NeverStop

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.

Accessing the TIB

The TIB can be accessed as an offset of segment register FS.

It is not common to access the TIB fields by an offset from FS:[0], but rather first getting a the linear self-referencing pointer to the stored at FS:[0x18]. That pointer is used in means of pointer arithmetics or 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:[18h]

        mov [pTib], EAX

    }

    return pTib;

}

Recent versions of the Microsoft C compiler also contain a compiler intrinsic __readfsword, which can be used to access the FS segment without requiring inline assembly.