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what does this mean?

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Copy and pasted from MSDN:

WM_USER
The WM_USER constant is used by applications to help define private messages for use by private window classes, usually of the form WM_USER+X, where X is an integer value.

#define WM_USER 0x0400
Remarks
The following are the ranges of message numbers.

Range Meaning
0 through WM_USER – 1 Messages reserved for use by the system.
WM_USER through 0x7FFF Integer messages for use by private window classes.
WM_APP through 0xBFFF Messages available for use by applications.
0xC000 through 0xFFFF String messages for use by applications.
Greater than 0xFFFF Reserved by the system for future use.


Message numbers in the first range (0 through WM_USER – 1) are defined by the system. Values in this range that are not explicitly defined are reserved for future use by the system.

Message numbers in the second range (WM_USER through 0x7FFF) can be defined and used by an application to send messages within a private window class. These values cannot be used to define messages that are meaningful throughout an application, because some predefined window classes already define values in this range. For example, predefined control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use these values. Messages in this range should not be sent to other applications unless the applications have been designed to exchange messages and to attach the same meaning to the message numbers.

Message numbers in the third range (0x8000 through 0xBFFF) are available for application to use as private messages. Message in this range do not conflict with system messages.

Message numbers in the fourth range (0xC000 through 0xFFFF) are defined at run time when an application calls the RegisterWindowMessage function to retrieve a message number for a string. All applications that register the same string can use the associated message number for exchanging messages. The actual message number, however, is not a constant and cannot be assumed to be the same between different sessions.

Message numbers in the fifth range (greater than 0xFFFF) are reserved for future use by the system.

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Guest Anonymous Poster
This definition that you''ve pasted here is a custom made windows message by the origional programmer.

WM_ messages are just numbers like:

#define running 1
#define walking 2
#define sitting 3

The WM_USER value is the limit of all current windows messages. Meaning that anything after this WM_USER value can be a customized windows message for the programmer to use and call. So WM_USER+1 is the first number that can be used for this purpose.

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