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Using Thread Local Storage in a Dynamic-Link Library

This section shows the use of a DLL entry-point function to set up a thread local storage (TLS) index to provide private storage for each thread of a multithreaded process.

The TLS index is stored in a global variable, making it available to all of the DLL functions. This example assumes that the DLL’s global data is not shared, because the TLS index is not necessarily the same for each process that loads the DLL.

The entry-point function uses the TlsAlloc function to allocate a TLS index whenever a process loads the DLL. Each thread can then use this index to store a pointer to its own block of memory.

When the entry-point function is called with the DLL_PROCESS_ATTACH value, the code performs the following actions:

Each time the process creates a new thread, the entry-point function is called with the DLL_THREAD_ATTACH value. The entry-point function then allocates a block of memory for the new thread and stores a pointer to it by using the TLS index.

When a function requires access to the data associated with a TLS index, specify the index in a call to the TlsGetValue function. This retrieves the contents of the TLS slot for the calling thread, which in this case is a pointer to the memory block for the data. When a process uses load-time linking with this DLL, the entry-point function is sufficient to manage the thread local storage. Problems can occur with a process that uses run-time linking because the entry-point function is not called for threads that exist before the LoadLibrary function is called, so TLS memory is not allocated for these threads. This example solves this problem by checking the value returned by the TlsGetValue function and allocating memory if the value indicates that the TLS slot for this thread is not set.

When each thread no longer needs to use a TLS index, it must free the memory whose pointer is stored in the TLS slot. When all threads have finished using a TLS index, use the TlsFree function to release the index.

When a thread terminates, the entry-point function is called with the DLL_THREAD_DETACH value and the memory for that thread is freed. When a process terminates, the entry-point function is called with the DLL_PROCESS_DETACH value and the memory referenced by the pointer in the TLS index is freed.

The following code demonstrates the use of the DLL functions defined in the previous example.

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GetGUIThreadInfo function (winuser.h)

Retrieves information about the active window or a specified GUI thread.

Syntax

Parameters

The identifier for the thread for which information is to be retrieved. To retrieve this value, use the GetWindowThreadProcessId function. If this parameter is NULL, the function returns information for the foreground thread.

A pointer to a GUITHREADINFO structure that receives information describing the thread. Note that you must set the cbSize member to sizeof(GUITHREADINFO) before calling this function.

Return value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

This function succeeds even if the active window is not owned by the calling process. If the specified thread does not exist or have an input queue, the function will fail.

This function is useful for retrieving out-of-context information about a thread. The information retrieved is the same as if an application retrieved the information about itself.

For an edit control, the returned rcCaret rectangle contains the caret plus information on text direction and padding. Thus, it may not give the correct position of the cursor. The Sans Serif font uses four characters for the cursor:

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To get the actual insertion point in the rcCaret rectangle, perform the following steps.

The function may not return valid window handles in the GUITHREADINFO structure when called to retrieve information for the foreground thread, such as when a window is losing activation.

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