[hackers] [dwm] Use sigaction(SA_NOCLDWAIT) for SIGCHLD handling || Chris Down

[git]

commit 712d6639ff8e863560328131bbb92b248dc9cde7
Author:     Chris Down <ch...@chrisdown.name>
AuthorDate: Sat Jan 28 12:53:48 2023 +0100
Commit:     Hiltjo Posthuma <hil...@codemadness.org>
CommitDate: Sat Jan 28 13:34:43 2023 +0100

    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

diff --git a/dwm.c b/dwm.c
index 03baf42..c2bd871 100644
--- a/dwm.c
+++ b/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);
@@ -1543,9 +1542,16 @@ 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);
 
        /* init screen */
        screen = DefaultScreen(dpy);
@@ -1638,14 +1644,6 @@ 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)
 {