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Posted 27 Nov 2011

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How to post a thread message to a console application

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4.94/5 (10 votes)
28 Nov 2011CPOL1 min read
Console threads can be promoted to GUI thread and receive thread messages
Although it is correct that console applications initially do not have a message queue... the first time a win32k.sys/GDI syscall greater or equal to 0x1000 is invoked... the Windows kernel invokes KiConvertToGuiThread and increases the thread stack-size which will convert the console thread into a GUI thread with a message queue.

How do you 'invoke a system call above 0x1000'?

WIN32K.SYS System Call Table[^]

As you can see in that table... the first GDI system call NtGdiAbortDoc is at index 0x1000. Because we know that the GetMessage function[^] results in a call to NtUserGetMessage and the DispatchMessage function[^] results in a call to NtUserDispatchMessage. We can infer that by simply creating a message loop... we will have in-fact promoted the thread into a GUI thread.

In other words... by simply attempting to pump the message queue... the thread is given one.

MSG msg;
while(GetMessage(&msg, NULL, 0, 0))

So what do you have now? Now you have a console process with a thread containing a message queue that has no window HANDLE so cannot receive window messages. Is this the end of the road?

No, window messages are not the only type of messages. You can still use the PostThreadMessage function[^] to post thread messages to the console.

Step 1: Create two console applications... Sender and Receiver.
Step 2: In the receiver, begin an infinite message pump (console thread will be promoted to GUI thread by the subsystem)
Step 3: In the sender, call CreateProcess with the CREATE_NEW_CONSOLE flag that creates the receiver process. (otherwise, messages will appear in senders console!)
Step 4: Using the PROCESS_INFORMATION.dwThreadId, begin posting messages with PostThreadMessage to receiver from sender.

A complete working sample on my blog:

This tip does not address issues such as unique application-defined messages[^]

Best wishes,
-David Delaune


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralReason for my vote of 5 Good stuff :) I loved the win32k.sys... Pin
Yiannis Spyridakis29-Nov-11 3:00
MemberYiannis Spyridakis29-Nov-11 3:00 

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