source: vendor/w32api/current/lib/kernel32.c

/* extern (library) versions of inline functions defined in winnt.h */

#if defined(__GNUC__)

void* GetCurrentFiber(void)
{
    void* ret;
    __asm__ volatile (
        "movl   %%fs:0x10,%0"
        : "=r" (ret) /* allow use of reg eax,ebx,ecx,edx,esi,edi */
        );
    return ret;
}

void* GetFiberData(void)
{
    void* ret;
    __asm__ volatile (            
        "movl   %%fs:0x10,%0\n"
        "movl   (%0),%0"
        : "=r" (ret) /* allow use of reg eax,ebx,ecx,edx,esi,edi */
        );
    return ret;
}

#elif !defined (__WATCOMC__)

void* GetCurrentFiber(void)
{
    void* res;
    _asm {
        mov     eax, dword ptr fs:0x10
        mov     res, eax
    };
    return res;
}

void* GetFiberData(void)
{
    void* res;
    _asm {
        mov     eax, dword ptr fs:0x10
        mov     eax, [eax]
        mov     res, eax
    };
    return res;
}

#endif /* __GNUC__ */