On Windows libc exports __cxa_init_primary_exception, std::uncaught_exception
and std::uncaught_exceptions only as _nid_postfix symbols (GCC_GLOBAL_ALIAS
is empty there), so host modules bind libstdc++-6.dll's versions:
- std::make_exception_ptr, which std::promise's destructor uses for
broken_promise, allocated the exception with libc and initialized it with
libstdc++'s __cxa_init_primary_exception, whose header layout differs.
A test that makes one and rethrows it dies with an access violation.
- std::uncaught_exceptions read libstdc++'s thread state, which libc's
throws never touch, so it returned 0 while an exception unwound. The
AGC driver's deferred-label guard relies on it to tell a failed record
from a finished one.
The three are now exported natively, like __cxa_throw and the rest.
__cxa_init_primary_exception also sets the reference count to 0, as the
Itanium ABI does: the exception_ptr created next takes the only
reference, so a made exception is destroyed once instead of leaking.
libc implements the C++ exception runtime and already defines
exception_ptr's out-of-line members, so host modules that link libc
reference-count libc's exceptions. Code inside libstdc++-6.dll does not
see those definitions on Windows: std::__future_base::_Result_base's
destructor is one of them, and it releases the stored exception with
libstdc++'s own exception_ptr::_M_release. That decrements a count 0xa0
bytes before the object, while libc keeps it 0x80 bytes before, so the
decrement lands in the heap memory in front of libc's allocation and
the exception is never freed.
VideoOutDriver reports the presentation thread's start-up failure
through a std::promise, so video_out_flip_lifetime sometimes ended with
0xc0000374 when SDL's offscreen driver refused the window.
libc now defines the base destructor, with a _nid_postfix twin so that
the NID-patched libc keeps its name, and modules bind it there.
Adrià
MarcZX100
ducloc080202
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