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Use sigaction(SA_NOCLDWAIT) for SIGCHLD handling

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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
This commit is contained in:
Chris Down 2023-01-28 12:53:48 +01:00 committed by Hiltjo Posthuma
parent 89f9905714
commit 712d6639ff
1 changed files with 9 additions and 11 deletions
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20
dwm.c
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@ -205,7 +205,6 @@ static void setmfact(const Arg *arg);
static void setup(void); static void setup(void);
static void seturgent(Client *c, int urg); static void seturgent(Client *c, int urg);
static void showhide(Client *c); static void showhide(Client *c);
static void sigchld(int unused);
static void spawn(const Arg *arg); static void spawn(const Arg *arg);
static void tag(const Arg *arg); static void tag(const Arg *arg);
static void tagmon(const Arg *arg); static void tagmon(const Arg *arg);
@ -1543,9 +1542,16 @@ setup(void)
int i; int i;
XSetWindowAttributes wa; XSetWindowAttributes wa;
Atom utf8string; Atom utf8string;
struct sigaction sa;
/* clean up any zombies immediately */ /* do not transform children into zombies when they terminate */
sigchld(0); 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 */ /* init screen */
screen = DefaultScreen(dpy); 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 void
spawn(const Arg *arg) spawn(const Arg *arg)
{ {