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. 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. 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

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

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
__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);
#endif
}

See also

• Structured Exception Handling

References

Further reading

• Pietrek, Matt (March 1996). Windows 95 Programming Secrets (PDF). IDG. pp. 136–138. ISBN 978-1-56884-318-6. Archived from the original (pdf) on 2011-05-14. Retrieved 2010-07-17.

External links

• TEB layout on NTinternals.net
• Structured Exception Handling and the TIB
• Description of the first slots of the TIB
• Description of TEB, field by field