From a709a0a2bd08c944968433ad984973b32cedc0ae Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 3 Sep 2025 16:49:31 -0700
Subject: [PATCH] diff: simplify sigaction configuration

* configure.ac: Remove checks for sigaction, sigprocmask, sigblock.
The sigprocmask and sigblock checks were not being used anyway.
* src/sdiff.c: Use (defined SA_NOCLDSTOP) instead of HAVE_SIGACTION.
This is what other modules do.
---
 configure.ac |  4 ----
 src/sdiff.c  | 16 +++++++++-------
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/configure.ac b/configure.ac
index 19bc5ef..ff79236 100644
--- a/configure.ac
+++ b/configure.ac
@@ -87,10 +87,6 @@ AC_HEADER_DIRENT
 AC_HEADER_SYS_WAIT
 AC_TYPE_PID_T
 
-AC_CHECK_FUNCS_ONCE([sigaction sigprocmask])
-if test $ac_cv_func_sigprocmask = no; then
-  AC_CHECK_FUNCS([sigblock])
-fi
 AC_FUNC_FORK
 
 dnl O_PATH exists since Linux 2.6.39, but is supported with fstat() only since
diff --git a/src/sdiff.c b/src/sdiff.c
index a21b081..278b5a2 100644
--- a/src/sdiff.c
+++ b/src/sdiff.c
@@ -100,7 +100,9 @@ enum
 typedef void (*sighandler) (int);
 static void signal_handler (int, sighandler);
 
-#if HAVE_SIGACTION
+/* Use SA_NOCLDSTOP as a proxy for whether the sigaction machinery is
+   present.  */
+#ifdef SA_NOCLDSTOP
 /* Prefer 'sigaction' if available, since 'signal' can lose signals.  */
 static struct sigaction initial_action[NUM_SIGS];
 static sighandler
@@ -714,21 +716,21 @@ static bool sigs_trapped;
 static void
 catchsig (int s)
 {
-#if ! HAVE_SIGACTION
+#ifndef SA_NOCLDSTOP
   signal (s, SIG_IGN);
 #endif
   if (! (s == SIGINT && ignore_SIGINT))
     signal_received = s;
 }
 
-#if HAVE_SIGACTION
+#ifdef SA_NOCLDSTOP
 static struct sigaction catchaction;
 #endif
 
 static void
 signal_handler (int sig, sighandler handler)
 {
-#if HAVE_SIGACTION
+#ifdef SA_NOCLDSTOP
   catchaction.sa_handler = handler;
   sigaction (sig, &catchaction, 0);
 #else
@@ -739,7 +741,7 @@ signal_handler (int sig, sighandler handler)
 static void
 trapsigs (void)
 {
-#if HAVE_SIGACTION
+#ifdef SA_NOCLDSTOP
   catchaction.sa_flags = SA_RESTART;
   sigemptyset (&catchaction.sa_mask);
   for (int i = 0;  i < NUM_SIGS;  i++)
@@ -748,7 +750,7 @@ trapsigs (void)
 
   for (int i = 0;  i < NUM_SIGS;  i++)
     {
-#if HAVE_SIGACTION
+#ifdef SA_NOCLDSTOP
       sigaction (sigs[i], nullptr, &initial_action[i]);
 #else
       initial_action[i] = signal (sigs[i], SIG_IGN);
@@ -773,7 +775,7 @@ untrapsig (int s)
     for (int i = 0;  i < NUM_SIGS;  i++)
       if ((! s || sigs[i] == s)  &&  initial_handler (i) != SIG_IGN)
         {
-#if HAVE_SIGACTION
+#ifdef SA_NOCLDSTOP
           sigaction (sigs[i], &initial_action[i], nullptr);
 #else
           signal (sigs[i], initial_action[i]);