Use sigaction(SA_NOCLDWAIT) for SIGCHLD handling

signal() semantics are pretty unclearly specified. For example, depending on OS
kernel and libc, the handler may be returned to SIG_DFL (hence the inner call
to read the signal handler). Moving to sigaction() means the behaviour is
consistently defined.

Using SA_NOCLDWAIT also allows us to avoid calling the non-reentrant function
die() in the handler.

Some addditional notes for archival purposes:

* NRK pointed out errno of waitpid could also theoretically get clobbered.
* The original patch was iterated on and modified by NRK and Hiltjo:
  * SIG_DFL was changed to SIG_IGN, this is required, atleast on older systems
    such as tested on Slackware 11.
  * signals are not blocked using sigprocmask, because in theory it would
    briefly for example also ignore a SIGTERM signal. It is OK if waitpid() is (in
    theory interrupted).

POSIX reference:
"Consequences of Process Termination":
https://pubs.opengroup.org/onlinepubs/9699919799/functions/_Exit.html#tag_16_01_03_01

1 files changed, 9 insertions, 11 deletions

diff --git a/dwm.c b/dwm.c
index a9d04b9..745c898 100644
--- a/dwm.c
+++ b/dwm.c
@@ -220,7 +220,6 @@ static void setmfact(const Arg *arg);
 static void setup(void);
 static void seturgent(Client *c, int urg);
 static void showhide(Client *c);
-static void sigchld(int unused);
 static void sighup(int unused);
 static void sigterm(int unused);
 static void spawn(const Arg *arg);
@@ -1711,9 +1710,16 @@ setup(void)
 	int i;
 	XSetWindowAttributes wa;
 	Atom utf8string;
+	struct sigaction sa;
 
-	/* clean up any zombies immediately */
-	sigchld(0);
+	/* do not transform children into zombies when they terminate */
+	sigemptyset(&sa.sa_mask);
+	sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;
+	sa.sa_handler = SIG_IGN;
+	sigaction(SIGCHLD, &sa, NULL);
+
+	/* clean up any zombies (inherited from .xinitrc etc) immediately */
+	while (waitpid(-1, NULL, WNOHANG) > 0);
 
 	signal(SIGHUP, sighup);
 	signal(SIGTERM, sigterm);
@@ -1810,14 +1816,6 @@ showhide(Client *c)
 }
 
 void
-sigchld(int unused)
-{
-	if (signal(SIGCHLD, sigchld) == SIG_ERR)
-		die("can't install SIGCHLD handler:");
-	while (0 < waitpid(-1, NULL, WNOHANG));
-}
-
-void
 sighup(int unused)
 {
 	Arg a = {.i = 1};