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.

           Added in version 245.

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 <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;
           }

HISTORY

       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.

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)