sd_event_child_handler_t(3) Add a child process state change event source to an event loop

Other Alias

sd_event_add_child, sd_event_source_get_child_pid


#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);



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.


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 WEXITED, WSTOPPED, and WCONTINUED.


A handler is already installed for this child process.


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.


These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.