good to print the exception error. However, wouldn't it be better to rethrow the exception ? I have the feeling that we are swallowing it here...
If an exception is uncaught, then the standard does not define whether the stack is unwound. So on some platforms destructors will be called, and on others the program will terminate immediately. Catching at the top level ensures that destructors are always called.
So, if you aren't running under the debugger, it's probably wise to catch everything: (...) as well as std::exception. Then your application code can clean up with RAII even on a fatal exception. In many such cases you don't actually need to clean up, since the OS will do it for you. But for instance you might prefer to disconnect cleanly from remote services where possible, and there might be resources external to the process, such as named pipes/mutexes, that you'd prefer to destroy rather than leaking.
Rethrowing the exception in main seems to me of limited use, since you've already lost the context in which it was originally thrown. I suppose that trapping an uncaught exception in the debugger is noisier than just logging the fault to std::cerr, so rethrowing would be the smart move if there's a chance of missing the logging.
If you want the debugger to trap unexpected conditions in debug mode, which in release mode throw an exception that eventually results in an exit, then there are other ways to do that than leaving the exception uncaught so that the debugger sees it. For example, you could use assert macros. Of course, that doesn't help with unexpected and unpredictable conditions, like hardware exceptions if you're using SEH on .NET.
Why do you say that the exception would be printed? This is not the typical behavior of the C++ runtime. At best, you can expect that its type gets printed.
In addition, this program leaves a "failure" status, whereas an exception might cause a termination-through-abort status (i.e. with a signal indicated in the exit code).
Try-catch in the main function hides the exception from debugger. I would say, it isn't good.
On the other hand, customers are not expected to have debuggers, so catching exceptions is nice. So it is good.
Personally, I catch all exceptions in main function, when making a release build, and I don't do that, when building a debug configuration.
A simple example of a situation where the stack did not unwind:
Why destructor is not called on exception?
A list of situations where exceptions may cause the application to terminate rather than unwind the stack.
Why destructor is not called on exception?
If an exception is not caught at any level and would escape main() then the implementation is allowed to call terminate() rather that unwinding the stack (yes this caught me by surprise as well).
As a result I always catch all exceptions in main().
int main()
{
try
{
}
catch(std::exception const& e)
{ /* LOG */
// optimally rethrow
}
catch(...) // Catch anything else.
{ /* LOG */
// optimally rethrow
}
}
To help with catching problems during debugging. Derive your exceptions from std::exception and then stick the break point in the constructor for std::exception.
This is a global catch block. It is common for displaying a nice and user understood message ('Internal error') instead of a cryptic exception print-out. This may be not evident from the specific code block, but it is in general a good idea.
Have a look at the C++ bible i.e. Stroustrup, he has an example which is also repeated in Applied C++ programming. The reasoning is:
int main(void)
{
try
{
// your code
}
catch ( /* YourPossibleExceptions i.e. barfs you expect may occur */ )
{
}
catch ( ... ) // unexpected errors, so you can exit gracefully
{
}
}
According the the Windows spec, the main isn't allowed to throw.
(practically it results into the message that asks if you want to send the bug-report to Microsoft)