hdc in shared mem

[Force Code]

Is there any way to create a memory device context that is valid between processes? If you do a HDC hdc = CreateCompatibleDC(...) its returning a pointer that's valid only for the current process address space. However, considering that DLL's can designate blocks of memory that are shared between processes, it seems you could have a mem DC shareable between processes as well.

[Mark Salsbery]

An HDC is only valid within the context of the process that created it. You can share the handle in a shared data segment but that won't make it valid to another process. What part(s) of the device context are you looking to share between processes? Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Force Code]

Mark Salsbery wrote:

An HDC is only valid within the context of the process that created it. You can share the handle in a shared data segment but that won't make it valid to another process. What part(s) of the device context are you looking to share between processes?

Just an HBITMAP, not fonts, or pens, etc. If one process does a hdc = CreateCompatibleDC(...) and SelectObject(hdc,hbmp), I want any other process to be able to BitBlt to/from it. This hdc is part of a C++ object I have defined in a shared memory segment of a DLL: G g_list __attribute__((section ("shared"), shared)); So, every process that accesses that DLL is accessing the exact same g_list and g_list members. The class doesn't contain pointers, with the exception of this hdc which I didn't realize was a local pointer.

[Waldermort]

Force Code wrote:

Just an HBITMAP

Sharing pointers between processes is a no-no, you need to share the data. In this case that data is called a "dib section".

Waldermort

[Mark Salsbery]

I was going to suggest the same thing as WalderMort - A DIB section through a named file mapping (in-memory). All the info you need to create the DIBSection and a memory DC in the second process can be shared. See CreateDIBSection()[^]. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Force Code]

Mark Salsbery wrote:

I was going to suggest the same thing as WalderMort - A DIB section through a named file mapping (in-memory). All the info you need to create the DIBSection and a memory DC in the second process can be shared. See CreateDIBSection()[^].

We must be looking at documentation near simultaneously, somehow. I saw the file mapping thing but ruled it out as too byzantine (for my tastes). What I did instead was to make hdc static in the class - which results in each process having its own copy - strangely. Since the class is in shared memory, making it nonstatic means every process has the same copy - static results in the opposite in this case. Then I have another new static member local_cnt. if local_cnt != (nonstatic)cnt then the static hdc is deleted and recreated (thus synchronizing with changes elsewhere.

[Waldermort]

Force Code wrote:

What I did instead was to make hdc static in the class

And in a few weeks you will be asking why your code is crashing unexpectedly.

Waldermort

[Mark Salsbery]

...or days :)

Mark Salsbery Microsoft MVP - Visual C++ :java: