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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLE | HISTORY | SEE ALSO | COLOPHON |
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SD_EVENT_ADD_CHILD(3) sd_event_add_child SD_EVENT_ADD_CHILD(3)
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
#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 *ret);
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);
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"). The event loop
internally makes use of pidfds to watch child processes,
regardless of whether 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 original file descriptor used for allocation. 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 via pidfd_send_signal(2). The
specified parameters match those of the underlying system call,
except that the info is never modified (and is thus declared
constant). Like for the underlying system call, the flags
parameter currently must be zero.
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.
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 1. Exit loop when the child terminates
/* SPDX-License-Identifier: MIT-0 */
#define _GNU_SOURCE 1
#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;
}
sd_event_add_child(), sd_event_child_handler_t(), and
sd_event_source_get_child_pid() were added in version 217.
sd_event_add_child_pidfd(), 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(), and
sd_event_source_send_child_signal() were added in version 245.
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)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SD_EVENT_ADD_CHILD(3)
Pages that refer to this page: sd-event(3), sd_event_add_defer(3), sd_event_add_inotify(3), sd_event_add_io(3), sd_event_add_memory_pressure(3), sd_event_add_signal(3), sd_event_add_time(3), sd_event_new(3), sd_event_run(3), sd_event_set_watchdog(3), sd_event_source_get_event(3), sd_event_source_get_pending(3), sd_event_source_set_description(3), sd_event_source_set_destroy_callback(3), sd_event_source_set_enabled(3), sd_event_source_set_exit_on_failure(3), sd_event_source_set_floating(3), sd_event_source_set_prepare(3), sd_event_source_set_priority(3), sd_event_source_set_userdata(3), sd_event_source_unref(3), sd_event_wait(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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