I have a c++ dll which I need to debug. Due to the circumstances in which I am using the dll, I am unable to debug it via the calling application.
So, I created a try -catch, where the catch writes the exception to a file.
The line which needs to be debugged involves imported classes from a 3rd party dll, so I have no way of knowing what type of exception it is. When I tried catch(exception e), no message was written to the file. So I tried catch(...), which did trigger something:
using std::exception::what, the only thing that got written to the file was "1".
using std::exception::exception, the file received the following code : "0579EF90".
Is there any way for me to retrieve meaningful info about the exception that was thrown?
TIA
CG
If you don't use catch(KnownExceptionType ex) and use your knwoledge about KnownExceptionType to extract info, no you can't.
When you catch with catch(...) you are pretty much lost, you know that you handled an exception but there is no type information there, there is little you can do.
You are in the worse case, an exception coming out from a library, you have no info on the exception, even if you had headers for the lib, that exception type doesn't need to be defined there.
If I understand you correctly, you've already narrowed down the source of the issue to a specific call to a 3rd party library, but you're not allowed to debug the application live (do I want to ask why?), and your question is "how can I debug the exception without any knowledge of what the exception is"
The answer is, you can't. As you observed, you can blindly guess and hope to catch the right thing. You can also catch(...), but that will tell you exactly nothing. If you could debug live, you could set the debugger to break when the exception is thrown and see what is going on there.
I think the right answer though is to contact the 3rd party library you've narrowed down the source of the issue to and ask them. It is very, very bad form to throw an exception and allow it to propogate across module boundries. It makes me suspect that it's a Windows SEH exception for a null pointer deref or something, and you're compiling in such a way that catch(...) catches those.
maybe try catching std::exception & e
std::cout << e.what() << endl;
see if you can cast it to std::logic_error, and std::runtime_error - that should give you some clue what you're dealing with
Firstly, you should always catch exceptions by const reference, in other words:
catch( const std::exception & ex ) {
...
}
Not doing so means that your exceptions will be of the exact type you catch and this may result in the loss of exception information.
However, it seems that your library is throwing something not derived from std::exception - you need to find out what the type (ideally the base type) is.
I'm a bit confused. On the one hand you write catch(std::exception) didn't work (you should use catch(const std::exception&), BTW), on the other hand you also write you invoked std::exception::what(). How did you do this if you didn't have a std::exception in the first place?
Anyway, once you have caught anything but ..., you could try to log the RTTI info:
#include <typeinfo>
try {
foreign_code(my_data);
} catch(const some_type& x) {
std::cerr << "Yikes! Caught exception of type '"
<< typeid(x).name()
<< "' with its hand in the cookie jar!\n";
std::abort();
}
While the standard doesn't make any assumptions of the result of std::type_info::name(), most (if not all) compilers will generate code that emits something that's at least somewhat useful.
When you're inside the VS debugger, you could also set it up so that it will stop at any exceptions thrown. That gives you a stack trace and thus might give you a clue as to what data passed to the DLL could have cause the problem.
Related
With Python, I could get the name of the exception easily as follows.
run the code, i.e. x = 3/0 to get the exception from python
"ZeroDivisionError: integer division or modulo by zero" tells me this is ZeroDivisionError
Modify the code i.e. try: x=3/0 except ZeroDivisionError: DO something
Is there any similar way to find the exception name with C++?
When I run the x = 3/0, the compiled binary just throws 'Floating point exception', which is not so useful compared to python.
While you can't easily ask for the name of the exception, if the exception derives from std::exception you can find out the specified reason it was shown with what():
try
{
...
}
catch (const std::exception &exc)
{
std::err << exc.what() << std::endl;
}
On a side note, dividing by 0 is not guaranteed to raise a C++ exception (I think the MS platforms may do that but you won't get that on Linux).
If you want to know the name of the exception class, you could use RTTI. However, the vast majority of C++ code will throw an exception derived from std::exception.
However, all you get is the exception data contained in std::exception::what, and you can get the name of the exception class from RTTI and catch that explicitly if you need more information (and it contains more information).
For most exceptions if you have the RTTI option set in your compiler, you can do:
catch(std::exception & e)
{
cout << typeid(e).name();
}
Unfortunately the exception thrown by a divide by zero does not derive from std::exception, so this trick will not work.
If this is a debugging issue, you may be able to set your compiler to break when it hits an exception, which can be infinitely useful.
I have some C++ code that uses a very standard exception pattern:
try {
// some code that throws a std::exception
}
catch (std::exception &e) {
// handle the exception
}
The problem is that the exceptions are not being caught and I cannot figure out why.
The code compiles to a static library in OS X (via Xcode). The library is linked into a Cocoa application, with a call to the function in question happening via an Objective-C++ thunk. I suspect that the interplay between Objective-C and C++ is the culprit but all my attempts to pin this down have failed.
I have not been able to create a simple example that reproduces this behavior in a simple example. When I take the relevant code out of the context of my big program everything works.
Can anyone suggest why my exceptions are not being caught?
C++ allows you a variety of options for catching: value, reference or pointer.
Note that this code only catches std::exceptions passed by reference or value:
try {
// some code that throws a std::exception
}
catch (std::exception &e) {
// handle the exception
}
It's likely that the exception is being passed by pointer:
catch (std::exception* e)
Check the code that is throwing the exception, and see how it's doing it.
As Mark points out, if you catch by value instead of by reference you risk slicing your object.
Try a catch(...) {} block, see if an exception is really thrown.
I suspect that the interplay between Objective-C and C++ is the culprit but all my attempts to pin this down have failed.
You're probably right, although it's hard to track down.
First, GCC explicitly does not allow you to throw exceptions in Objective C++ and catch them in C++ ("when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time. This means you cannot #throw an exception from Objective-C and catch it in C++, or vice versa (i.e., throw ... #catch).")
However, I think you're describing a case where Objective C++ calls C++ code, the C++ code throws and you're hoping for C++ code to catch the exception. Unfortunately I'm having difficulty finding documentation for this specific case. There is some hope because, "It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area." If they can do it for Java, there is a chance they can do it for Objective C++.
At the very least, you'll need to specify -fexceptions at compile time ("you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++"). Again, that doesn't specifically mention Objective C++ but it may apply.
One little known gotcha with exceptions relates to the access of the base class.
If you are actually throwing a class that derives privately from std::exception then the std::exception handler will not be chosen.
For example:
#include <iostream>
class A { };
class B : private A { } ;
int main ()
{
try
{
throw B ();
}
catch (A & )
{
std::cout << "Caught an 'A'" << std::endl;
}
catch (B & )
{
std::cout << "Caught an 'B'" << std::endl;
}
}
Usually, such an order of handlers would result in the 'B' handler never being selected, but in this case 'B' dervies from 'A' privately and so the catch handler for type 'A' is not considered.
I can offer two theories:
the exception gets caught before it comes your catch clause; any function on the stack might be the culprit. As Michael proposes, try catching everything.
exception unwinding fails to locate your handler. To analyze this in more detail, you would have to step through the exception unwinding code, which is very hairy. See whether compiling the Objective-C code with -fobjc-exceptions helps.
This might be a long shot, but in Visual Studio's compiler settings there is an option to switch off exceptions entirely. Perhaps there's something similar in GCC / XCode.
C++ exceptions can be just about anything, quite frequently a char*. As suggested before add catch (...) to at least get it to break and see what's going on.
Thanks for the input from everyone. Those are good suggestions for anyone who runs into a similar problem. It's working now, but I'm not 100% sure which of various changes I made caused things to become sane again. Once again, the approach of simplifying down to something that works and building back up from there paid off.
One thing that wasn't mentioned in the responses, and which I think was part of my confusion, is to make sure that the handler makes it obvious that it actually caught the exception. I think that in some of my formulations of the handler it was masking that fact and passing the exception on to a higher level handler.
I have a problem in catching an exception. I am trying to rethrow an exception and I get a message:
There is no source code available for the current location.
The code is very simple:
#include <exception>
using namespace std;
try {
throw exception("Asas");
}
catch (const exception& e) {
cout<< "Error msg" << e.what() << endl;
throw; //This the error message I get from the IDE.
}
It also repeats if I try to throw a string message and try to rethrow it.
It seems like you are stepping into the re-throw, and you probably don't have source code for the part of the code you stepped into. You will get those messages when the IDE can't find source code for the current line of code.
Your question is so misleading, it's very hard to give you back anything but more questions.
You write you get this message when you're trying to rethrow, but it's very unclear what you mean: Is this a compiler error, a run-time error, or something you get while you're debugging? If the latter (which I assume), why are you debugging? Isn't the thrown exception caught? If so, what's the code that attempts to catch it?
I suggest you change your question to show a small, but complete piece of code (ideally self-contained and compilable) plus a exact description of A) what happens and B) what you think should happen instead.
Meanwhile, all I can say is that the error message you quote might mean that you are running/debugging something that isn't compiled from the code you look at. But that#s about as vague as your question...
Is there a c++ equivalent of Java's
try {
...
}
catch (Throwable t) {
...
}
I am trying to debug Java/jni code that calls native windows functions and the virtual machine keeps crashing. The native code appears fine in unit testing and only seems to crash when called through jni. A generic exception catching mechanism would prove extremely useful.
try{
// ...
} catch (...) {
// ...
}
will catch all C++ exceptions, but it should be considered bad design. You can use c++11's new current_exception mechanism, but if you don't have the ability to use c++11 (legacy code systems requiring a rewrite), then you have no named exception pointer to use to get a message or name. You may want to add separate catch clauses for the various exceptions you can catch, and only catch everything at the bottom to record an unexpected exception. E.g.:
try{
// ...
} catch (const std::exception& ex) {
// ...
} catch (const std::string& ex) {
// ...
} catch (...) {
// ...
}
Someone should add that one cannot catch "crashes" in C++ code. Those don't throw exceptions, but do anything they like. When you see a program crashing because of say a null-pointer dereference, it's doing undefined behavior. There is no std::null_pointer_exception. Trying to catch exceptions won't help there.
Just for the case someone is reading this thread and thinks he can get the cause of the program crashes. A Debugger like gdb should be used instead.
This is how you can reverse-engineer the exception type from within catch(...) should you need to (may be useful when catching unknown from a third party library) with GCC:
#include <iostream>
#include <exception>
#include <typeinfo>
#include <stdexcept>
int main()
{
try {
throw ...; // throw something
}
catch(...)
{
std::exception_ptr p = std::current_exception();
std::clog <<(p ? p.__cxa_exception_type()->name() : "null") << std::endl;
}
return 1;
}
and if you can afford using Boost you can make your catch section even simpler (on the outside) and potentially cross-platform
catch (...)
{
std::clog << boost::current_exception_diagnostic_information() << std::endl;
}
try {
// ...
} catch (...) {
// ...
}
Note that the ... inside the catch is a real ellipsis, ie. three dots.
However, because C++ exceptions are not necessarily subclasses of a base Exception class, there isn't any way to actually see the exception variable that is thrown when using this construct.
it is not possible (in C++) to catch all exceptions in a portable manner. This is because some exceptions are not exceptions in a C++ context. This includes things like division by zero errors and others. It is possible to hack about and thus get the ability to throw exceptions when these errors happen, but it's not easy to do and certainly not easy to get right in a portable manner.
If you want to catch all STL exceptions, you can do
try { ... } catch( const std::exception &e) { ... }
Which will allow you do use e.what(), which will return a const char*, which can tell you more about the exception itself. This is the construct that resembles the Java construct, you asked about, the most.
This will not help you if someone is stupid enough to throw an exception that does not inherit from std::exception.
In short, use catch(...). However, note that catch(...) is meant to be used in conjunction with throw; basically:
try{
foo = new Foo;
bar = new Bar;
}
catch(...) // will catch all possible errors thrown.
{
delete foo;
delete bar;
throw; // throw the same error again to be handled somewhere else
}
This is the proper way to use catch(...).
it is possible to do this by writing:
try
{
//.......
}
catch(...) // <<- catch all
{
//.......
}
But there is a very not noticeable risk here: you can not find the exact type of error that has been thrown in the try block, so use this kind of catch when you are sure that no matter what the type of exception is, the program must persist in the way defined in the catch block.
You can use
catch(...)
but that is very dangerous. In his book Debugging Windows, John Robbins tells a war story about a really nasty bug that was masked by a catch(...) command. You're much better off catching specific exceptions. Catch whatever you think your try block might reasonably throw, but let the code throw an exception higher up if something really unexpected happens.
Let me just mention this here: the Java
try
{
...
}
catch (Exception e)
{
...
}
may NOT catch all exceptions! I've actually had this sort of thing happen before, and it's insantiy-provoking; Exception derives from Throwable. So literally, to catch everything, you DON'T want to catch Exceptions; you want to catch Throwable.
I know it sounds nitpicky, but when you've spent several days trying to figure out where the "uncaught exception" came from in code that was surrounded by a try ... catch (Exception e)" block comes from, it sticks with you.
Well, if you would like to catch all exception to create a minidump for example...
Somebody did the work on Windows.
See http://www.codeproject.com/Articles/207464/Exception-Handling-in-Visual-Cplusplus
In the article, he explains how he found out how to catch all kind of exceptions and he provides code that works.
Here is the list you can catch:
SEH exception
terminate
unexpected
pure virtual method call
invalid parameter
new operator fault
SIGABR
SIGFPE
SIGILL
SIGINT
SIGSEGV
SIGTERM
Raised exception
C++ typed exception
And the usage:
CCrashHandler ch;
ch.SetProcessExceptionHandlers(); // do this for one thread
ch.SetThreadExceptionHandlers(); // for each thred
By default, this creates a minidump in the current directory (crashdump.dmp)
Be aware
try{
// ...
} catch (...) {
// ...
}
catches only language-level exceptions, other low-level exceptions/errors like Access Violation and Segmentation Fault wont be caught.
A generic exception catching mechanism
would prove extremely useful.
Doubtful. You already know your code is broken, because it's crashing. Eating exceptions may mask this, but that'll probably just result in even nastier, more subtle bugs.
What you really want is a debugger...
Can you run your JNI-using Java application from a console window (launch it from a java command line) to see if there is any report of what may have been detected before the JVM was crashed. When running directly as a Java window application, you may be missing messages that would appear if you ran from a console window instead.
Secondly, can you stub your JNI DLL implementation to show that methods in your DLL are being entered from JNI, you are returning properly, etc?
Just in case the problem is with an incorrect use of one of the JNI-interface methods from the C++ code, have you verified that some simple JNI examples compile and work with your setup? I'm thinking in particular of using the JNI-interface methods for converting parameters to native C++ formats and turning function results into Java types. It is useful to stub those to make sure that the data conversions are working and you are not going haywire in the COM-like calls into the JNI interface.
There are other things to check, but it is hard to suggest any without knowing more about what your native Java methods are and what the JNI implementation of them is trying to do. It is not clear that catching an exception from the C++ code level is related to your problem. (You can use the JNI interface to rethrow the exception as a Java one, but it is not clear from what you provide that this is going to help.)
For the real problem about being unable to properly debug a program that uses JNI (or the bug does not appear when running it under a debugger):
In this case it often helps to add Java wrappers around your JNI calls (i.e. all native methods are private and your public methods in the class call them) that do some basic sanity checking (check that all "objects" are freed and "objects" are not used after freeing) or synchronization (just synchronize all methods from one DLL to a single object instance). Let the java wrapper methods log the mistake and throw an exception.
This will often help to find the real error (which surprisingly is mostly in the Java code that does not obey the semantics of the called functions causing some nasty double-frees or similar) more easily than trying to debug a massively parallel Java program in a native debugger...
If you know the cause, keep the code in your wrapper methods that avoids it. Better have your wrapper methods throw exceptions than your JNI code crash the VM...
If you are looking for Windows-specific solution then there is structured exception handling:
https://learn.microsoft.com/en-us/cpp/cpp/try-except-statement
The code looks as follows
__try
{
// code here may throw or make access violation
}
__except( EXCEPTION_EXECUTE_HANDLER )
{
// after exception code here, e.g. log the error
}
It will catch not only C++ exceptions but also access violations or other system exceptions.
Well this really depends on the compiler environment.
gcc does not catch these.
Visual Studio and the last Borland that I used did.
So the conclusion about crashes is that it depends on the quality of your development environment.
The C++
specification says that catch(...) must catch any exceptions, but it doesn't in all cases.
At least from what I tried.
I'm looking for an answer in MS VC++.
When debugging a large C++ application, which unfortunately has a very extensive usage of C++ exceptions. Sometimes I catch an exception a little later than I actually want.
Example in pseudo code:
FunctionB()
{
...
throw e;
...
}
FunctionA()
{
...
FunctionB()
...
}
try
{
Function A()
}
catch(e)
{
(<--- breakpoint)
...
}
I can catch the exception with a breakpoint when debugging. But I can't trace back if the exception occurred in FunctionA() or FunctionB(), or some other function. (Assuming extensive exception use and a huge version of the above example).
One solution to my problem is to determine and save the call stack in the exception constructor (i.e. before it is caught). But this would require me to derive all exceptions from this base exception class. It would also require a lot of code, and perhaps slow down my program.
Is there an easier way that requires less work? Without having to change my large code base?
Are there better solutions to this problem in other languages?
You pointed to a breakpoint in the code. Since you are in the debugger, you could set a breakpoint on the constructor of the exception class, or set Visual Studio debugger to break on all thrown exceptions (Debug->Exceptions Click on C++ exceptions, select thrown and uncaught options)
If you are just interested in where the exception came from, you could just write a simple macro like
#define throwException(message) \
{ \
std::ostringstream oss; \
oss << __FILE __ << " " << __LINE__ << " " \
<< __FUNC__ << " " << message; \
throw std::exception(oss.str().c_str()); \
}
which will add the file name, line number and function name to the exception text (if the compiler provides the respective macros).
Then throw exceptions using
throwException("An unknown enum value has been passed!");
There's an excellent book written by John Robbins which tackles many difficult debugging questions. The book is called Debugging Applications for Microsoft .NET and Microsoft Windows. Despite the title, the book contains a host of information about debugging native C++ applications.
In this book, there is a lengthy section all about how to get the call stack for exceptions that are thrown. If I remember correctly, some of his advice involves using structured exception handling (SEH) instead of (or in addition to) C++ exceptions. I really cannot recommend the book highly enough.
Put a breakpoint in the exception object constructor. You'll get your breakpoint before the exception is thrown.
There is no way to find out the source of an exception after it's caught, unless you include that information when it is thrown. By the time you catch the exception, the stack is already unwound, and there's no way to reconstruct the stack's previous state.
Your suggestion to include the stack trace in the constructor is your best bet. Yes, it costs time during construction, but you probably shouldn't be throwing exceptions often enough that this is a concern. Making all of your exceptions inherit from a new base may also be more than you need. You could simply have the relevant exceptions inherit (thank you, multiple inheritance), and have a separate catch for those.
You can use the StackTrace64 function to build the trace (I believe there are other ways as well). Check out this article for example code.
Here's how I do it in C++ using GCC libraries:
#include <execinfo.h> // Backtrace
#include <cxxabi.h> // Demangling
vector<Str> backtrace(size_t numskip) {
vector<Str> result;
std::vector<void*> bt(100);
bt.resize(backtrace(&(*bt.begin()), bt.size()));
char **btsyms = backtrace_symbols(&(*bt.begin()), bt.size());
if (btsyms) {
for (size_t i = numskip; i < bt.size(); i++) {
Aiss in(btsyms[i]);
int idx = 0; Astr nt, addr, mangled;
in >> idx >> nt >> addr >> mangled;
if (mangled == "start") break;
int status = 0;
char *demangled = abi::__cxa_demangle(mangled.c_str(), 0, 0, &status);
Str frame = (status==0) ? Str(demangled, demangled+strlen(demangled)) :
Str(mangled.begin(), mangled.end());
result.push_back(frame);
free(demangled);
}
free(btsyms);
}
return result;
}
Your exception's constructor can simply call this function and store away the stack trace. It takes the param numskip because I like to slice off the exception's constructor from my stack traces.
There's no standard way to do this.
Further, the call stack must typically be recorded at the time of the exception being thrown; once it has been caught the stack has unrolled, so you no longer know what was going on at the point of being thrown.
In VC++ on Win32/Win64, you might get usable-enough results by recording the value from the compiler intrinsic _ReturnAddress() and ensuring that your exception class constructor is __declspec(noinline). In conjunction with the debug symbol library, I think you could probably get the function name (and line number, if your .pdb contains it) that corresponds to the return address using SymGetLineFromAddr64.
In native code you can get a shot at walking the callstack by installing a Vectored Exception handler. VC++ implements C++ exceptions on top of SEH exceptions and a vectored exception handler is given first shot before any frame based handlers. However be really careful, problems introduced by vectored exception handling can be difficult to diagnose.
Also Mike Stall has some warnings about using it in an app that has managed code. Finally, read Matt Pietrek's article and make sure you understand SEH and vectored exception handling before you try this. (Nothing feels quite so bad as tracking down a critical problem to code you added help track down critical problems.)
I believe MSDev allows you to set break points when an exception is thrown.
Alternatively put the break point on the constructor of your exception object.
If you're debugging from the IDE, go to Debug->Exceptions, click Thrown for C++ exceptions.
Other languages? Well, in Java you call e.printStackTrace(); It doesn't get much simpler than that.
In case anyone is interested, a co-worker replied to this question to me via email:
Artem wrote:
There is a flag to MiniDumpWriteDump() that can do better crash dumps that will allow seeing full program state, with all global variables, etc. As for call stacks, I doubt they can be better because of optimizations... unless you turn (maybe some) optimizations off.
Also, I think disabling inline functions and whole program optimization will help quite a lot.
In fact, there are many dump types, maybe you could choose one small enough but still having more info
http://msdn.microsoft.com/en-us/library/ms680519(VS.85).aspx
Those types won't help with call stack though, they only affect the amount of variables you'll be able to see.
I noticed some of those dump types aren't supported in dbghelp.dll version 5.1 that we use. We could update it to the newest, 6.9 version though, I've just checked the EULA for MS Debugging Tools -- the newest dbghelp.dll is still ok to redistribute.
I use my own exceptions. You can handle them quite simple - also they contain text. I use the format:
throw Exception( "comms::serial::serial( )", "Something failed!" );
Also I have a second exception format:
throw Exception( "comms::serial::serial( )", ::GetLastError( ) );
Which is then converted from a DWORD value to the actual message using FormatMessage. Using the where/what format will show you what happened and in what function.
By now, it has been 11 years since this question was asked and today, we can solve this problem using only standard C++11, i.e. cross-platform and without the need for a debugger or cumbersome logging.
You can trace the call stack that led to an exception
Use std::nested_exception and std::throw_with_nested
This won't give you a stack unwind, but in my opinion the next best thing.
It is described on StackOverflow here and here, how you can get a backtrace on your exceptions inside your code without need for a debugger or cumbersome logging, 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 (i.e. functions without this will not appear in your trace).
You could automate this with macros, reducing the amount of code you have to write/change.
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, 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"