Win32 Thread Information Block

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In computing, the Win32 Thread Information Block (TIB) is a data structure in Win32 on x86 that stores information about the currently running thread. This structure is also known as the Thread Environment Block (TEB).[1]

The TIB is officially undocumented for Windows 9x. The Windows NT series DDK includes a struct NT_TIB in winnt.h that documents the subsystem independent part. Wine includes declarations for the extended (subsystem-specific part of) TIB.[citation needed] Yet so many Win32 programs use these undocumented fields that they are effectively a part of the API. The first field, in particular, is directly referenced by the code produced by Microsoft's own compiler.[1]

The TIB can be used to get a lot of information on the process without calling Win32 API. Examples include emulating GetLastError(), GetVersion(). Through the pointer to the PEB one can obtain access to the import tables (IAT), process startup arguments, image name, etc.

Contents of the TIB (32-bit Windows)[edit]

Position Bytes Windows Versions Description
FS:[0x00] 4 Win9x and NT Current Structured Exception Handling (SEH) frame
FS:[0x04] 4 Win9x and NT Stack Base / Bottom of stack (high address)
FS:[0x08] 4 Win9x and NT Stack Limit / Ceiling of stack (low address)
FS:[0x0C] 4 NT SubSystemTib
FS:[0x10] 4 NT Fiber data
FS:[0x14] 4 Win9x and NT Arbitrary data slot
FS:[0x18] 4 Win9x and NT Linear address of TEB
---- End of NT subsystem independent part ----
FS:[0x1C] 4 NT Environment Pointer
FS:[0x20] 4 NT Process ID (in some windows distributions this field is used as 'DebugContext')
FS:[0x24] 4 NT Current thread ID
FS:[0x28] 4 NT Active RPC Handle
FS:[0x2C] 4 Win9x and NT Linear address of the thread-local storage array
FS:[0x30] 4 NT Linear address of Process Environment Block (PEB)
FS:[0x34] 4 NT Last error number
FS:[0x38] 4 NT Count of owned critical sections
FS:[0x3C] 4 NT Address of CSR Client Thread
FS:[0x40] 4 NT Win32 Thread Information
FS:[0x44] 124 NT, Wine Win32 client information (NT), user32 private data (Wine), 0x60 = LastError (Win95), 0x74 = LastError (WinME)
FS:[0xC0] 4 NT Reserved for Wow64. Contains a pointer to FastSysCall in Wow64.
FS:[0xC4] 4 NT Current Locale
FS:[0xC8] 4 NT FP Software Status Register
FS:[0xCC] 216 NT, Wine Reserved for OS (NT), kernel32 private data (Wine)
herein: FS:[0x124] 4 NT Pointer to KTHREAD (ETHREAD) structure
FS:[0x1A4] 4 NT Exception code
FS:[0x1A8] 18 NT Activation context stack
FS:[0x1BC] 24 NT, Wine Spare bytes (NT), ntdll private data (Wine)
FS:[0x1D4] 40 NT, Wine Reserved for OS (NT), ntdll private data (Wine)
FS:[0x1FC] 1248 NT, Wine GDI TEB Batch (OS), vm86 private data (Wine)
FS:[0x6DC] 4 NT GDI Region
FS:[0x6E0] 4 NT GDI Pen
FS:[0x6E4] 4 NT GDI Brush
FS:[0x6E8] 4 NT Real Process ID
FS:[0x6EC] 4 NT Real Thread ID
FS:[0x6F0] 4 NT GDI cached process handle
FS:[0x6F4] 4 NT GDI client process ID (PID)
FS:[0x6F8] 4 NT GDI client thread ID (TID)
FS:[0x6FC] 4 NT GDI thread locale information
FS:[0x700] 20 NT Reserved for user application
FS:[0x714] 1248 NT Reserved for GL
FS:[0xBF4] 4 NT Last Status Value
FS:[0xBF8] 532 NT Static UNICODE_STRING buffer
FS:[0xE0C] 4 NT Pointer to deallocation stack
FS:[0xE10] 256 NT TLS slots, 4 byte per slot
FS:[0xF10] 8 NT TLS links (LIST_ENTRY structure)
FS:[0xF18] 4 NT VDM
FS:[0xF1C] 4 NT Reserved for RPC
FS:[0xF28] 4 NT Thread error mode (RtlSetThreadErrorMode)

FS maps to a TIB which is embedded in a data block known as the TDB (thread data base). The TIB contains the thread-specific exception handling chain and pointer to the TLS (thread local storage.) The thread local storage is not the same as C local storage.

Note: The above description ONLY refers to 32-bit Windows on x86. On x86-64 (64-bit) Windows, GS (and not FS) is used as the segment register that points to the TIB. Additionally some of the variable slots in the structure above have a different size (typically 8 instead of 4 bytes for pointer-sized data slots).

Accessing the TIB[edit]

The TIB of the current thread can be accessed as an offset of segment register FS (x86) or GS (x64).

It is not common to access the TIB fields by an offset from FS:[0], but rather first getting a linear self-referencing pointer to it stored at FS:[0x18]. That pointer can be used with pointer arithmetics or be cast to a struct pointer.

Example in C inlined-assembly for 32-bit x86:

// gcc (AT&T-style inline assembly).
void *getTIB() {
    void *pTIB;
    __asm__("movl %%fs:0x18, %0" : "=r" (pTIB) : : );
    return pTIB;
// Microsoft C
void *getTIB() {
    void *pTIB;
    __asm {
        mov EAX, FS:[0x18]
        mov pTIB, EAX
    return pTIB;
// Using Microsoft's intrinsics instead of inline assembly
void *getTIB() {
    return (void *)__readfsdword(0x18);

See also[edit]


  1. ^ a b Pietrek, Matt (May 1996). "Under The Hood". Microsoft Systems Journal. Retrieved 2010-07-07. 

Further reading[edit]

External links[edit]