Question

I SEEM to be having an issue with GlobalLock in my application. I say seem because I haven’t been able to witness the issue by stepping through yet but when I let it run it breaks in one of two locations.

The app has multiple threads (say 2) simultaneously reading and writing bitmaps from PDF files. each thread handles a different file.

The first location it breaks I am reading a dib from the pdf to be OCRed. OCR is reading the characters on the bitmap and turning them into string data. The second location is when a new PDF is being created with the string data being added over the bitmap.

GlobalLock is being used on a HANDLE created by the following:

GlobalAlloc(GMEM_MOVEABLE, uBytes);

I either get an AccessViolationError (always in the first instance) or I get GlobalLock returning a NULL pointer. (The second occurance)

It seems like one file is being read and another is having a copy written at the same time. There seems to be no pattern to which files it happens on.

Now I understand that the VC++ runtime has been multithreaded since 2005 (I am using VS2010 with 2008 toolchain). But is GlobalLock part of the runtime? It seems to me more like a platform independent thing.

I want to avoid just putting a CRITICAL_SECTION around globallock and globalunlock to get them to work, or at least not know why I am doing so.

Can anyone inform me better about GlobalLock/Unlock?
-A fish out of water

Answer 1

First, the Global* heap routines are provided for compatibility with 16-bit windows. They still work, but there’s no real reason to use them anymore, except for compatibility with routines that still use global heap object handles. Note that GlobalLock/GlobalUnlock are not threading locks – they prevent the memory from moving, but multiple threads can GlobalLock the same object at the same time.

That said, they are otherwise thread-safe; they take a heap lock internally, so there is no need to wrap your own locking around every Global* call. If you are having problems like this, it suggests you may be trying to GlobalLock a freed object, or you may be corrupting the heap (heap overflows, use-after-free, etc). You may also be missing thread synchronization on the contents of the heap object – the Global* API does not prevent multiple threads from accessing or modifying the same object at once.