sashoalm sashoalm - 2 months ago 14
C++ Question

Qt Creator can't break on thrown exceptions (when using CDB as debugger)

I set Qt Creator to break when a C++ exception is thrown:

enter image description here

I then tested it with this code:

try {
throw std::runtime_error("error");
} catch (std::exception &e) {
qDebug("%s", e.what());
}


But it didn't break on
throw std::runtime_error("error");
. I'm using CDB, not GDB, because I'm using the MSVC Kit.

Edit: There is another question where CDB is working for the OP, even if slowly. So it should work in principle. My configuration is: Qt Creator 3.3.0, compiling with Qt4/MSVC 9.0 (x86), the debugger is CDB 6.2.9200.16384.

Edit 2: This is what I'm getting in the CDB log window (I made a diff between the CDB log with and without the breakpoint):

<bu100400 CxxThrowException
<!qtcreatorcdbext.breakpoints -t 1 -v
<!qtcreatorcdbext.pid -t 2
dATTEMPT SYNC
d*** Bp expression 'CxxThrowException' contains symbols not qualified with module name.
1 breakpoint(s) pending...
*** Unable to resolve unqualified symbol in Bp expression 'CxxThrowException' from module 'C:\Windows\WinSxS\x86_microsoft.windows.common-controls_6595b64144ccf1df_5.82.7601.18201_none_ec80f00e8593ece5\comctl32.dll'.


Full CDB log (in case needed): http://pastebin.com/jhNRy9bE

Edit 3: @HansPassant's explained why it fails in the comments:


Keep in mind that you are using a very old version of MSVC++, big changes at VS2012. The pastebin shows it being out of sync pretty badly, never getting to the DLL that contains __CxxThrowException@8 (MSVCR90D.dll) before the exception is thrown. It is simple with the sxe debugger command, automatic break when any exception is thrown.. Maybe you shouldn't be using QT's UI at all, it looks too gimpy. – Hans Passant 10 hours ago

Just look at the trace, the debugger shows what DLLs it is searching for "CxxThrowException". It never gets to msvcr90d.dll. And the exception is thrown while it is searching for the symbol after which it all ends. Completely out of sync. – Hans Passant 56 mins ago

Answer

I'll just write up why this is going wrong, a workaround is going to be difficult to find. The debugger trace in your pastebin tells the tale.

The basic issue is that the communication between the debugger and the QT front-end is rather poor. And in your case it gets out of sync, QT doesn't wait long enough for the debugger to complete the command. QT tries to set a breakpoint on the msvcr90d.dll!__CxxThrowException@8 function, the one that raises a C++ exception in the Microsoft CRT. This function can be present in more than one executable file if the program uses multiple CRTs. A pretty common mishap, caused by building with /MT. And sometimes intentional if you use a well-isolated DLL that you interface with by using COM.

This takes a while as you might imagine, the complaint in the linked question, the debugger has to plow through the symbol information for every DLL that's loaded. It will take especially long if the PDB for the DLL needs to be downloaded from the symbol server and doesn't otherwise get cached so it is available the next time you debug. Not your issue afaict, it does setup the cache location to C:\Users\sasho\AppData\Local\Temp\symbolcache. Go have a look-see to verify that you do see PDBs for the operating system DLLs there.

This operation is tricky, the debugger doesn't give a good signal that it is done searching the DLLs. What the QT should do is verify the debugger feedback against the list of DLLs it obtained. It does not do that, it issues the g command before the debugger is done searching. Could be a timeout that is too short but it actually looks like QT doesn't count on the debugger performing this command in the background. A convenience to a human, not exactly very helpful here :)

There ought to be a way to configure CDB to not perform this search in the background. This is well-hidden, I don't see anything in the debugger.chm help file but it probably wasn't updated in a while. Google doesn't help either. I'd recommend you ask a question about it. Most significantly perhaps is that you have a rather major mismatch in version numbers. The compiler you use is 2008 vintage, the debugger is quite new, SDK 8.0 version, I can't tell what QT version you use.

So a possible workaround is to intentionally use an older version of CDB, one that's more likely to have been tested with the QT front-end version you use. Download the corresponding SDK version, version 6.0 matches the VS2008 time frame. I think the "Debugging Tools for Windows" was still a separate download back then and not yet included in the SDK. Another workaround is to stop relying on the friendly QT front-end and learn to drive CDB from the command prompt. Moderately more useful is WINDBG, uses the same debugging engine but has a GUI interface. Just moderate, it is still mostly prompt driven. You do lose several days of your life learning the commands however. Getting the debugger to break when an exception is thrown is trivial, use the sxe command.