Hi I got couple of questions regarding debugging Qt Code using QtCreator.
I primarily use mac for Qt development but also use win32. So to debug Qt code do I need to install debug library on both?
Say in my application I'm deleting a pointer twice. In small application I can just scan the code and figure out (at least I'll give it a try). In a big application I know I'm deleting location 003E0000 twice. Then what is the best way of determining what is being deleted and who are trying to delete it (preferably using debugger that comes with QtCreator).
Yes you have to...
I suggest you to use valgrind memcheck. It works on linux and mac (I don't know whether it has windows version) and it replaces malloc and free methods and shows much information about memory handling.
valgrind --tool=memcheck
Related
I'm trying to understand c++, but i had some compatibility problems between valgrind and mac os mojave so i decide to use xcode.
First of all my doubt was, how can i detect
memory leaks(invoke a new without a delete) with a very simple command line program?
I have read that someone use Instruments panel so i start with it but in the call tree nothing appears.
I specify that my c++ program is too short but i think it isn't a problem.
Are there some flags to set up in build schemes?
is there another way to do that?
Does anyone use xcode? please help me!
my Qt (QML/C++) application crashes and I can not find the reason why. I tried to output a lot of information but some signal/slot connection probably causes a crash. I spent many hours trying to find the reason but I failed.
The only good point is that I can reproduce the crash whenever I want.
Unfortunately I don't know hot to use the included GDB debugger. This is the output I got:
How do I find from this what happened and where? I need to find at least the function, in which my application crashed.
Or what else could I try? Unfortunately I can not disable the signal/slot connections or the associated functions, because then I can not get to the point, where it crashes.
Qt has detailed documentation on how to install a debugger found here: QtCreator Debugger
MingW does have a GDB that can be used to debug the application better. You can also use CDB to debug, just depends on your preference.
Once that is installed, you'll be able to set breakpoints and check variable information to see where your program is crashing using the Debugger view in QtCreator.
Tools->Options->Build & Run
If you have Qt version kit like this you need to check debuggers.
https://i.stack.imgur.com/LaY1p.png
https://i.stack.imgur.com/8kTG6.png
You need to install MinGW and after install you will be have debugger. After install press F5 to start debuging.
I set Qt Creator to break when a C++ exception is thrown:
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
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.
I am using Qt Creator to develop a C++ application, and I am trying to track down some memory leaks.
I have followed these instructions*, and have Valgrind isntalled on my Ubuntu machine.
I begin my application from the Analyze Memory section, and after clicking the stop button I am told that Tool Analyze Memory finished, 928 issues found. However, no results appear in the analysis section, which remains blank.
Am I doing something wrong? Where can I find the results of the memory analysis?
*Confusingly, the instructions ask for a 'release' build configuration, but selecting this causes a warning in Qt saying that the memory analysis tool requires a debug configuration. I have tried both ways round.
I've checked it in my Qt Creator and it works. I'm using version 2.3.1 with Qt 4.7.4 and Valgrind 3.7.0. I've tested it on dropsite example with Debug build configuration. I've also tested it with my CMake based project (compiled with -g and -O0 flags) and it also worked. Make sure, that next to start button Valgrind Memory Analyzer is chosen. On the same bar you'll find arrow icons. Next to them is icon of funnel (I think). Click it and make sure that types of errors you're interested in are checked.
If it won't work, and you don't want to use Valgrind tools from console, there are other Valgrind GUIs like Valkyrie.
EDIT: I've updated Qt Creator to version 2.4.0rc1 and checked it with my CMake project. It also showed no results, while reporting few hundred issues. Solution to this was checking External errors after clicking funnel icon.
EDIT2: In final 2.4.0 issue was fixed.
You did nothing wrong. In the Valgrind menu, press the Filter icon and select External Errors. You will see all issues appearing. Those are most likely not from your application, but Qt- or else-related, so you can often ignore them.
X issues found refers to the total number of issues; when External Errors is unselected, only those related to your app show up. If none appears, Valgrind has not detected any memory leak in your app.
I'm trying to generate a release build for a C++ application that I've written. The application runs fine (debug & release) when you run it from within VS2008; but when you run the executable it crashes nearly every single time.
Now, is there a hack so I can run this application as a standalone application without having to run through all of the code and finding the bug that is causing it?
Thanks in advance.
In short, no.
you will have to find the bug, if it works within VS, then I'd hazard a guess that it is a timing issue, possibly you're overwriting shared thread data, this would be less likely (though still possible to see) inside VS as its being run in a debug environment which slows it down a bit.
If you want help finding your bug, then tell us more. Otherwise, build your release with debug symbols (pdbs), install DrWatson as the system debugger and run it standalone. When it crashes DrWatson will create a minidump file, load this into WinDbg (my favourite) and you'll be able to see exactly where your bug is (it'll even tell you that the dump contains an exception and show you it by default. You need to add your source code path and path to your symbols in WinDbg to get it to do this correctly).
Then you will also know how to diagnose crashes when the app is run on-site too.
Are you loading external resources? If you are check that your relative paths are correct in the C++ program.
One possibility is that your program uses uninitialized heap data. Launching a program from the debugger enables the NT debug heap, which causes the heap allocator to fill new memory blocks with a fill pattern, and also enables some heap checking. Launching the same program from outside the debugger leaves the NT debug heap disabled, but if the program was linked against the debug version of the C runtime, then the CRT debug heap will still be enabled.
A much less likely possibility is that your program requires SeDebugPrivilege to be set in its process token. The debugger enables this privilege in its process token, which has the side effect that all programs launched from the debugger inherit this privilege. If your program tries to use OpenProcess()/ReadProcessMemory()/WriteProcessMemory() and doesn't handle errors correctly, it's conceivable that it could crash.
There are a few possibilities. Besides what has already been mentioned, running an app from Visual Studio will execute in the same security context as the Visual Studio instance. So if, for instance, you are working on Vista, you might be hitting an unhandled security violation if you're trying to access protected files, or the registry.
What if you build a debug version and run it standalone? Does it crash? If so, you can usually break into the debugger from there and get a call stack to see what the malfunction is.
From the details you've given, it sounds like there may be a library issue. Are you running the program on the same computer? If not then you'll also have to deploy the appropriate libraries for your application. If you are running on the same computer but outside of the dev environment, ensure that your application can see the appropriate libraries.
Best way i have found to debug in release is to create a crash dump when an crash happens and the dump then allows me to load debug symbols on my dev computer and find out whats going on. More info here: http://www.debuginfo.com/articles/effminidumps.html
You can also go to file => open in Visual Studio and open the .exe, so you are not starting it under the debugger per se. Not sure if it will help.
http://blogs.msdn.com/saraford/archive/2008/08/21/did-you-know-you-can-debug-an-executable-that-isn-t-a-part-of-a-visual-studio-project-without-using-tools-attach-to-process-296.aspx