C++ – How to build a C++ Dll wrapper that catches all exceptions


Like the title says, we’re looking for a way to catch all exceptions from a piece of C++ code, and wrap this in a dll. This way we can shield of the application that uses this dll, from any errors occurring in this dll.

However, this does not seem possible with C++ under Windows.


void function()
        std::list<int>::iterator fd_it;
    } catch(...) {}

The exception that occurs is not caught by the standard C++ try/catch block, nor by any SEH translator function set by _set_se_translator(). Instead, the DLL crashes, and the program that uses the DLL is aborted. We compiled with Visual C++ 2005, with the option /SHa. Does anyone know if it’s possible in C++/Win32 to catch these kind of problems and make a rocksolid DLL wrapper?

Best Solution

The only way to make a rock solid DLL wrapper is to load the buggy DLL in another process, so that if it crashes it doesn't take your primary process down with it.

Catching all C++ exceptions seems reasonable, but catching all structured exceptions is another story. SEH might seem to get you most of the way there, because it allows you to catch access violations, divide-by-zero exceptions, etc.

But what if the buggy DLL happens to touch an uncommitted page from another thread's stack? The memory access will page fault, the exception handler will be invoked, and now that page is no longer a guard page. When that thread needs to grow the stack, it will get an access violation, and the process will crash. (These posts describe this case in more detail.)

Another likely problem: the buggy DLL crashes while holding a synchronization object, but you use SEH to catch the exception. If your process attempts to acquire the same synchronization object, then it deadlocks instead of crashing. The shared synchronization object may be part of the C runtime or the OS: what if buggy DLL 1 loads buggy DLL 2, which crashes in its DllMain() while buggy DLL 1 is holding the loader lock? Will your process deadlock the next time it loads a DLL?

For more information on why this (and functions like IsBadReadPtr(), which have similar problems) is a misuse of SEH:

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