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How to catch a panic

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Created: tote_b5 (26 Mar 2007)
Last edited: hamishwillee (12 Jan 2012)

Yes, a panic. Although panics, by their nature, are uncatchable there is a way to prevent them from severely influencing your program.

First off, why would you do it? What is the use-case? Typically a panic draws attention to a programmatic error that should have already been corrected in the program. It always terminates the program execution and it's on purpose: programs panic from an unrecoverable stage. So why would we want to catch a panic if we were able to do so?

Well, there is at least one special use-case: in a test framework used for automated testing. In a typical unit testing framework, for example, where test suites containing test cases are being performed it's always a nice feature if the execution of tests doesn't get interrupted even in case of a fatal error. If the severe programming error doesn't harm other test cases, then it's ideal if the termination of a test case doesn't influence the life-time of other test cases.

Can panics be caught? At first sight, it may seem that User::SetExceptionHandler() would do. But it doesn't: exceptions are not the same as panics. So installing a new exception handler will not enable us to catch panics, even though it makes it possible to catch a small portion of ALL panics (e.g. an access violation aka KERN-EXE 3).

The ultimate solution is to let your test case run in a new thread. A new thread that you have control over: you create, destroy and most importantly monitor it. You can request notification when this thread dies normally or otherwise via RThread::Logon(). Combined this with RThread::ExitReason(), you can get a clue when and how a thread has exited. Even in case of a panic your main thread will not be affected at all so your - test - framework can continue to run.

The following example shows a console based exe which monitors other thread's death, displaying type and reason:

#ifndef THREADNOTIFIER_H
#define THREADNOTIFIER_H
 
#include <e32base.h>
 
class CThreadNotifier : public CActive
{
public:
CThreadNotifier();
~CThreadNotifier();
void ConstructL();
 
void IssueRequest();
 
protected:
void RunL();
void DoCancel();
TInt RunError(TInt aError);
 
RUndertaker iUndertaker;
TInt iThreadHandle;
};
 
#endif
#include "ThreadNotifier.h"
 
#include <e32cons.h>
 
_LIT(KPanicMsg, "THREAD-NOTIFIER");
 
LOCAL_D CConsoleBase* console;
 
CThreadNotifier::CThreadNotifier()
: CActive(CActive::EPriorityStandard)
{
CActiveScheduler::Add(this);
}
 
void CThreadNotifier::ConstructL()
{
User::LeaveIfError(iUndertaker.Create());
}
 
CThreadNotifier::~CThreadNotifier()
{
Cancel();
iUndertaker.Close();
}
 
void CThreadNotifier::IssueRequest()
{
__ASSERT_ALWAYS(!IsActive(), User::Panic(KPanicMsg, 0));
 
iUndertaker.Logon(iStatus, iThreadHandle);
SetActive();
}
 
void CThreadNotifier::RunL()
{
if (iStatus == KErrDied)
{
RThread thread;
thread.SetHandle(iThreadHandle);
console->Printf(_L("Thread %S (%d) died (Type: %d, reason %d)\n"),
&thread.Name(), (int)thread.Id(), thread.ExitType(), thread.ExitReason());
thread.Close();
}
 
IssueRequest();
}
 
void CThreadNotifier::DoCancel()
{
iUndertaker.LogonCancel();
}
 
TInt CThreadNotifier::RunError(TInt /*aError*/)
{
return KErrNone;
}
 
 
LOCAL_C void callExampleL()
{
console = Console::NewL(_L("Thread Notifier"), TSize(KDefaultConsWidth,
KDefaultConsHeight));
CleanupStack::PushL(console);
 
CActiveScheduler* scheduler = new(ELeave) CActiveScheduler;
CleanupStack::PushL(scheduler);
CActiveScheduler::Install(scheduler);
 
CThreadNotifier* notifier = new(ELeave) CThreadNotifier;
CleanupStack::PushL(notifier);
notifier->ConstructL();
notifier->IssueRequest();
 
CActiveScheduler::Start();
 
CleanupStack::PopAndDestroy(3, console); // console, scheduler, notifier
}
 
GLDEF_C TInt E32Main()
{
__UHEAP_MARK;
CTrapCleanup* cleanup = CTrapCleanup::New();
TRAPD(error, callExampleL());
__ASSERT_ALWAYS(!error, User::Panic(KPanicMsg, error));
delete cleanup;
__UHEAP_MARKEND;
return 0;
}

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This page was last modified on 12 January 2012, at 06:02.
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