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 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.
TlsGetValue always sets a thread's last error. In some cases, an application (such as those with custom heaps that support malloc) may need to call GetLastError before calling TlsGetValue to save the thread's last error (followed by SetLastError to restore the saved error). Unfortunately, this can incur a non-trivial performance cost on certain CPUs.
Windows 11 24H2 and later: Use the TlsGetValue2 function, which is identical to TlsGetValue except that it doesn't set the thread's last error. Applications calling TlsGetValue2 should avoid using 0 as a valid value because GetLastError cannot be called to check if TlsGetValue2 failed.
Examples
For an example, see Using Thread Local Storage or Using Thread Local Storage in a Dynamic-Link Library.
Requirements
| Requirement | Value |
|---|---|
| Minimum supported client | Windows XP [desktop apps | UWP apps] |
| Minimum supported server | Windows Server 2003 [desktop apps | UWP apps] |
| Target Platform | Windows |
| Header | processthreadsapi.h (include Windows.h on Windows Vista, Windows 7, Windows Server 2008 Windows Server 2008 R2) |
| Library | Kernel32.lib; WindowsPhoneCore.lib on Windows Phone 8.1 |
| DLL | KernelBase.dll on Windows Phone 8.1; Kernel32.dll |