https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=0f3e6315712428fcbfc0b6ffeec7c850d9b7206a

commit 0f3e6315712428fcbfc0b6ffeec7c850d9b7206a
Author: Corinna Vinschen <[email protected]>
Date:   Tue Jun 30 20:43:50 2015 +0200

    Add comment to sigaltstack to explain setting ss_flags here
    
        * signal.cc (sigaltstack): Add comment.
    
    Signed-off-by: Corinna Vinschen <[email protected]>

Diff:
---
 winsup/cygwin/ChangeLog | 4 ++++
 winsup/cygwin/signal.cc | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 31b429b..01bb3eb 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,7 @@
+2015-06-30  Corinna Vinschen  <[email protected]>
+
+       * signal.cc (sigaltstack): Add comment.
+
 2015-06-27  Corinna Vinschen  <[email protected]>
 
        * exceptions.cc (_cygtls::call_signal_handler): Drop manipulating
diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc
index 467c919..2b9cfda 100644
--- a/winsup/cygwin/signal.cc
+++ b/winsup/cygwin/signal.cc
@@ -670,6 +670,10 @@ sigaltstack (const stack_t *ss, stack_t *oss)
        {
          char stack_marker;
          memcpy (oss, &me.altstack, sizeof *oss);
+         /* Check if the current stack is the alternate signal stack.  If so,
+            set ss_flags accordingly.  We do this here rather than setting
+            ss_flags in _cygtls::call_signal_handler since the signal handler
+            calls longjmp, so we never return to reset the flag. */
          if (!me.altstack.ss_flags && me.altstack.ss_sp)
            {
              if (&stack_marker >= (char *) me.altstack.ss_sp

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