dynamic window manager
git clone git://git.suckless.org/dwm
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commit 712d6639ff8e863560328131bbb92b248dc9cde7
parent 89f9905714c1c1b2e8b09986dfbeca15b68d8af8
Author: Chris Down <chris@chrisdown.name>
Date:   Sat, 28 Jan 2023 12:53:48 +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":

Mdwm.c | 20+++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)

diff --git 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); @@ -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)