05.03.2016 01:22, Andy Lutomirski пишет:

On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev [off-list ref] wrote:

quoted

This patch implements the SS_AUTODISARM flag that can be ORed with
SS_ONSTACK when forming ss_flags.
When this flag is set, sigaltstack will be disabled when entering
the signal handler; more precisely, after saving sas to uc_stack.
When leaving the signal handler, the sigaltstack is restored by
uc_stack.
When this flag is used, it is safe to switch from sighandler with
swapcontext(). Without this flag, the subsequent signal will corrupt
the state of the switched-away sighandler.

This looks reasonable to me with one exception: how does a user
program detect the presence of this feature?

Compile-time detection:
#ifdef SS_AUTODISARM
   # I have this feature
   ...
#endif

Run-time detection:
int err = sigaltstack(SS_ONSTACK | SS_AUTODISARM);
if (err == EINVAL) {
   i_dont_have_this_feature = 1;
   err = sigaltstack(SS_ONSTACK);
}

Note: if you want to keep such detection for the future
additions, the mask can be enlarged to, say, ((1 << 24) - 1),
and whenever someone adds a new flag, he can lower the
mask by one bit.
But I think this would be an overkill in that particular case.