Use sigaction(SA_NOCLDWAIT) for SIGCHLD handling 712d6639 
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
Chris Down · 2023-01-28 12:53 1 file(s) · +9 −11
dwm.c +9 −11
205 205 
static void setup(void);
206 206 
static void seturgent(Client *c, int urg);
207 207 
static void showhide(Client *c);
208 - 
static void sigchld(int unused);
209 208 
static void spawn(const Arg *arg);
210 209 
static void tag(const Arg *arg);
211 210 
static void tagmon(const Arg *arg);
1543 1542 
	int i;
1544 1543 
	XSetWindowAttributes wa;
1545 1544 
	Atom utf8string;
1545 + 
	struct sigaction sa;
1546 1546 

            

            
            

                1547
                
                -
                
	/* clean up any zombies immediately */

            

            
            

                1548
                
                -
                
	sigchld(0);

            

            
            

                
                1547
                +
                
	/* do not transform children into zombies when they terminate */

            

            
            

                
                1548
                +
                
	sigemptyset(&sa.sa_mask);

            

            
            

                
                1549
                +
                
	sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;

            

            
            

                
                1550
                +
                
	sa.sa_handler = SIG_IGN;

            

            
            

                
                1551
                +
                
	sigaction(SIGCHLD, &sa, NULL);

            

            
            

                
                1552
                +
                

            

            
            

                
                1553
                +
                
	/* clean up any zombies (inherited from .xinitrc etc) immediately */

            

            
            

                
                1554
                +
                
	while (waitpid(-1, NULL, WNOHANG) > 0);

            

            
            

                1549
                1555
                 
                

            

            
            

                1550
                1556
                 
                
	/* init screen */

            

            
            

                1551
                1557
                 
                
	screen = DefaultScreen(dpy);

            

            
        
            

            
            

                1636
                1642
                 
                
		showhide(c->snext);

            

            
            

                1637
                1643
                 
                
		XMoveWindow(dpy, c->win, WIDTH(c) * -2, c->y);

            

            
            

                1638
                1644
                 
                
	}

            

            
            

                1639
                
                -
                
}

            

            
            

                1640
                
                -
                

            

            
            

                1641
                
                -
                
void

            

            
            

                1642
                
                -
                
sigchld(int unused)

            

            
            

                1643
                
                -
                
{

            

            
            

                1644
                
                -
                
	if (signal(SIGCHLD, sigchld) == SIG_ERR)

            

            
            

                1645
                
                -
                
		die("can't install SIGCHLD handler:");

            

            
            

                1646
                
                -
                
	while (0 < waitpid(-1, NULL, WNOHANG));

            

            
            

                1647
                1645
                 
                
}

            

            
            

                1648
                1646
                 
                

            

            
            

                1649
                1647
                 
                
void