signal.7: Clarify that siginfo_t isn't changed on coalescing

signal.7: Clarify that siginfo_t isn't changed on coalescing

Confirmed by experiment by mtk:

$ cat siginfo_nonqueuing.c

                        } while (0)

static void
grimReaper(int sig, siginfo_t *si, void *ucontext)
{
    printf("caught signal %d\n", sig);

    printf("    si_pid=%ld, si_uid=%ld, si_status=%d\n",
            (long) si->si_pid, (long) si->si_uid, si->si_status);
}

static void
child(int sleepTime, uid_t uid, int status)
{
    switch (fork()) {
    case -1:
        errExit("fork");
    case 0:
        sleep(sleepTime);
        if (geteuid() == 0)
            setuid(uid);

        printf("Child %ld with UID %ld exiting with status %d\n",
                (long) getpid(),(long) getuid(), status);
        exit(status);
    default:
        return;
    }
}

int
main(int argc, char *argv[])
{
    struct sigaction sa;
    sigset_t blocking;

    sa.sa_sigaction = grimReaper;
    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);

    if (sigaction(SIGCHLD, &sa, NULL) == -1)
        errExit("sigaction");

    sigemptyset(&blocking);
    sigaddset(&blocking, SIGCHLD);
    if (sigprocmask(SIG_BLOCK, &blocking, NULL) == -1)
        errExit("sigprocmask");

    child(2, 20000, 20);
    child(3, 30000, 30);
    child(1, 10000, 10);

    sleep(5);

    if (sigprocmask(SIG_UNBLOCK, &blocking, NULL) == -1)
        errExit("sigprocmask");

    exit(EXIT_SUCCESS);
}
$ ./siginfo_nonqueuing
Child 4042 with UID 1000 exiting with status 10
Child 4040 with UID 1000 exiting with status 20
Child 4041 with UID 1000 exiting with status 30
caught signal 17
    si_pid=4042, si_uid=1000, si_status=10

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Lennart Poettering <lennart@poettering.net>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Michal Sekletar <msekleta@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>