C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How do I catch exceptions thrown from C++ functions called w          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Paul Harman Guest Posted: Fri Jul 16, 2004 1:36 pm    Post subject: How do I catch exceptions thrown from C++ functions called w I'm on Solaris 8, using g++ to compile. Versions availabe if required... I have googled for this, but not come up with any useful answers... I'm writing a C++ (ish) application that uses dynamic libraries loaded using dlopen and functions called by dlsym. Because of the name mangling this involves, all of the functions are prototyped inside an extern C block. These library functions then call object methods that are defined back in the main application. Unfortunately, the code inside the library is unable to catch any exceptions thrown. I presume this has something to do with namespaces or some other kind of name mangling, since other methods in the main application that call these methods *are* able to catch the exceptions. Is there an obvious way around this... other than rewriting all of my C++ code so that it doesn't throw exceptions for errors? Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Richter Guest Posted: Mon Jul 19, 2004 5:29 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call Hi, Quote: I'm writing a C++ (ish) application that uses dynamic libraries loaded using dlopen and functions called by dlsym. Because of the name mangling this involves, all of the functions are prototyped inside an extern C block. Quote: These library functions then call object methods that are defined back in the main application. Unfortunately, the code inside the library is unable to catch any exceptions thrown. I presume this has something to do with namespaces or some other kind of name mangling, since other methods in the main application that call these methods *are* able to catch the exceptions. Quote: Is there an obvious way around this... other than rewriting all of my C++ code so that it doesn't throw exceptions for errors? The C++ standard itself doesn't say a word about dynamic libraries, or what has to happen if you throw an exception in a function called by a library, so in what follows I can only present some observations: Do you have access to the library source? Is this C or C++? g++ requires a special stack-layout to be able to perform automatic stack-unwinding, and this might not be available for the in-between library code. If this library is your code, you might want to try to compile it as C++. This might help the compiler to forward exceptions properly. Another (arkward) approach might be to build a tiny wrapper around the library calls and around your callee: Before calling the library, use "setjmp" to collect the current position, catch errors here and generate the apropriate exceptions for your front-end. In the callee, add another wrapper around your functions, catch exceptions there and forward them to the caller by means of "longjmp" (where they are thrown again). Depending on what the library does, this may or may not work, as all resources allocated on the way down to the callee are not properly released (no stack unwinding!). Thus, your milage may vary. So long, Thomas Quote: Paul Quote: [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul Harman Guest Posted: Tue Jul 20, 2004 6:11 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call Thomas Richter <thor (AT) cleopatra (DOT) math.tu-berlin.de> wrote Quote: Do you have access to the library source? Is this C or C++? Yes, I also write the dynamic libraries involved. It's C++, compiled using the following: g++ -Wall -O -g -fPIC -c <whatever.cpp> -o <whatever.o> ld -G <whatever.o> -o <whatever.so> Quote: g++ requires a special stack-layout to be able to perform automatic stack-unwinding, and this might not be available for the in-between library code. If this library is your code, you might want to try to compile it as C++. This might help the compiler to forward exceptions properly. Hmmm - see above. Of course all of the functions in the library are defined as "extern C", so that dlsym can find them - I expect that this is mangling the ability to unwind the stack. Quote: Another (arkward) approach might be to build a tiny wrapper around the library calls and around your callee: Before calling the library, use "setjmp" to collect the current position, catch errors here and generate the apropriate exceptions for your front-end. In the callee, add another wrapper around your functions, catch exceptions there and forward them to the caller by means of "longjmp" (where they are thrown again). The software architect in me is screaming out in horror... yes, that might well work, but before going to those extremes I think I'd rather write versions of the utility functions that don't throw exceptions and instead return NULL pointers, or whatever is appropriate. Thanks, Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Tue Jul 20, 2004 9:22 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call Paul Harman wrote: Quote: I'm on Solaris 8, using g++ to compile. Versions availabe if required... I have googled for this, but not come up with any useful answers... I'm writing a C++ (ish) application that uses dynamic libraries loaded using dlopen and functions called by dlsym. Because of the name mangling this involves, all of the functions are prototyped inside an extern C block. These library functions then call object methods that are defined back in the main application. Unfortunately, the code inside the library is unable to catch any exceptions thrown. I presume this has something to do with namespaces or some other kind of name mangling, since other methods in the main application that call these methods *are* able to catch the exceptions. I think extern "C" is irrelevant to this. It shouldn't affect exception handling at all. I suspect that the cause of the problem is that the DSOs (dynamic libraries) and the main executable are each using their own separate copies of the type information for the exception classes. If the exception-handling code compares type information by address then it would see the exception thrown in the main executable as having a different type from the type expected by the handler in the DSO. Quote: Is there an obvious way around this... other than rewriting all of my C++ code so that it doesn't throw exceptions for errors? There should be some way to sort this out, but it appears to be a platform-specific problem that you should ask about on a Unix or Solaris programming group. I don't know enough about DSOs and C++ to offer a solution. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Gershnik Guest Posted: Tue Jul 20, 2004 9:32 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call Paul Harman wrote: Quote: I'm on Solaris 8, using g++ to compile. Versions availabe if required... I have googled for this, but not come up with any useful answers... I'm writing a C++ (ish) application that uses dynamic libraries loaded using dlopen and functions called by dlsym. Because of the name mangling this involves, all of the functions are prototyped inside an extern C block. These library functions then call object methods that are defined back in the main application. Unfortunately, the code inside the library is unable to catch any exceptions thrown. I have seen the same with Sun's compiler and the reason there was that the shared library and the main application used different versions of C++ runtime. Make sure you compile all parts of your application (main and shared libs) using the same compiler settings and standard libraries. -- Eugene [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Wed Jul 21, 2004 11:57 am    Post subject: Re: How do I catch exceptions thrown from C++ functions call Ben Hutchings <do-not-spam-benh (AT) bwsint (DOT) com> writes: Quote: I think extern "C" is irrelevant to this. It shouldn't affect exception handling at all. I suspect that the cause of the problem is that the DSOs (dynamic libraries) and the main executable are each using their own separate copies of the type information for the exception classes. If the exception-handling code compares type information by address then it would see the exception thrown in the main executable as having a different type from the type expected by the handler in the DSO. I was going to say that, but when I had this sort of problem on GNU/Linux their people investigated Solaris and found that it *didn't* occur there. Sun CC compares typeinfo by comparing hashes and then comparing strings if the hashes collide. So this is a real mystery. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul Harman Guest Posted: Wed Jul 21, 2004 7:05 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call "Eugene Gershnik" <gershnik (AT) hotmail (DOT) com> wrote Quote: I have seen the same with Sun's compiler and the reason there was that the shared library and the main application used different versions of C++ runtime. Make sure you compile all parts of your application (main and shared libs) using the same compiler settings and standard libraries. Indeed. I've fixed this now. I was using 'ld' to link the libraries, but 'g++' to compile & link the main application. Now that I'm using 'g++' to link the libraries, the exceptions all seem to work properly. I'm also passing RTLD_GLOBAL to dlopen()... not sure if that is helping or not, but it seems good advice. Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joerg Richter Guest Posted: Wed Jul 21, 2004 8:13 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call Hi Paul, Quote: Yes, I also write the dynamic libraries involved. It's C++, compiled using the following: g++ -Wall -O -g -fPIC -c <whatever.cpp> -o <whatever.o ld -G Try using g++ to build your shared libs (and executables): g++ -shared -o Joerg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Wed Jul 21, 2004 8:15 pm    Post subject: Re: How do I catch exceptions thrown from C++ functions call [email]chatterbox (AT) doctorwhowebguide (DOT) net[/email] (Paul Harman) writes: Quote: "Eugene Gershnik" <gershnik (AT) hotmail (DOT) com> wrote I have seen the same with Sun's compiler and the reason there was that the shared library and the main application used different versions of C++ runtime. Make sure you compile all parts of your application (main and shared libs) using the same compiler settings and standard libraries. Indeed. I've fixed this now. I was using 'ld' to link the libraries, but 'g++' to compile & link the main application. Now that I'm using 'g++' to link the libraries, the exceptions all seem to work properly. I'm also passing RTLD_GLOBAL to dlopen()... not sure if that is helping or not, but it seems good advice. Yes, if you're using g++, it's helping. Not sure if it's good advice -- it's really a double-edged sword. With g++ it fixes the typeinfo/exception problems, but dlopen is usually used for plugin type architectures, and RTLD_GLOBAL makes you vulnerable to symbol collisions from plugin authors. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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