x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
SD_EVENT_ADD_CHILD(3) sd_event_add_child SD_EVENT_ADD_CHILD(3)
NAME
sd_event_add_child, sd_event_add_child_pidfd,
sd_event_source_get_child_pid, sd_event_source_get_child_pidfd,
sd_event_source_get_child_pidfd_own,
sd_event_source_set_child_pidfd_own,
sd_event_source_get_child_process_own,
sd_event_source_set_child_process_own,
sd_event_source_send_child_signal, sd_event_child_handler_t - Add a
child process state change event source to an event loop
SYNOPSIS
#include <systemd/sd-event.h>
typedef struct sd_event_source sd_event_source;
typedef int (*sd_event_child_handler_t)(sd_event_source *s,
const siginfo_t *si,
void *userdata);
int sd_event_add_child(sd_event *event, sd_event_source **source,
pid_t pid, int options,
sd_event_child_handler_t handler,
void *userdata);
int sd_event_add_child_pidfd(sd_event *event, sd_event_source **source,
int pidfd, int options,
sd_event_child_handler_t handler,
void *userdata);
int sd_event_source_get_child_pid(sd_event_source *source, pid_t *pid);
int sd_event_source_get_child_pidfd(sd_event_source *source);
int sd_event_source_get_child_pidfd_own(sd_event_source *source);
int sd_event_source_set_child_pidfd_own(sd_event_source *source,
int own);
int sd_event_source_get_child_process_own(sd_event_source *source);
int sd_event_source_set_child_process_own(sd_event_source *source,
int own);
int sd_event_source_send_child_signal(sd_event_source *source, int sig,
const siginfo_t *info,
unsigned flags);
DESCRIPTION
sd_event_add_child() adds a new child process state change event source
to an event loop. The event loop object is specified in the event
parameter, the event source object is returned in the source parameter.
The pid parameter specifies the PID of the process to watch, which must
be a direct child process of the invoking process. The options
parameter determines which state changes will be watched for. It must
contain an OR-ed mask of WEXITED (watch for the child process
terminating), WSTOPPED (watch for the child process being stopped by a
signal), and WCONTINUED (watch for the child process being resumed by a
signal). See waitid(2) for further information.
The handler must be a function to call when the process changes state
or NULL. The handler function will be passed the userdata pointer,
which may be chosen freely by the caller. The handler also receives a
pointer to a siginfo_t structure containing information about the child
process event. The handler may return negative to signal an error (see
below), other return values are ignored. If handler is NULL, a default
handler that calls sd_event_exit(3) will be used.
Only a single handler may be installed for a specific child process.
The handler is enabled for a single event (SD_EVENT_ONESHOT), but this
may be changed with sd_event_source_set_enabled(3). If the handler
function returns a negative error code, it will either be disabled
after the invocation, even if the SD_EVENT_ON mode was requested
before, or it will cause the loop to terminate, see
sd_event_source_set_exit_on_failure(3).
To destroy an event source object use sd_event_source_unref(3), but
note that the event source is only removed from the event loop when all
references to the event source are dropped. To make sure an event
source does not fire anymore, even when there's still a reference to it
kept, consider setting the event source to SD_EVENT_OFF with
sd_event_source_set_enabled(3).
The SIGCHLD signal must be blocked in all threads before this function
is called (using sigprocmask(2) or pthread_sigmask(3)).
If the second parameter of sd_event_add_child() is passed as NULL no
reference to the event source object is returned. In this case the
event source is considered "floating", and will be destroyed implicitly
when the event loop itself is destroyed.
Note that the handler function is invoked at a time where the child
process is not reaped yet (and thus still is exposed as a zombie
process by the kernel). However, the child will be reaped automatically
after the function returns. Child processes for which no child process
state change event sources are installed will not be reaped by the
event loop implementation.
If the handler parameter to sd_event_add_child() is NULL, and the event
source fires, this will be considered a request to exit the event loop.
In this case, the userdata parameter, cast to an integer, is passed as
the exit code parameter to sd_event_exit(3).
If both a child process state change event source and a SIGCHLD signal
event source is installed in the same event loop, the configured event
source priorities decide which event source is dispatched first. If the
signal handler is processed first, it should leave the child processes
for which child process state change event sources are installed
unreaped.
sd_event_add_child_pidfd() is similar to sd_event_add_child() but takes
a file descriptor referencing the process ("pidfd") instead of the
numeric PID. A suitable file descriptor may be acquired via
pidfd_open(2) and related calls. The passed file descriptor is not
closed when the event source is freed again, unless
sd_event_source_set_child_pidfd_own() is used to turn this behaviour
on. Note that regardless which of sd_event_add_child() and
sd_event_add_child_pidfd() is used for allocating an event source, the
watched process has to be a direct child process of the invoking
process. Also in both cases SIGCHLD has to be blocked in the invoking
process.
sd_event_source_get_child_pid() retrieves the configured PID of a child
process state change event source created previously with
sd_event_add_child(). It takes the event source object as the source
parameter and a pointer to a pid_t variable to return the process ID
in.
sd_event_source_get_child_pidfd() retrieves the file descriptor
referencing the watched process ("pidfd") if this functionality is
available. On kernels that support the concept the event loop will make
use of pidfds to watch child processes, regardless if the individual
event sources are allocated via sd_event_add_child() or
sd_event_add_child_pidfd(). If the latter call was used to allocate the
event source, this function returns the file descriptor used for
allocation. On kernels that do not support the pidfd concept this
function will fail with EOPNOTSUPP. This call takes the event source
object as the source parameter and returns the numeric file descriptor.
sd_event_source_get_child_pidfd_own() may be used to query whether the
pidfd the event source encapsulates shall be closed when the event
source is freed. This function returns zero if the pidfd shall be left
open, and positive if it shall be closed automatically. By default this
setting defaults to on if the event source was allocated via
sd_event_add_child() and off if it was allocated via
sd_event_add_child_pidfd(). The sd_event_source_set_child_pidfd_own()
function may be used to change the setting and takes a boolean
parameter with the new setting.
sd_event_source_get_child_process_own() may be used to query whether
the process the event source watches shall be killed (with SIGKILL) and
reaped when the event source is freed. This function returns zero if
the process shell be left running, and positive if it shall be killed
and reaped automatically. By default this setting defaults to off. The
sd_event_source_set_child_process_own() function may be used to change
the setting and takes a boolean parameter with the new setting. Note
that currently if the calling process is terminated abnormally the
watched process might survive even thought the event source ceases to
exist. This behaviour might change eventually.
sd_event_source_send_child_signal() may be used to send a UNIX signal
to the watched process. If the pidfd concept is supported in the
kernel, this is implemented via pidfd_send_signal(2) and otherwise via
rt_sigqueueinfo(2) (or via kill(2) in case info is NULL). The specified
parameters match those of these underlying system calls, except that
the info is never modified (and is thus declared constant). Like for
the underlying system calls, the flags parameter currently must be
zero.
RETURN VALUE
On success, these functions return 0 or a positive integer. On failure,
they return a negative errno-style error code.
Errors
Returned errors may indicate the following problems:
-ENOMEM
Not enough memory to allocate an object.
-EINVAL
An invalid argument has been passed. This includes specifying an
empty mask in options or a mask which contains values different
than a combination of WEXITED, WSTOPPED, and WCONTINUED.
-EBUSY
A handler is already installed for this child process, or SIGCHLD
is not blocked.
-ESTALE
The event loop is already terminated.
-ECHILD
The event loop has been created in a different process, library or
module instance.
-EDOM
The passed event source is not a child process event source.
-EOPNOTSUPP
A pidfd was requested but the kernel does not support this concept.
NOTES
Functions described here are available as a shared library, which can
be compiled against and linked to with the libsystemd pkg-config(1)
file.
The code described here uses getenv(3), which is declared to be not
multi-thread-safe. This means that the code calling the functions
described here must not call setenv(3) from a parallel thread. It is
recommended to only do calls to setenv() from an early phase of the
program when no other threads have been started.
EXAMPLE
Example 1. Exit loop when the child terminates
/* SPDX-License-Identifier: MIT-0 */
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <systemd/sd-event.h>
int main(int argc, char **argv) {
pid_t pid = fork();
assert(pid >= 0);
/* SIGCHLD signal must be blocked for sd_event_add_child to work */
sigset_t ss;
sigemptyset(&ss);
sigaddset(&ss, SIGCHLD);
sigprocmask(SIG_BLOCK, &ss, NULL);
if (pid == 0) /* child */
sleep(1);
else { /* parent */
sd_event *e = NULL;
int r;
/* Create the default event loop */
sd_event_default(&e);
assert(e);
/* We create a floating child event source (attached to 'e').
* The default handler will be called with 666 as userdata, which
* will become the exit value of the loop. */
r = sd_event_add_child(e, NULL, pid, WEXITED, NULL, (void*) 666);
assert(r >= 0);
r = sd_event_loop(e);
assert(r == 666);
sd_event_unref(e);
}
return 0;
}
SEE ALSO
systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3),
sd_event_add_io(3), sd_event_add_time(3), sd_event_add_signal(3),
sd_event_add_inotify(3), sd_event_add_defer(3),
sd_event_source_set_enabled(3), sd_event_source_set_priority(3),
sd_event_source_set_userdata(3), sd_event_source_set_description(3),
sd_event_source_set_floating(3), waitid(2), sigprocmask(2),
pthread_sigmask(3), pidfd_open(2), pidfd_send_signal(2),
rt_sigqueueinfo(2), kill(2)
systemd 254 SD_EVENT_ADD_CHILD(3)
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