sd_event_add_child(3) — Linux manual page


SD_EVENT_ADD_CHILD(3)      sd_event_add_child      SD_EVENT_ADD_CHILD(3)

NAME         top

       sd_event_add_child, sd_event_add_child_pidfd,
       sd_event_source_get_child_pid, sd_event_source_get_child_pidfd,
       sd_event_source_send_child_signal, sd_event_child_handler_t - Add
       a child process state change event source to an event loop

SYNOPSIS         top

       #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);

                                                 sd_event_source_get_child_process_own(sd_event_source *source);

                                                 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         top

       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

       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

       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

       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

       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         top

       On success, these functions return 0 or a positive integer. On
       failure, they return a negative errno-style error code.

       Returned errors may indicate the following problems:

           Not enough memory to allocate an object.

           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

           A handler is already installed for this child process, or
           SIGCHLD is not blocked.

           The event loop is already terminated.

           The event loop has been created in a different process,
           library or module instance.

           The passed event source is not a child process event source.

           A pidfd was requested but the kernel does not support this

           Added in version 245.

NOTES         top

       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

EXAMPLE         top

       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;
             sigaddset(&ss, SIGCHLD);
             sigprocmask(SIG_BLOCK, &ss, NULL);

             if (pid == 0)  /* child */

             else {         /* parent */
               sd_event *e = NULL;
               int r;

               /* Create the default event loop */

               /* 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);


             return 0;

HISTORY         top

       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_set_child_process_own(), and
       sd_event_source_send_child_signal() were added in version 245.

SEE ALSO         top

       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_floating(3), waitid(2), sigprocmask(2),
       pthread_sigmask(3), pidfd_open(2), pidfd_send_signal(2),
       rt_sigqueueinfo(2), kill(2)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-13.)  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

systemd 257~devel                                  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)