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We are using Google Test as our C++ unit testing framework. But I ran into a painful situation and don't know how to deal with.
Basically, when there is an uncaught exception in the code, I got the following error message printed in the console and get a FAILED. Obviously, the exception is captures by google test. However, I have no information at all where is the exception was throw.
unknown file: error: SEH exception with code 0xc000005 thrown in the test body.
What I can do is debug and step through the code and I will eventually figure out where the problem is. But this is not very efficient as the project is big.
I want the debugger to stop at the line of uncaught exception and give me a nice call stack. Is there any settings in google test that I don't know of? Any other work around or suggestions will be very much appreciated.
Edit: I am looking for something like the following under Windows
Finally according to the answers, I found this settings for visual studio and everything works as the way I want now :)
At work the approach I use is to run only the failing testcase using gdb like so:
gdb /path/to/test
catch throw
r --gtest_filter='Test.Testcase' --gmock_verbose=info
bt
With visual studio, I suspect you should be able to start your binary with arguments as above, and set a breakpoint to any throw, then take a look at the backtrace.
An SEH Exception is NOT a C++ exception.
It is a windows exception that is throw outside of the standard C++ framework for exception handing (there is a different syntax for catching them).
The best way to find the location is to run this inside DevStudio. Its been a while but I am sure DevStudio has an option to break when SEH exception is thrown. Just turn this on and your debugger will stop at the throw point and allow you to debug.
See: https://msdn.microsoft.com/en-us/library/d14azbfh.aspx
As noted by #MatthiasVegh you should pass the name of the test as well so you don't have to run through all the tests.
That is not possible since C++ doesn't keep stack trace in the exception object and even if gtest had some smart catching mechanism it would have no means to know where the exception came from. The best you can do is to store some information in the exception yourself and check it in the test case.
This C++ program is a CGI script, I have no desire to deal with exceptions. I'd rather get a marginal performance boost and let the OS (Linux) handle cleanup after the process dies.
I am using the Standard C++ Library, and want any function to die like in Perl: Whenever it throws an exception. Without unwinding, or running any further code in my process.
How does -fno-exceptions work? If I have no catch at all in my code, and basically pretend like exceptions do no exist. but I do use std:: c++ library which can throw()?
Option #1: Simply never catch exceptions.
Exceptions don't have much overhead when they're not thrown or caught; if you're throwing and not prepared to catch, well, you're doing to die anyway, so the performance impact at that point is trivial. Note also that stack unwinding will not be performed if an exception is not handled; the program will simply terminate without performing stack unwinding.
It's important to note that, in G++, exceptions have almost no overhead when not actually thrown. G++ generates extra information sufficient to trace back the execution of the program through the stack, and some extra code to invoke destructors, etc - however none of this extra code or data is ever used until an exception is actually thrown. So you should not see a performance difference between code with exceptions enabled but not used and code with exceptions disabled (through whatever mechanism).
Option #2: Pass -fno-exceptions.
This flag instructs G++ to do two things:
All exception handling in STL libraries are removed; throws are replaced with abort() calls
Stack unwind data and code is removed. This saves some code space, and may make register allocation marginally easier for the compiler (but I doubt it'll have much performance impact). Notably, however, if an exception is thrown, and the library tries to unwind through -fno-exceptions code, it will abort at that point, as there is no unwind data.
This will, effectively, turn all exceptions into abort()s, as you would like. Note, however, that you will not be allowed to throw - any actual throws or catchs in your code will result in a compile-time error.
Option #3: (Nonportable and not recommended!) Hook __cxa_allocate_exception.
C++ exceptions are implemented using (among others) the __cxa_allocate_exception and __cxa_throw internal library functions. You can implement a LD_PRELOAD library that hooks these functions to abort():
void __cxa_allocate_exception() { abort(); }
void __cxa_throw() { abort(); }
WARNING: This is a horrible hack. It should work on x86 and x86-64, but I strongly recommend against this. Notably, it won't actually improve performance or save code space, as -fno-exceptions might. However, it will allow the throw syntax, while turning throws into abort()s.
-fno-exceptions turns all standard library throw's into a call to std::abort(). That handles the part you can't modify directly, the rest is to not use them at all in your code.
Of course, I really doubt your justification in doing this. You only "lose" performance when you actually throw, and you're throwing out a significant and helpful bit of the language.
In case anyone stumbles upon this question, I'd like to correct what #GManNickG and (https://stackoverflow.com/a/7249460/157344) and #bdonlan (https://stackoverflow.com/a/7249442/157344) said in their answers. Unfortunately the part about "-fno-exception" removing all exception handling code and turning all throws into aborts is wrong. Well - partially wrong. This is true when you compile the library in question (libstdc++v3) with this flag, but not true if you use this library (as an .a or .so or .dll or whatever) in your own code compiled with this flag. In the latter case the exception handling code in YOUR code is forbidden, but all the calls to exception handling inside the library remain (because the library was compiled WITHOUT this flag, with exceptions enabled), so if you use new then your executable WILL have exception handling code - the only difference is that you cannot anything about these exceptions with a catch() (which is forbidden in your code), so all throws effectively end up as abort(), but only because no one catches them.
Quote:
This C++ program is a CGI script, I have no desire to deal with exceptions.
Then don't. Simple. The exception will get to the top of the stack very quickly.
But I would urge you to do so. To do so means that you are thinking of the things that can go wrong.
Just don't catch them anywhere in your code. In that case, a termination handler will be called and your program will "crash".
Today, in my C++ multi-platform code, I have a try-catch around every function. In every catch block I add the current function's name to the exception and throw it again, so that in the upmost catch block (where I finally print the exception's details) I have the complete call stack, which helps me to trace the exception's cause.
Is it a good practice, or are there better ways to get the call stack for the exception?
What you are doing is not good practice. Here's why:
1. It's unnecessary.
If you compile your project in debug mode so that debugging information gets generated, you can easily get backtraces for exception handling in a debugger such as GDB.
2. It's cumbersome.
This is something you have to remember to add to each and every function. If you happen to miss a function, that could cause a great deal of confusion, especially if that were the function that caused the exception. And anyone looking at your code would have to realize what you are doing. Also, I bet you used something like __FUNC__ or __FUNCTION__ or __PRETTY_FUNCTION__, which sadly to say are all non-standard (there is no standard way in C++ to get the name of the function).
3. It's slow.
Exception propagation in C++ is already fairly slow, and adding this logic will only make the codepath slower. This is not an issue if you are using macros to catch and rethrow, where you can easily elide the catch and rethrow in release versions of your code. Otherwise, performance could be a problem.
Good practice
While it may not be good practice to catch and rethrow in each and every function to build up a stack trace, it is good practice to attach the file name, line number, and function name at which the exception was originally thrown. If you use boost::exception with BOOST_THROW_EXCEPTION, you will get this behavior for free. It's also good to attach explanatory information to your exception that will assist in debugging and handling the exception. That said, all of this should occur at the time the exception is constructed; once it is constructed, it should be allowed to propagate to its handler... you shouldn't repeatedly catch and rethrow more than stricly necessary. If you need to catch and rethrow in a particular function to attach some crucial information, that's fine, but catching all exceptions in every function and for the purposes of attaching already available information is just too much.
No, it is deeply horrible, and I don't see why you need a call stack in the exception itself - I find the exception reason, the line number and the filename of the code where the initial exception occurred quite sufficient.
Having said that, if you really must have a stack trace, the thing to do is to generate the call stack info ONCE at the exception throw site. There is no single portable way of doing this, but using something like http://stacktrace.sourceforge.net/ combined with and a similar library for VC++ should not be too difficult.
One solution which may be more graceful is to build a Tracer macro/class. So at the top of each function, you write something like:
TRACE()
and the macro looks something like:
Tracer t(__FUNCTION__);
and the class Tracer adds the function name to a global stack on construction, and removes itself upon destruction. Then that stack is always available to logging or debugging, maintenance is much simpler (one line), and it doesn't incur exception overhead.
Examples of implementations include things like http://www.drdobbs.com/184405270, http://www.codeproject.com/KB/cpp/cmtrace.aspx, and http://www.codeguru.com/cpp/v-s/debug/tracing/article.php/c4429. Also Linux functions like this http://www.linuxjournal.com/article/6391 can do it more natively, as described by this Stack Overflow question: How to generate a stacktrace when my gcc C++ app crashes. ACE's ACE_Stack_Trace may be worth looking at too.
Regardless, the exception-handling method is crude, inflexible, and computationally expensive. Class-construction/macro solutions are much faster and can be compiled out for release builds if desired.
The answer to all your problems is a good debugger, usually http://www.gnu.org/software/gdb/ on linux or Visual Studio on Windows. They can give you stack traces on demand at any point in the program.
Your current method is a real performance and maintenance headache. Debuggers are invented to accomplish your goal, but without the overhead.
There's a nice little project that gives a pretty stack trace:
https://github.com/bombela/backward-cpp
Look at this SO Question. This might be close to what you're looking for. It isn't cross-platform but the answer gives solutions for gcc and Visual Studio.
One more project for stack-trace support: ex_diag. There are no macros, cross-platform is present, no huge code needs, tool is fast, clear and easy in use.
Here you need only wrap objects, which are need to trace, and they will be traced if exception occurs.
Linking with the libcsdbg library (see https://stackoverflow.com/a/18959030/364818 for original answer) looks like the cleanest way of getting a stack trace without modifying your source code or 3rd party source code (ie STL).
This uses the compiler to instrument the actual stack collection, which is really want you want to do.
I haven't used it and it is GPL tainted, but it looks like the right idea.
While quite a few counter-arguments have been made in the answers here, I want to note that since this question was asked, with C++11, methods have been added which allow you to get nice backtraces in a cross-platform way and without the need for a debugger or cumbersome logging:
Use std::nested_exception and std::throw_with_nested
It is described on StackOverflow here and here, how you can get a backtrace on your exceptions inside your code by simply writing a proper exception handler which will rethrow nested exceptions.
It will, however, require that you insert try/catch statements at the functions you wish to trace.
Since you can do this with any derived exception class, you can add a lot of information to such a backtrace!
You may also take a look at my MWE on GitHub or my "trace" library, where a backtrace would look something like this:
Library API: Exception caught in function 'api_function'
Backtrace:
~/Git/mwe-cpp-exception/src/detail/Library.cpp:17 : library_function failed
~/Git/mwe-cpp-exception/src/detail/Library.cpp:13 : could not open file "nonexistent.txt"
An exception that isn't handled is left for the calling function to handle. That continues until the exception is handled. This happens with or without try/catch around a function call. In other words, if a function is called that isn't in a try block, an exception that happens in that function will automatically be passed up to call stack. So, all you need to do is put the top-most function in a try block and handle the exception "..." in the catch block. That exception will catch all exceptions. So, your top-most function will look something like
int main()
{
try
{
top_most_func()
}
catch(...)
{
// handle all exceptions here
}
}
If you want to have specific code blocks for certain exceptions, you can do that too. Just make sure those occur before the "..." exception catch block.
I've managed to get through my C++ game programming career so far virtually never touching exceptions but recently I've been working on a project with the Ogre engine and I'm trying to learn properly. I've found a lot of good questions and answers here on the general usage of C++ exceptions but I'd like to get some outside opinions from here on whether Ogre's usage is good and how best to work with them.
To start with, quoting from Ogre's documentation of it's own Exception class:
OGRE never uses return values to indicate errors. Instead, if an error occurs, an exception is thrown, and this is the object that encapsulates the detail of the problem. The application using OGRE should always ensure that the exceptions are caught, so all OGRE engine functions should occur within a try{} catch(Ogre::Exception& e) {} block.
Really? Every single Ogre function could throw an exception and be wrapped in a try/catch block? At present this is handled in our usage of it by a try/catch in main that will show a message box with the exception description before exiting. This can be a bit awkward for debugging though as you don't get a stack trace, just the function that threw the error - more important is the function from our code that called the Ogre function. If it was an assert in Ogre code then it would go straight to the code in the debugger and I'd be able to find out what's going on much easier - I don't know if I'm missing something that would allow me to debug exceptions already?
I'm starting to add a few more try/catch blocks in our code now, generally thinking about whether it matters if the Ogre function throws an exception. If it's something that will stop everything working then let the main try/catch handle it and exit the program. If it's not of great importance then catch it just after the function call and let the program continue. One recent example of this was building up a vector of the vertex/fragment program parameters for materials applied to an entity - if a material didn't have any parameters then it would throw an exception, which I caught and then ignored as it didn't need to add to my list of parameters. Does this seem like a reasonable way of dealing with things? Any specific advice for working with Ogre is much appreciated.
You don't need to wrap every last call to Ogre in try { ... } catch. You do it wherever you can meaningfully deal with the exception. This may be at the individual call site in some cases, or it could be in a high-level loop of some sort. If you can't deal with it meaningfully anywhere, don't catch it at all; let the debugger take over.
In particular, you shouldn't catch exceptions in main() for precisely the reason you cite (at least, not during development; you should in production).
I don't know anything about Ogre, I'm afraid, but the general rule with exception handling is that you catch the exception as far as possible from the throw site, but no further. However, this is only possible if the code that throws the exception uses RAII to look after allocated resources. If the code uses dynamic allocation to plain pointers, or other forms of manual resource management, then you need try-blocks at the call site. If this is the case, I'd say use of exceptions is a bad idea.
You seem to be unaware of how to debug exceptions.
Either
Bring up the VS Debug/Exceptions
dialog and tick the C++ Exceptions
box. This will give you an
opportunity (a dialog appears) to debug when an
exception is thrown.
or
If you've built Ogre source, set a
breakpoint in the Ogre::Exception
constructor and when it attempts to throw one you'll break with
a call stack where the next level up is the throw site.
Is there a way catch an exception like access violation and get information about on which line an exception occurred? This would be very good for debugging purposes, especially for testers..
My environment is Windows with VC++ on VS2008
An access violation is not an exception in C++ terms, so the answer is in general "no". Concieveably, your specific implementation might have a feature that turns access violations into C++ exceptions - you need to specify what compiler & platform you are using.
In case you really want to log info about C++ exceptions (AV is not one) thrown from your code, you can use macros __FILE__ and __LINE__ within constructors of your exception types.
If you catch the SEH exception using a __catch handler you can then access the thread context at the time of the exception and then use StackWalk64 to dump the call stack at the point where the exception was generated. Note that as I mentioned here: Printing the stack trace in C++ (MSVC)? StackWalker by Jochen Kalmbach [MVP VC++] and available on codeproject is probably the easiest way to do this. It wraps up all of the details of dealing with the underlying StackWalk64 API.
Alternatively you could the same solution that I proposed here: How to catch divide-by-zero error in Visual Studio 2008 C++? to convert the Windows Structured Exceptions into C++ exceptions and capture the stack trace as shown above at the point where you translate the exception. This would give you a C++ exception with a stack trace like you get in C# or Java.
I'm not sure why testers would need to know which line an exception occurred.
The developers might want to know, though. But a better approach is the following:
Include the information about class and method with each PROGRAM exception. These are exceptions that should NOT have happened.
This should be output by whatever logs your exceptions. Your program does catch and log every exception, doesn't it? If not, it should.
Make sure your methods are small enough that the above is enough information to easy track down a bug. If you need line information as well, then your methods are too large and your exceptions are not specific enough.
In MSVC you can set the debugger to break when any exception or things like an access violation happen buy going to debug->Exceptions and checking the appropriate box.
Note that access violations are not C++ exceptions and will be handled differently on different systems. In Windows you can trap them with a structured exception handler. In unixy systems it will usually core dump. You can usually get a stacktrace from that.
For msdev / Windows, during development, always have your linker generate a MAP file.
When Windows throws up an access violation, and it's your code (not library), you can use the address to match up function / data in the MAP file and get within a few lines of the offender. At a minimum, you'll know the function/method.
That's really a compiler question.
If you want to know if it is possible for a compiler to provide, the answer is yes. I know multiple Ada compilers that provide tracebacks for unhandled exceptions, so it is clearly possible. This includes the gcc-based Gnat, so if the C++ compiler uses any of the same facilities for its exceptions, the support for doing that should already be there.
On unix type systems a Access violation generates a SEGV or BUS signal. This normally causes the application to core dump. You could always write your own signal handler. From there you could probably generate a stack-dump using the glibc before core dumping.
A core dump generally give you all this for wasy analysis in gdb.
My answer refers to Windows only. You have quite a few options:
Without changing your code -
Avoid catching exceptions (at least ones you do not expect), and let your app just crash. Configure good ol' dr. watson (drwtsn32.exe) to create a crash dump, and open that dump in your debugger. You'll get the exact line of code there.
Use adPlus (from the windbg installation) to monitor your app's run, and create a dump when an exception is thrown.
From within your code, you can -
Easily walk the stack when a structured exception is thrown. See Jochen Kalmbach's stack walker for example.
Use this weird hack to walk the stack when a C++ exception is thrown.
Finally, quite a lot of questions have been asked here on this issue. Look around, and you'll get additional answers.
On one project I worked on, the root class of the exception hierarchy used captured a callstack in the constructor in case that information was needed later for debugging after the exception was caught. It was a decent idea, although if you're catching and 'handling' the exception, exactly when it was thrown shouldn't really matter all that much.
To put it another way, if you care about this info (context of "who" threw it), you probably shouldn't be using an exception to report this condition, or potentially you shouldn't be catching the exception in the first place. Uncaught exceptions cause crashes, giving you a crash dump at the point the exception was thrown.
On windows, the C runtime system function _set_se_translator() takes a simple static function with signature
void f(int, EXCEPTION_POINTERS*)
neither of which argument you totally need. In the body of f, throw your favorite exception. Call the function near the beginning of your program. The documentation is reasonable for microsoft.
You can do all manner of additional stuff with this function.
The google breakpad project has many good things. Among them are means to convert a crash address to file, line, and function name using build symbols and the debuginfo dll.