On platforms with sigaltstack, TXR programs can now catch the

segmentation fault that occurs when running out of stack space,
and escape by throwing an exception.

Also, bugfix: save and restore the gc enable/disable state.  Without
this, if we are ever running cod in a gc disabled state and it jumps
out, gc stays disabled.

* configure: added check for sigaltstack.

* gc.h (gc_enabled): Declaration added for existing variable.

* signal.c (is_cpu_exception): New static function.
(sig_handler): For cpu_exception type signals that pertain
to the execution of some instruction, turn on async_sig_enabled,
so that the signal is not deferred. Otherwise we will just
return without calling the user-defined handler, restart
the instruction and get into a loop. Also, disable gc around
the handler just in case. One issue is that we might be on
an alternate stack, which gc won't like.
(setup_alt_stack, teardown_alt_stack): New static functions.
(set_sig_handler): If we have sigaltstack, and are asked
to set up a SEGV handler, then set it up on the alternate
stack.

* signal.h (extended_jmp_buf): Adding new member, gc.
(extended_setjmp, extended_longjmp): use gc member
to save and restore the gc_enable state across
setjmp and longjmp.

1 files changed, 22 insertions, 0 deletions

diff --git a/configure b/configure
index c897c7e2..ff7f67fa 100755
--- a/configure
+++ b/configure
@@ -1611,6 +1611,28 @@ else
   printf "no\n"
 fi
 
+printf "Checking for sigaltstack ... "
+
+cat > conftest.c <<!
+#include <signal.h>
+#include <stdlib.h>
+
+int main(void)
+{
+  stack_t ss;
+  ss.ss_sp = malloc(SIGSTKSZ);
+  ss.ss_size = SIGSTKSZ;
+  ss.ss_flags = 0;
+  return sigaltstack(&ss, 0);
+}
+!
+if conftest ; then
+  printf "yes\n"
+  printf "#define HAVE_SIGALTSTACK 1\n" >> config.h
+else
+  printf "no\n"
+fi
+
 printf "Checking for makedev ... "
 
 cat > conftest.c <<!