TlsAlloc

The TlsAlloc function allocates a thread local storage (TLS) index. Any thread of the process can subsequently use this index to store and retrieve values that are local to the thread.

DWORD TlsAlloc(VOID);

Parameters

This function has no parameters.

Return Values

If the function succeeds, the return value is a TLS index initialized to zero.

If the function fails, the return value is -1. To get extended error information, call GetLastError.

Remarks

The threads of the process can use the TLS index in subsequent calls to the TlsFree, TlsSetValue, or TlsGetValue functions.

TLS indexes are typically allocated during process or dynamic-link library (DLL) initialization. Once allocated, each thread of the process can use a TLS index to access its own TLS storage slot. To store a value in its slot, a thread specifies the index in a call to TlsSetValue. The thread specifies the same index in a subsequent call to TlsGetValue, to retrieve the stored value.

The constant TLS_MINIMUM_AVAILABLE defines the minimum number of TLS indexes available in each process. This minimum is guaranteed to be at least 64 for all systems.

Windows 2000: There is a limit of 1088 TLS indexes per process.
Windows NT 4.0 and earlier: There is a limit of 64 TLS indexes per process.

TLS indexes are not valid across process boundaries. A DLL cannot assume that an index assigned in one process is valid in another process.

A DLL might use TlsAlloc, TlsSetValue, TlsGetValue, and TlsFree as follows:

1. When a DLL attaches to a process, the DLL uses TlsAlloc to allocate a TLS index. The DLL then allocates some dynamic storage and uses the TLS index in a call to TlsSetValue to store the address in the TLS slot. This concludes the per-thread initialization for the initial thread of the process. The TLS index is stored in a global or static variable of the DLL.
2. Each time the DLL attaches to a new thread of the process, the DLL allocates some dynamic storage for the new thread and uses the TLS index in a call to TlsSetValue to store the address in the TLS slot. This concludes the per-thread initialization for the new thread.
3. Each time an initialized thread makes a DLL call requiring the data in its dynamic storage, the DLL uses the TLS index in a call to TlsGetValue to retrieve the address of the dynamic storage for that thread.

For additional information on thread local storage, see Thread Local Storage.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Processes and Threads Overview, Process and Thread Functions, TlsFree, TlsGetValue, TlsSetValue