How do I get at the exception information when using MiniDumpWriteDump out-of-process?

Question

When using the MiniDumpWriteDump function to create a core dump of a process on Windows, it is recommended (e.g. here, and here) that the MiniDumpWriteDump is run from another "watchdog" process because it may well not work when called from within the same process.

At the moment, our application is calling it in-process on an unhandled exception (we do it from a watchdog thread). Since we sometimes have problems with it not working, we'd like to move it to a separate process.

Now, signalling the other process to start writing the dump is trivial (just use an event, semaphore, you name it) but how do I pass the LPEXCEPTION_POINTERS info I get for the callback function I register with SetUnhandledExceptionFilter to the other process so that it can be passed to MiniDumpWriteDumps ExceptionParam argument??

Answer 1

You also need the MINIDUMP_EXCEPTION_INFORMATION.ThreadId value. The simplest way, and the way I made it work, is to use a memory-mapped file to transfer both the ThreadId and the ExceptionPointers. And a named event to wake up the watchdog. It doesn't matter that the pointer is not valid in the context of the watchdog process.

Use CreateFileMapping + MapViewOfFile in the watched process as part of its initialization, OpenFileMapping + MapViewOfFile in the watchdog. Your SetUnhandledExceptionFilter should then only call GetCurrentThreadId() and copy the tid and the pExcept to the memory mapped file view, call SetEvent() to wake up the watchdog and block forever until the watchdog terminates it.