sd_event_add_child

systemd
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 *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 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 before.

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 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.

`-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.

## 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