Date: Mon, 20 Nov 1995 11:08:52 +1100 (EST) From: John Birrell <cimaxp1!jb@werple.net.au> To: bde@zeta.org.au Cc: hackers@FreeBSD.org, jb@cimlogic.com.au Subject: Re: int type in jmpbuf Message-ID: <199511200005.LAA25969@werple.net.au>
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>
>>typedef struct {
>> char _jb0[_THE_FULL_SIZEOF_JMP_BUF];
>>} jmp_buf[1];
>
>The latter is only guaranteed to be aligned on 1-byte boundaries.
And it needs to be aligned so that the asm code can access it?
>It's supposed to be opaque to stop that :-).
Or was it opaque 'cause the only thing that accessed it was asm code which
couldn't use a structure anyway?
Would it be evil if it were _not_ opaque under FreeBSD?
>You should copy setjmp.S
>and adapt it. Note that it isn't possible to do preemptive context
>switching for threads using only setjmp() - setjmp() doesn't preserve
>all of the floating point state in FreeBSD.
I don't agree with this. gcc under FreeBSD appears to re-load floating point
"registers" after a function call (like setjmp). This means that setjmp/longjmp
don't need to save the floating point state.
When a signal occurs, though, the floating point state is not saved. [We think
it should be saved. Is there anything that says that floating point
calculations in a signal handler can ruin your whole day?! 8-)]. This is
true of FreeBSD and NetBSD/i386, but it is _not_ true of NetBSD/Alpha which
saves floating point registers routinely.
Preemptive context switching in user-space threads under FreeBSD needs to
save the floating point state when entering the scheduler from the signal
handler. Before calling sigreturn to re-start an interrupted thread (as
opposed to one that was simply context switched using setjmp), the floating
point state is restored. This means that the behavio[u]r of a signal handler
under a threaded program will not corrupt the floating point state of a
program as it will with a non-threaded program.
>
>Bruce
--
John Birrell CIMlogic Pty Ltd
jb@cimlogic.com.au 119 Cecil Street
Ph +61 3 9690 9600 South Melbourne Vic 3205
Fax +61 3 9690 6650 Australia
Mob +61 18 353 137
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