TlsGetValue function (processthreadsapi.h)

Retrieves the value in the calling thread's thread local storage (TLS) slot for the specified TLS index. Each thread of a process has its own slot for each TLS index.

Syntax

LPVOID TlsGetValue(
  [in] DWORD dwTlsIndex
);

Parameters

[in] dwTlsIndex

The TLS index that was allocated by the TlsAlloc function.

Return value

If the function succeeds, the return value is the value stored in the calling thread's TLS slot associated with the specified index. If dwTlsIndex is a valid index allocated by a successful call to TlsAlloc, this function always succeeds.

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

The data stored in a TLS slot can have a value of 0 because it still has its initial value or because the thread called the TlsSetValue function with 0. Therefore, if the return value is 0, you must check whether GetLastError returns ERROR_SUCCESS before determining that the function has failed. If GetLastError returns ERROR_SUCCESS, then the function has succeeded and the data stored in the TLS slot is 0. Otherwise, the function has failed.

Functions that return indications of failure call SetLastError when they fail. They generally do not call SetLastError when they succeed. The TlsGetValue function is an exception to this general rule. The TlsGetValue function calls SetLastError to clear a thread's last error when it succeeds. That allows checking for the error-free retrieval of zero values.

Remarks

Windows Phone 8.1: This function is supported for Windows Phone Store apps on Windows Phone 8.1 and later. When a Windows Phone Store app calls this function, it is replaced with an inline call to FlsGetValue. Refer to FlsGetValue for function documentation.

Windows 8.1, Windows Server 2012 R2, and Windows 10, version 1507: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and Windows 10, version 1507. When a Windows Store app calls this function, it is replaced with an inline call to FlsGetValue. Refer to FlsGetValue for function documentation.

Windows 10, version 1511 and Windows 10, version 1607: This function is fully supported for Universal Windows Platform (UWP) apps, and is no longer replaced with an inline call to FlsGetValue.

TLS indexes are typically allocated by the TlsAlloc function during process or DLL initialization. After a TLS index is allocated, each thread of the process can use it to access its own TLS slot for that index. A thread specifies a TLS index in a call to TlsSetValue to store a value in its slot. The thread specifies the same index in a subsequent call to TlsGetValue to retrieve the stored value.

TlsGetValue was implemented with speed as the primary goal. The function performs minimal parameter validation and error checking. In particular, it succeeds if dwTlsIndex is in the range 0 through (TLS_MINIMUM_AVAILABLE– 1). It is up to the programmer to ensure that the index is valid and that the thread calls TlsSetValue before calling TlsGetValue.

Examples

For an example, see Using Thread Local Storage or Using Thread Local Storage in a Dynamic-Link Library.

Requirements

See also

Process and Thread Functions

Thread Local Storage

TlsAlloc

TlsFree

TlsSetValue

Vertdll APIs available in VBS enclaves