sd_event_add_child

systemd
More text Zbigniew Jędrzejewski-Szmek zbyszek@in.waw.pl
sd_event_add_child 3
sd_event_add_child sd_event_source_get_child_pid 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_source_get_child_pid` sd_event_source *source pid_t *pid

## 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. 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.

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_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.

## 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.

`-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.

## 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_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, waitid 2