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);

       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         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 handler must reference a function to call when the process
       changes state. 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 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.

       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 be disabled
       after the invocation, even if the SD_EVENT_ON mode was requested

       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

       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 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

       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

       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

       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         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 WCONTINUED.

           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.

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

           A pidfd was requested but the kernel does not support this

NOTES         top

       These APIs are implemented as a shared library, which can be compiled
       and linked to with the libsystemd pkg-config(1) file.

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_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)

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 2020-09-18.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-09-18.)  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 246                                            SD_EVENT_ADD_CHILD(3)

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