I have a process A that generates HBITMAP GDI objects to be painted on the screen. I have another process B which wants to display the content of images that process A creates.
I plan to do the communications/talking using Point-to-Point message queue or by using other message passing; and use shared memory (along with mutex and or events) to share data.
How do I share image data? I read here that the handles of GDI objects are not guaranteed to be shared amongst processes. Sharing using files is not really an option since the images keep changing (but I still consider it if there is no other way).
Adding more to the ingredients, process A is written using eVC4 (PPC2003 SDK) and process B is written using VS2005 (WM 6 Pro SDK). I have working source code of both applications so I can make some modifications to both but not migrating app A to VS2005 for the time being.
Is COM IImage an option?
I prefer native codes but also consider managed codes.
Use CreateDIBSection with a shared memory object HANDLE as the section handle. The HBITMAP you get back from CreateDIBSection may not be sharable with the other process (I don't know), but if the other process ALSO Creates a DIB Section from the same shared memory object, then the bitmap bits will be shared.
Related
How to create a device independent bitmap in windows using win32/mfc that will be shared among all processes running on the machine?
Looking for a best and fastest way of sharing a DIB between all processes on windows XP/7 machine. Processes should be able to lock the contents of bitmap and make drawing on this bitmap and other processes can use this bitmap for reading also.
For e.g. initially this DIB will be created by a main application. When some other process want to draw something on this process, can lock the contents of this DIB and draw on this bitmap. If some other process want to read the contents of this bitmap, can lock the bitmap and read the bitmap.
Please suggest a best way both in time complexity and space complexity manner. Space complexity means, a process which wants to draw contents on this bitmap should not need to copy all contents on local memory and draw, it should directly be able to take this bitmap in device context and draw in the bitmap.
There is, unfortunately*, no supported way to share GDI handles (such as to bitmaps) between processes.
There is a supported way however to get multiple bitmaps (in multiple processes) to share the same storage.
In your primary process, create a memory section using the CreateFileMapping API.
You have several ways to get the section handle to your various processes -
The simplest of which (as pointed out by Hans Passant) is to simply name the section when calling CreateFileMapping... and then using OpenFileMapping.
If you have a main process that launches the other processes ensure that the section is created with the bInheritHandle of the SECURITY_ATTRIBUTES set to TRUE and the handle will be automatically duplicated into any sub processes - its usual to pass the handle value on the command line of the new process.
Otherwise use DuplicateHandle function to copy the handle into other processes - but you will still need some other kind of IPC to get the handle to the process.
However it happens - You can then call CreateDIBSection in each process to create GDI bitmaps that are backed by the same memory section. Note the comments on synchronizing access to the bitmap. If you have multiple processes trying to write to the bitmap you might need to serialize access at that level.
As an ironic note: Because Win32 is based on Win16 there are a lot of Win16 legacy APIs that deal with window messages and the clipboard that do in fact expect HBITMAP's to be usable from multiple processes. Also (as an implementation detail) on Windows NT 5.x and 6.x bitmaps are allocated by kernel mode drivers from a single system wide handle table and are thus technically valid in any process - However, GDI also stores the process ID of each GDI object in that table and GDI API calls explicitly check the process ID and fail out if called on a GDI handle belonging to another process.
I'm building a dll in C++ to be imported into a VB project. The dll exposes a function that will extract the preview image of a specified document through the Win Shell, and then writes it into a buffer to be used by the VB app.
So far, I'm able to get the HBITMAP handle without issue. I was also able to extract the bitmap data using the GetBitmapBits function. However, this will not work as the object that requires the bit data needs it to be in a device independent format.
I've read that GetDiBits is the function to use for this purpose, but I'm having a hard time figuring out how to call the function successfully.
What device context should I be using? Also, how can I ensure that it writes out the data in device independent format?
Thanks in advance.
If your HBITMAP references a device dependent bitmap, and you need device independent bits, you have to convert them. Try creating a CImage and call the BitBlt method to put device independent bits in it.
Goal
Pass images generated by one process efficiently and at very high speed to another process. The two processes run on the same machine and on the same desktop. The operating system may be WinXP, Vista and Win7.
Detailed description
The first process is solely for controlling the communication with a device which produces the images. These images are about 500x300px in size and may be updated up to several hundred times per second. The second process needs these images to process them. The first process uses a third party API to paint the images from the device to a HDC. This HDC has to be provided by me.
Note: There is already a connection open between the two processes. They are communicating via anonymous pipes and share memory mapped file views.
Thoughts
How would I achieve this goal with as little work as possible? And I mean both work for the computer and me (of course ;)). I am using Delphi, so maybe there is some component available for doing this? I think I could always paint to any image component's HDC, save the content to memory stream, copy the contents via the memory mapped file, unpack it on the other side and paint it there to the destination HDC. I also read about a IPicture interface which can be used to marshal images. I need it as quick as possible, so the less overhead the better. I don't want the machine to be stressed just by copying some images.
What are your ideas? I appreciate every thought on this!
Use a Memory Mapped File.
For a Delphi reference see Memory-mapped Files in Delphi and Shared Memory in Delphi.
For a more versatile approach you can look at using pipes or sending bitmap data via TCP. This would allow you to distribute the image data between nodes more easily, if necessary.
Use shared memory to pass the image data, and something else (named pipes, sockets, ...) to coordinate the handover.
In some cases, you can pass HBITMAP handles across processes. I've seen it done before (yes, on XP/Vista), and was surprised as everyone else on the team when one of my co-workers showed me.
If memory serves me correctly, I believe it will work if the HBITMAP was allocated with one of the GDI function (CreateBitmap, CreateCompatibleBitmap,CreateDIBitmap,etc...) HBIMAP handles created by LoadBitmap will not work as it's just a pointer to an in-proc resource.
That, and I think when you share the HBITMAP across to the other process, don't try to do anything special with it other than normal BitBlt operations.
At least that's what I remember. We got lucky because our graphic libraries were already written to manage all images as HBITMAPs.
YMMV
Ok it seems as if memory mapped files and pipes are the right way to go. That is not too bad because the two processes already share a MMF and two pipes (for bidirectional communication). The only thing left to solve was how to pass the data with as little copy operations as possible.
The design which works quite well looks as follows (sequential flow):
Process 1 (wants image)
give signal to process 2 (via pipe 1) to store image in shared memory
go to sleep and wait for response (blocking read from pipe 2)
Process 2 (provides images)
on signal (via pipe 1) wake up and tell hardware device to paint to HDC 1 (this is backed by shared memory, see below)
give signal to process 1 (via pipe 2)
go to sleep and wait for new job (via pipe 1)
Process 1 (wants image)
on signal (via pipe 2) wake up and paint from shared memory to destination HDC 2
Now for the image transfer via shared memory (my goal was to use not more than one additional copy operation):
Process 2 creates a HBITMAP via CreateDIBSection and provides the handle of the file mapping and the offset of the mapped view. Thus the image data lives in the shared memory. This creates an HBITMAP which is selected into HDC 1 (which is also created by process 2) and which will be used from now on by process 2.
Process 1 uses StretchDIBits with a pointer to the mapped view's memory (as described here). This seems to be the only function for getting bits from memory directly into another HDC (in this case HDC 2). Other functions would copy them first into some intermediary buffer before you could transfer them from there to the final HDC.
So in the end it seems the bits needed to be transferred are about twice as much as in the beginning. But I think this is as good as it gets unless sharing GDI handles between processes would be possible.
Note: I used pipes instead of signals because I need to transfer some additional data, too.
As I can see this, you have two options:
Pass only the image handle / pointer to other process, so both processes work only on one collection of images.
Copy the image content to other process and work on a copy from then on.
Which approach is best depends on your design. Best tool for both approaches would be "memory mapped files", or "named pipes". This are the fastest you can get. Memory mapped files are probaly the fastest form of inter process communication but have the donwside that there is no "client-server" paradigm build into them. So you have to synchronize the acces to MMF yourself. Named pipes on the other hand are almost as fast but have the client-server paradigm build right into them. The difference in speed comes mainly from that.
Now because of the share speed of the updates, the first approach could be better, but then you have to watch out for synchronization between processes, so they do not read / write to single image at the same time. Also some sort of caching or other smart tehniques could be used, so you reduce your traffic to minimum. When facing such high level of communications there is always advisable to look for means of reducing that level if possible.
For a very fast implementation of IPC based on named pipes you can use my IPC implementation. It is message oriented so you do not have to worry about pipe technical details. It also uses thread pool behind the scenes and has mininal additional overhead.
You can stress test it and see for yourself (a typical message takes 0.1 ms for full client-server request-response cycle).
I am writing some kind of IPC functionality and need to pass certain resources from one process to another. This works well for Pipe handles etc. which can be duplicated via DuplicateHandle. Now I need to pass a HDC from one process to the other. Is this even possible? If yes: how?
Sub-Question: I am assuming passing window handles (HWND) from one process to the other is safe. Is this assumption correct?
HWNDs can be shared between processes, SendMessage() wouldn't work otherwise. They are however scoped to a specific desktop, a desktop is associated with a session. There is one session for each logged-in user. And session 0 is special, the session in which services run. And there's a secure desktop, the one you see at login or when you press Ctrl+Alt+Del, you cannot mess with the password entry box. But as long as both processes run in the same desktop you won't have any trouble.
HDCs are murky, never tried that. I wouldn't recommend it. You can always create one from a HWND with GetDC().
All GDI handles are stored in a table that is mapped into every process. The entries in the table contain the process id of the owning process, and this is checked on every GDI access to the handle.
So, (ironically), GDI handles - including HDCs - are valid system wide. But can only be used from the process that created them.
This Page documents the process affinity of GDI objects. Of course, as a counter point it is worth noting that some COM functions, and window messages like WM_PRINT do not have any interprocess restrictions and they ARE passed HDC's, so they clearly have to do something behind the scenes to marshal the HDC from one process to the next.
Assuming, that you want to paint onto a HDC belonging to one process from another process (e.g. by using BitBlt) then as pointed out by nobugz and Chris Becke you cannot share that HDC across process boundaries. But, further assuming that the HDC of that one process belongs to a window (and your intention is to finally draw onto that window) then you can pass that window handle to the other process and in this process use GetDc to obtain a HDC. With this HDC you can then paint onto the window of the other process.
I'll try to be brief, but fully descriptive:
This is Windows-specific. Using the Windows Driver Development Kit (DDK).
I am writing a Kernel Mode Driver (KMD) for the first time, having no prior experience in Kernel Mode. I am playing around currently with the "scanner" mini-filter sample which comes with the DDK, and expanding upon it for practice. The "scanner" mini-filter is a basic outline for a generic "anti-virus" type scanning driver which hooks file creates/closes and operates on the associated file to scan for a "bad word" before approving/denying the requested operation.
The end goal is to scan the file with the user-mode application when it is opened, deciding whether or not the mini-filter should allow the operation to complete, without noticeable slow-down to the process or user which is attempting to open the file. I will also want to scan the entire file again when a save is attempted to decide whether or not to allow the save to complete successfully or deny the save. The mini-filter sample lays out the groundwork for how to hook these calls, but is a bit weak in the actually "scanning" portion.
I am looking at expanding the sample to scan the entire file that has been opened, such as to generate a hash, rather than just the first 1k (the sample's limit). I have modified the sample to read the entirety of the file and send it using the same mechanisms within the original sample. This method uses FltReadFile to read the file within the KMD and FltSendMessage to send the buffer to the user-mode component. The user-mode application is using GetQueuedCompletionStatus to grab the notifications from the KMD and process the buffers.
However, I'm noticing that this process seems to be pretty slow compared to a normal open/read in C++ using the standard library (fstream). This method is taking between approximately 4-8 times longer than simplying opening and reading the file in a simple C++ user app. I have adjusted buffer sizes to see if it makes for a noticeable improvement, and while it can help slightly, the benefits have not appeared to be very significant.
Since I am looking to scan files in 'real-time', this rate of transfer is highly disappointing and prohibitive. Is there a faster way to transfer a file's contents from a Kernel-Mode Driver to a User-Mode Application?
I can suggest several solutions:
Use DeviceIoControl with METHOD_OUT_DIRECT transfer type to pass large amounts of data.
Create memory section and map it to your process (remember about limited address space on 32-bit platforms).
Pass file path to your application and open it there.