'\" t
.TH "SD_EVENT_ADD_CHILD" "3" "" "systemd 232" "sd_event_add_child"
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.SH "NAME"
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
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-event\&.h>
.fi
.ft
.sp
.ft B
.nf
typedef struct sd_event_source sd_event_source;
.fi
.ft
.HP \w'typedef\ int\ (*sd_event_child_handler_t)('u
.BI "typedef int (*sd_event_child_handler_t)(sd_event_source\ *" "s" ", const\ siginfo_t\ *" "si" ", void\ *" "userdata" ");"
.HP \w'int\ sd_event_add_child('u
.BI "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" ");"
.HP \w'int\ sd_event_source_get_child_pid('u
.BI "int sd_event_source_get_child_pid(sd_event_source\ *" "source" ", pid_t\ *" "pid" ");"
.SH "DESCRIPTION"
.PP
\fBsd_event_add_child()\fR
adds a new child process state change event source to an event loop\&. The event loop object is specified in the
\fIevent\fR
parameter, the event source object is returned in the
\fIsource\fR
parameter\&. The
\fIpid\fR
parameter specifies the PID of the process to watch\&. The
\fIhandler\fR
must reference a function to call when the process changes state\&. The handler function will be passed the
\fIuserdata\fR
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
\fIoptions\fR
parameter determines which state changes will be watched for\&. It must contain an OR\-ed mask of
\fBWEXITED\fR
(watch for the child process terminating),
\fBWSTOPPED\fR
(watch for the child process being stopped by a signal), and
\fBWCONTINUED\fR
(watch for the child process being resumed by a signal)\&. See
\fBwaitid\fR(2)
for further information\&.
.PP
Only a single handler may be installed for a specific child process\&. The handler is enabled for a single event (\fBSD_EVENT_ONESHOT\fR), but this may be changed with
\fBsd_event_source_set_enabled\fR(3)\&. If the handler function returns a negative error code, it will be disabled after the invocation, even if the
\fBSD_EVENT_ON\fR
mode was requested before\&.
.PP
To destroy an event source object use
\fBsd_event_source_unref\fR(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\*(Aqs still a reference to it kept, consider setting the event source to
\fBSD_EVENT_OFF\fR
with
\fBsd_event_source_set_enabled\fR(3)\&.
.PP
If the second parameter of
\fBsd_event_add_child()\fR
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\&.
.PP
Note that the
\fIhandler\fR
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\&.
.PP
If both a child process state change event source and a
\fBSIGCHLD\fR
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\&.
.PP
\fBsd_event_source_get_child_pid()\fR
retrieves the configured PID of a child process state change event source created previously with
\fBsd_event_add_child()\fR\&. It takes the event source object as the
\fIsource\fR
parameter and a pointer to a
\fBpid_t\fR
variable to return the process ID in\&.
.SH "RETURN VALUE"
.PP
On success, these functions return 0 or a positive integer\&. On failure, they return a negative errno\-style error code\&.
.SH "ERRORS"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-ENOMEM\fR
.RS 4
Not enough memory to allocate an object\&.
.RE
.PP
\fB\-EINVAL\fR
.RS 4
An invalid argument has been passed\&. This includes specifying an empty mask in
\fIoptions\fR
or a mask which contains values different than a combination of
\fBWEXITED\fR,
\fBWSTOPPED\fR, and
\fBWCONTINUED\fR\&.
.RE
.PP
\fB\-EBUSY\fR
.RS 4
A handler is already installed for this child process\&.
.RE
.PP
\fB\-ESTALE\fR
.RS 4
The event loop is already terminated\&.
.RE
.PP
\fB\-ECHILD\fR
.RS 4
The event loop has been created in a different process\&.
.RE
.PP
\fB\-EDOM\fR
.RS 4
The passed event source is not a child process event source\&.
.RE
.SH "NOTES"
.PP
These APIs are implemented as a shared library, which can be compiled and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1),
\fBsd-event\fR(3),
\fBsd_event_new\fR(3),
\fBsd_event_now\fR(3),
\fBsd_event_add_io\fR(3),
\fBsd_event_add_time\fR(3),
\fBsd_event_add_signal\fR(3),
\fBsd_event_add_defer\fR(3),
\fBsd_event_source_set_enabled\fR(3),
\fBsd_event_source_set_priority\fR(3),
\fBsd_event_source_set_userdata\fR(3),
\fBsd_event_source_set_description\fR(3),
\fBwaitid\fR(2)