75 lines
3 KiB
Text
75 lines
3 KiB
Text
Contributed by Max Kaehn
|
|
|
|
All cygwin threads have separate context in an object of class _cygtls. The
|
|
storage for this object is kept on the stack in the bottom CYGTLS_PADSIZE
|
|
bytes. Each thread references the storage via the Thread Environment Block
|
|
(aka Thread Information Block), which Windows maintains for each user thread
|
|
in the system, with the address in the FS segment register. The memory
|
|
is laid out as in the NT_TIB structure from <w32api/winnt.h>:
|
|
|
|
typedef struct _NT_TIB {
|
|
struct _EXCEPTION_REGISTRATION_RECORD *ExceptionList;
|
|
PVOID StackBase;
|
|
PVOID StackLimit;
|
|
PVOID SubSystemTib;
|
|
_ANONYMOUS_UNION union {
|
|
PVOID FiberData;
|
|
DWORD Version;
|
|
} DUMMYUNIONNAME;
|
|
PVOID ArbitraryUserPointer;
|
|
struct _NT_TIB *Self;
|
|
} NT_TIB,*PNT_TIB;
|
|
|
|
Cygwin sees it like this:
|
|
|
|
extern exception_list *_except_list asm ("%fs:0"); // exceptions.cc
|
|
extern char *_tlsbase __asm__ ("%fs:4"); // cygtls.h
|
|
extern char *_tlstop __asm__ ("%fs:8"); // cygtls.h
|
|
|
|
And accesses cygtls like this:
|
|
|
|
#define _my_tls (((_cygtls *) _tlsbase)[-1]) // cygtls.h
|
|
|
|
|
|
Initialization always goes through _cygtls::init_thread(). It works
|
|
in the following ways:
|
|
|
|
* In the main thread, _dll_crt0() provides CYGTLS_PADSIZE bytes on the stack
|
|
and passes them to initialize_main_tls(), which calls _cygtls::init_thread().
|
|
It then calls dll_crt0_1(), which terminates with cygwin_exit() rather than
|
|
by returning, so the storage never goes out of scope.
|
|
|
|
If you load cygwin1.dll dynamically from a non-cygwin application, it is
|
|
vital that the bottom CYGTLS_PADSIZE bytes of the stack are not in use
|
|
before you call cygwin_dll_init(). See winsup/testsuite/cygload for
|
|
more information.
|
|
|
|
* Threads other than the main thread receive DLL_THREAD_ATTACH messages
|
|
to dll_entry() (in init.cc).
|
|
- dll_entry() calls munge_threadfunc(), which grabs the function pointer
|
|
for the thread from the stack frame and substitutes threadfunc_fe(),
|
|
- which then passes the original function pointer to _cygtls::call(),
|
|
- which then allocates CYGTLS_PADSIZE bytes on the stack and hands them
|
|
to call2(),
|
|
- which allocates an exception_list object on the stack and hands it to
|
|
init_exceptions() (in exceptions.cc), which attaches it to the end of
|
|
the list of exception handlers, changing _except_list (aka
|
|
tib->ExceptionList), then passes the cygtls storage to init_thread().
|
|
call2() calls ExitThread() instead of returning, so the storage never
|
|
goes out of scope.
|
|
|
|
Note that the padding isn't necessarily going to be just where the _cygtls
|
|
structure lives; it just makes sure there's enough room on the stack when the
|
|
CYGTLS_PADSIZE bytes down from there are overwritten.
|
|
|
|
|
|
Debugging
|
|
|
|
You can examine the segment registers in gdb via "info w32 selector $fs"
|
|
(which is using GetThreadSelectorEntry()) to get results like this:
|
|
|
|
Selector $fs
|
|
0x03b: base=0x7ffdd000 limit=0x00000fff 32-bit Data (Read/Write, Exp-up)
|
|
Priviledge level = 3. Byte granular.
|
|
|
|
"x/3x 0x7ffdd000" will give you _except_list, _tlsbase, and _tlstop.
|