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
typedef struct sd_event_source sd_event_source;
|const siginfo_t *si,|
|const siginfo_t *info,|
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
parameter specifies the PID of the process to watch, which must be a direct child process of the invoking
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
parameter determines which state changes will be watched for. It must contain an OR-ed mask of
WEXITED (watch for the child process terminating),
(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
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
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
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
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
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_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.
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_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
and off if it was allocated via
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
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
currently must be zero.
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
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 concept.
These APIs are implemented as a shared
library, which can be compiled and linked to with the
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)