The TlsGetValue function retrieves the value in the calling thread’s thread local storage (TLS) slot for a specified TLS index. Each thread of a process has its own slot for each TLS index.
LPVOID TlsGetValue(
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DWORD dwTlsIndex |
// TLS index to retrieve value for |
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); |
If the function succeeds, the return value is the value stored in the calling thread’s TLS slot associated with the specified index.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
Note that the data stored in a TLS slot can have a value of zero. In this case, the return value is zero and GetLastError returns NO_ERROR.
TLS indexes are typically allocated by the TlsAlloc function during process or DLL initialization. Once allocated, each thread of the process can use a TLS index to access its own TLS storage slot for that index. The storage slot for each thread is initialized to NULL. 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.
TlsSetValue and TlsGetValue were implemented with speed as the primary goal. These functions perform minimal parameter validation and error checking. In particular, this function 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.
Win32 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 NULL values.