Win32 Thread Information Block

In computing, the Win32 Thread Information Block (TIB) is a data structure in Win32 on x86 that stores information about the currently running thread. It is also known as the Thread Environment Block (TEB) for Win32. It descended from, and is backward-compatible on 32-bit systems with, a similar structure in OS/2.[1]

The TIB is officially undocumented for Windows 9x. The Windows NT series DDK (as well as the MinGW/ReactOS implementation) includes a struct NT_TIB in winnt.h that documents the subsystem independent part. Even before TIB was effectively documented, many applications have already started using its fields that they are effectively a part of the API. The first field containing the SEH frame, in particular, is directly referenced by the code produced by Microsoft's own compiler.[1] The Win32 subsystem-specific part of the TEB is undocumented, but Wine includes a TEB definition in winternl.h.[2]

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 on 32-bit Windows and GS on 64-bit Windows.

Contents of the TIB on Windows

This table is based on Wine's work on Microsoft Windows internals.[2]

Bytes/
Type		 offset (32-bit, FS) offset (64-bit, GS) Windows Versions Description
pointer FS:[0x00] GS:[0x00] Win9x and NT Current Structured Exception Handling (SEH) frame

Note: the 64-bit version of Windows uses stack unwinding done in kernel mode instead.

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; below are Win32-dependent
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] GS:[0x50] NT Active RPC Handle
4 FS:[0x2C] GS:[0x58] 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] GS:[0x6C] NT Count of owned critical sections
4 FS:[0x3C] GS:[0x70] NT Address of CSR Client Thread
4 FS:[0x40] GS:[0x78] NT Win32 Thread Information
124 FS:[0x44] GS:[0x80] NT, Wine Win32 client information (NT), user32 private data (Wine), 0x60 = LastError (Win95&98), 0x74 = LastError (WinME)
4 FS:[0xC0] GS:[0x100] NT Reserved for Wow64. Contains a pointer to FastSysCall in Wow64.
4 FS:[0xC4] GS:[0x108] NT Current Locale
4 FS:[0xC8] GS:[0x10C] NT FP Software Status Register
216 FS:[0xCC] GS:[0x110] NT, Wine Reserved for OS (NT), kernel32 private data (Wine)
herein: FS:[0x124] 4 NT Pointer to KTHREAD (ETHREAD) structure
4 FS:[0x1A4] GS:[0x2C0] NT Exception code
18 FS:[0x1A8] GS:[0x2C8] NT Activation context stack
24 FS:[0x1BC] GS:[0x2E8] NT, Wine Spare bytes (NT), ntdll private data (Wine)
40 FS:[0x1D4] GS:[0x300] NT, Wine Reserved for OS (NT), ntdll private data (Wine)
1248 FS:[0x1FC] GS:[0x350] NT, Wine GDI TEB Batch (OS), vm86 private data (Wine)
4 FS:[0x6DC] GS:[0x838] NT GDI Region
4 FS:[0x6E0] GS:[0x840] NT GDI Pen
4 FS:[0x6E4] GS:[0x848] NT GDI Brush
4 FS:[0x6E8] GS:[0x850] NT Real Process ID
4 FS:[0x6EC] GS:[0x858] NT Real Thread ID
4 FS:[0x6F0] GS:[0x860] NT GDI cached process handle
4 FS:[0x6F4] GS:[0x868] NT GDI client process ID (PID)
4 FS:[0x6F8] GS:[0x86C] NT GDI client thread ID (TID)
4 FS:[0x6FC] GS:[0x870] NT GDI thread locale information
20 FS:[0x700] GS:[0x878] NT Reserved for user application
1248 FS:[0x714] GS:[0x890] NT Reserved for GL (See wine ref for internals)[2]
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 Also known as DeallocationStack, it establishes the real start address of the stack buffer, hence the real stack limit: it is a few pages less than the stack limit field (which hides the guard pages used to detect stack overflows).
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] GS:[0x1690] NT VDM
4 FS:[0xF1C] GS:[0x1698] NT Reserved for RPC
4 FS:[0xF28] GS:[0x16B0] NT Thread error mode (RtlSetThreadErrorMode)
This is not the full table; see wine ref for all fields until FS:[0xfb4] / GS:[17c8].[2] Newer Windows versions extend the size of TIB further, up to 0x1000/0x1838 in Windows 10. Some of the fields appended are removed, leading to conflicting definitions.[3]

FS (for 32-bit) or GS (for 64-bit) 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.

Accessing the TIB

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:[18h]. That pointer can be used with pointer arithmetic or be cast to a struct pointer.

Using Microsoft Windows SDK or similar, a programmer could use an inline function defined in winnt.h named NtCurrentTeb which returns the address of the current Thread Information Block as NT_TIB *.[4]

Alternative methods of access for IA-32 architectures are as follows:

// gcc (AT&T-style inline assembly).
void *getTIB(void) {
    register void *pTIB;
#if defined(__x86_64__) || defined(__amd64__)
    __asm__("movq %%gs:0x30, %0" : "=r" (pTIB));
#elif defined(__i386__)
    __asm__("movl %%fs:0x18, %0" : "=r" (pTIB));
#else
#error unsupported architecture
#endif
    return pTIB;
}

// gcc (named address spaces, same as the inline assembly version on -O1 or -ftree-ter).
void *getTIB(void) {
#if defined(__x86_64__) || defined(__amd64__)
#ifndef __SEG_GS
#error unsupported GCC version
#endif
    return *(void *__seg_gs *) 0x30;
#elif defined(__i386__)
#ifndef __SEG_FS
#error unsupported GCC version
#endif
    return *(void *__seg_fs *) 0x18;
#else
#error unsupported architecture
#endif
}

// Microsoft C
__declspec(naked)
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);
#else
#error unsupported architecture
#endif
}
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See also
  • Structured Exception Handling

References
  1. Pietrek, Matt (May 1996). "Under The Hood". Microsoft Systems Journal. Archived from the original on 2009-06-14. Retrieved 2010-07-07.
  2. "wine winternl.h: typedef struct _TEB". GitHub. wine-mirror. 29 October 2019.
  3. Chapell, Geoff. "TEB".
  4. "NtCurrentTeb function". Microsoft Docs. Retrieved 20 November 2019.

Further reading
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