commit 712d6639ff8e863560328131bbb92b248dc9cde7
from: Chris Down <chris@chrisdown.name>
via: Hiltjo Posthuma <hiltjo@codemadness.org>
date: Sat Jan 28 12:34:43 2023 UTC

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

commit - 89f9905714c1c1b2e8b09986dfbeca15b68d8af8
commit + 712d6639ff8e863560328131bbb92b248dc9cde7
blob - 03baf42b350240f1ab7907169f85725f05d6fe70
blob + c2bd8710544eb4b4e7eaa4a1307e1f1dfd8d16ba
--- dwm.c
+++ dwm.c
@@ -205,7 +205,6 @@ 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 spawn(const Arg *arg);
 static void tag(const Arg *arg);
 static void tagmon(const Arg *arg);
@@ -1543,10 +1542,17 @@ 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);
+
 	/* init screen */
 	screen = DefaultScreen(dpy);
 	sw = DisplayWidth(dpy, screen);
@@ -1639,14 +1645,6 @@ 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
 spawn(const Arg *arg)
 {
 	if (arg->v == dmenucmd)