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In addition to what Michael posted, both APIs are adding messages to the thread's message queue, thus can be retrieved using GetMessage() and PeekMessage(). As MSDN says, on Vista there's a small exception for this behaviour:
"Microsoft Windows Vista and later.
...
If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine.
..."
Best regards,
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What about the other part of the question
Also if we want to direct a message to another process will both PostMessage/SendMessage work. What is the difference?
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Do you understand English? PostMessage() returns without waiting for the thread to process the message. SendMessage() does not return until the window procedure has processed the message. PostMessage() is a non-blocking call. SendMessage() is a blocking call. Regardless of Microsoft's implementation details, both calls go to the same place -- the window procedure of the window to which you are sending (or posting) the message.
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hi Tom,
did u get the answer for your question,
"if we want to direct a message to another process will both PostMessage/SendMessage work?"
If so, can you reply that to me?
I dont think postmessage/sendmessage works across processes.
it that right?
Thanks in advance.
cheers....!
Balaji
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tom groezer wrote: MSDN states that sending a message is done directly to a message windows procedure rather than message queue.
Hi again,
It seems others tend to paste MSDN snippets, however I'll try to explain it as I understand it. Lets just hope that I understand it correctly 
Its quite complicated... and this is only a summary.
SendMessage() ... It will completely bypass the message que and ultimately call the window Proc directly. The message is never placed into the message que. Instead it is placed into a seperate FIFO stack of "nonqueued messages" which is technically not the normal message que.
The following functions generate nonqueued messages:
SendMessage, SendMessageCallback, SendMessageTimeout, SendNotifyMessage, BroadcastSystemMessage, BroadcastSystemMessageEx... maybe more I dunno.
PostMessage()... always places the message into the standard windows message que and returns immediately.
tom groezer wrote: If that is true then is it true that GetMessage/PeekMessage is valid only for messages that are placed into message queue by postmessage.
GetMessage() and PeekMessage() checks for both queued and nonqueued messages. Meaning regardless of whether they are sent by SendMessage() or PostMessage() you can filter them.
tom groezer wrote: Also if we want to direct a message to another process will both PostMessage/SendMessage work. What is the difference?
You should use PostMessage(). If you use SendMessage() and the other application is blocked... your SendMessage() will never return. But there are alternatives... SendMessageTimeout() and such.
Best Wishes,
-Randor (David Delaune)
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David, in my opinion, your interpretation does not make any sense. Why? Because when SendMessage is used across thread boundaries, it cannot be processed in the sender's thread, thus... must be queued, and processed by the receiving window's thread, whenever it is reached. That's why SendMessage may hang if the receiver thread (other than sending one) is busy... not processing queued messages. If fact, I can tell you from experience that it happen once to redraw a window, but to return the wrong value from the window procedure, thus the paint message was not removed from the queue. In that case, any other thread using SendMessage to communicate with this window was blocked, not only the thread that created the window.
Your explaination with a secondary FIFO doesn't hold, because a thread's context can't change "suddenly". If there is a message queue (also FIFO) which GetMessage() and PeekMessage() are using, when and how a secondary FIFO can be processed? No OS would suspend a thread's execution while processing a message from the queue, or even while waiting for a new message, and "suddenly" call a window procedure with a message sent through SendMessage! Remember, I'm not talking here about sending a message to a window belonging to the same thread where the call is made.
Indeed, when the window belongs to the same thread, it makes perfect sense to call the window procedure immediatelly, because the developer would expect that SendMessage() returns only after the message is processed, so... the thread execution depends on the processing of that message, before any other message already existing in the queue at that point in time. I don't even understand why MSDN says that such behaviour is for "Vista or later", because, when I think about it,... it cannot be otherwise.
Regards,
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Thanks David for the links... especially the second one!
It is interesting to see different opinions about an "old new subject" such as message queueing. We might fight over nothing here, since I kinda agree with one of the conclusions mentioned in that discussion thread: it seems to be an implementation detail. Point taken regarding the non-queueing messages! Indeed, sent messages are different since are not stored as messages posted, and also not returned by GetMessage. My logic still says that they are "somehow" queued... and how is indeed "an implementation" detail. All this confusion is set by MSDN, which is not comprehensive on the subject, and also contradicting itself in the documentation of GetQueueStatus()[^], when describing QS_SENDMESSAGE. As you well described, sent messages are different then posted ones, having higher priority, and still being queued (maybe separately, or could be inserted ahead of the posted ones) because they can't be processed by receiving thread "immediately" (as MSDN is misleading us).
Please have a look at "The Algorithm for Extracting Messages from a Thread's Queue" section at "Waking a Thread[^]" sub-chapter from a book called "Programming Applications for MS Windows, 4th edition" by Jeffrey Richter, and tell me if you agreed with what says there... at least partially. It was published by Microsoft Press[^]!
Thanks,
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Hi All,
I am debugging my application by printing the Working Set at the specific point with the function provided in psapi.h
My purpose is to check which part of my application used up the memory.
However, just look at the Working Set is not very conclusive. I found if allocated 1k memory in the app but the Working Set may not grow up 1k accordingly. It will grow eventually(say allocated 10 times 1k, Working Set will grow up around 10k.)
In the psapi.h, there are several values such PageFault, PagedPool, Pagefile... i can see those values will change each time when allocating memory. I guess the result of memory allocation action has reflected in those values.
What i want to ask is any formula to calculate the exact memory usage of the application? say, i do "new char[128]" the memory usage result will grow up 128byte immediately.
Any reply will be appreciated!
Thanks!
Jim
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When you allocate 128 bytes through "new char[128]", you may not commit any new page of memory!!!
My point is that memory allocation is done by the heap manager used by your module. For example, if CRT is the heap manager (msvcrt.dll) you're linking with, all "malloc / new" kind of instructions will be handled by that library, which may reuse previously released memory, or return an address of already commited memory. The heap manager keeps track of your allocations, and commits pages of memory only when needed. For example, on first allocation, a number of pages are commited to support the request. On those pages there's still room for more allocations. Thus, next allocation, if small enough, will not commit more pages, but use the existing commited pages.
There're plenty of online resources explaining how a "heap manager" implementation works:
http://blogs.technet.com/askperf/archive/2007/06/26/what-a-heap-of-part-one.aspx[^]
http://en.wikipedia.org/wiki/Chunking_(computing)[^]
http://www.blackhat.com/presentations/bh-usa-06/BH-US-06-Marinescu.pdf[^]
http://www.freshpatents.com/Methods-systems-and-computer-program-products-for-managing-a-memory-by-storing-external-objects-in-a-dynamic-heap-dt20051215ptan20050278496.php[^]
Best regards,
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Does WM_COPYDATA use some kind of shared memory concept when it uses the structure COPYDATASTRUCT?
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When sending WM_COPYDATA message to another application, lParam must point to a COPYDATASTRUCT, and the passed data must not contain any addresses (pointers or references) that would not be available in the other process. Until the thread sending the message returns from the call, the data structure should be either locked or not modified by other threads of the calling process. The data received by the other process cannot modify / return any of the structure's content. The reason behind all these is that the OS is copying the given structure as a block, from the calling process to the receiving process. It is like calling WriteProcessMemory, on some temp variable (thread stack address) in the target process. The communication is one way!
Regards,
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While using the structure could we say that the OS implements one as a shared memory?
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Yes.
The way I understand it... when you post a WM_COPYDATA message, the kernel creates a temporary memory mapped file containing the bytes at the location specified relative to the senders address space. This memory mapped file is accessable to the recieving application. Upon SendMessage() returning the memory mapped file is marked for deletion.
Best Wishes,
Randor (David Delaune)
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Hmm,... reading a few times what MSDN, I understood that you cannot return data in the buffer given by the sending application... but, since I've never tried myself, I would consider David's interpretation. Have already found some sample code on CP using such technique to share memory... 2 ways.
Well, it wouldn't be the first time when MSDN is missleading!
Still, the opinions found are divided almost 50-50. If you're planning to try it out, please let us know about your findings. It would be great if you can try it out on different OS versions.
While searching more info about it, I came across an interesting aspect:
http://www.flounder.com/wm_copydata.htm[^]
Hope you'll find this idea useful, and couldn't hurt using a GUID as explained there.
Best wishes,
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hi all
i wana add my control icon into vc++ toolbox control. ex: i have my own button class drived from CButton. is there any way to put my button's icon in toolbox and than make vc++ ide to produce my button's source code? i mean the new button class will be drived from mybutton class. (sorry for my pure english  )
thx much.
EmptyBox
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Try Project --> Add to Project --> Components and Controls.
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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thx DavidCrow.
i think it works in vc++ 6.0. does it work in 2003, too?
EmptyBox
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eagle51 wrote: does it work in 2003, too?
I don't know. Mark Salsbery will have to answer that one.
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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not sure if this is the right place, but i need a script to login and download some results on HyperTerminal. is there a script that can be used?
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What are the best ways to send messages from one process to other process?
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The "best" way depends on the message type/size/etc.
For messages between windows or threads that can fit in two INT_PTR-sized variables there's
PostMessage()/PostThreadMessage() (note you can't pass pointers).
For larger "messages" between windows there's the WM_COPYDATA message.
For anything else...
Interprocess Communications[^]
This episode brought to you by the letter N
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What does a notepad open two similar process when invoked on a single file. What is the rational. It looks logical to do that when doing it over a network between two machines.
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tom groezer wrote: What does a notepad open two similar process when invoked on a single file.
Like other text editors/word processors, Notepad does not actually keep the file open. Therefore, when opening the same file more than once, Notepad does not communicate with other instances of itself to see what they are doing.
"A good athlete is the result of a good and worthy opponent." - David Crow
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Is there a way to set a size for message queue? Also is there any performance consideration to be made while selecting PeekMessage or GetMessage.
-- modified at 5:13 Sunday 8th July, 2007
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