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. It is accessed from the FS segment register when operating on 32 bits, and from GS in 64 bits.

Contents of the TIB on Windows[edit]




(32 bits, FS)


(64 bits, GS)

Windows Versions Description
pointer FS:[0x00] GS:[0x00] Win9x and NT Current Structured Exception Handling (SEH) frame
pointer FS:[0x04] GS:[0x08] Win9x and NT Stack Base / Bottom of stack (high address)
pointer FS:[0x08] GS:[0x10] Win9x and NT Stack Limit / Ceiling of stack (low address)
pointer FS:[0x0C] GS:[0x18] NT SubSystemTib
pointer FS:[0x10] GS:[0x20] NT Fiber data
pointer FS:[0x14] GS:[0x28] Win9x and NT Arbitrary data slot
pointer FS:[0x18] GS:[0x30] Win9x and NT Linear address of TEB
---- End of NT subsystem independent part ----
pointer FS:[0x1C] GS:[0x38] NT Environment Pointer
pointer FS:[0x20] GS:[0x40] NT Process ID (in some windows distributions this field is used as 'DebugContext')
4 FS:[0x24] GS:[0x48] NT Current thread ID
4 FS:[0x28] NT Active RPC Handle
4 FS:[0x2C] Win9x and NT Linear address of the thread-local storage array
4 FS:[0x30] GS:[0x60] NT Linear address of Process Environment Block (PEB)
4 FS:[0x34] GS:[0x68] NT Last error number
4 FS:[0x38] NT Count of owned critical sections
4 FS:[0x3C] NT Address of CSR Client Thread
4 FS:[0x40] NT Win32 Thread Information
124 FS:[0x44] NT, Wine Win32 client information (NT), user32 private data (Wine), 0x60 = LastError (Win95), 0x74 = LastError (WinME)
4 FS:[0xC0] NT Reserved for Wow64. Contains a pointer to FastSysCall in Wow64.
4 FS:[0xC4] NT Current Locale
4 FS:[0xC8] NT FP Software Status Register
216 FS:[0xCC] NT, Wine Reserved for OS (NT), kernel32 private data (Wine)
herein: FS:[0x124] 4 NT Pointer to KTHREAD (ETHREAD) structure
4 FS:[0x1A4] NT Exception code
18 FS:[0x1A8] NT Activation context stack
24 FS:[0x1BC] NT, Wine Spare bytes (NT), ntdll private data (Wine)
40 FS:[0x1D4] NT, Wine Reserved for OS (NT), ntdll private data (Wine)
1248 FS:[0x1FC] NT, Wine GDI TEB Batch (OS), vm86 private data (Wine)
4 FS:[0x6DC] NT GDI Region
4 FS:[0x6E0] NT GDI Pen
4 FS:[0x6E4] NT GDI Brush
4 FS:[0x6E8] NT Real Process ID
4 FS:[0x6EC] NT Real Thread ID
4 FS:[0x6F0] NT GDI cached process handle
4 FS:[0x6F4] NT GDI client process ID (PID)
4 FS:[0x6F8] NT GDI client thread ID (TID)
4 FS:[0x6FC] NT GDI thread locale information
20 FS:[0x700] NT Reserved for user application
1248 FS:[0x714] NT Reserved for GL
4 FS:[0xBF4] GS:[0x1250] NT Last Status Value
532 FS:[0xBF8] GS:[0x1258] NT Static UNICODE_STRING buffer
pointer FS:[0xE0C] GS:[0x1478] NT Address of memory allocated for stack
pointer[] FS:[0xE10] GS:[0x1480] NT TLS slots, 4/8 bytes per slot, 64 slots
8 FS:[0xF10] GS:[0x1680] NT TLS links (LIST_ENTRY structure)
4 FS:[0xF18] NT VDM
4 FS:[0xF1C] NT Reserved for RPC
4 FS:[0xF28] 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() {
    __asm mov EAX, FS:[18h]
// Using Microsoft's intrinsics instead of inline assembly (works for both X86 and X64 architectures)
void *getTIB() {
#ifdef _M_IX86
    return (void *)__readfsdword(0x18);
#elif _M_AMD64
    return (void *)__readgsqword(0x30);

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]