Index · Directives systemd 250


sd_event_add_signal, sd_event_source_get_signal, sd_event_signal_handler_t — Add a UNIX process signal event source to an event loop


#include <systemd/sd-event.h>
typedef struct sd_event_source sd_event_source;
typedef int (*sd_event_signal_handler_t)(sd_event_source *s,
 const struct signalfd_siginfo *si,
 void *userdata);
int sd_event_add_signal(sd_event *event,
 sd_event_source **source,
 int signal,
 sd_event_signal_handler_t handler,
 void *userdata);
int sd_event_source_get_signal(sd_event_source *source);


sd_event_add_signal() adds a new UNIX process signal event source to an event loop. The event loop object is specified in the event parameter, and the event source object is returned in the source parameter. The signal parameter specifies the numeric signal to be handled (see signal(7)). The handler parameter must reference a function to call when the signal is received or be NULL. 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 signalfd_siginfo structure containing information about the received signal. See signalfd(2) for further information.

Only a single handler may be installed for a specific signal. The signal must be blocked in all threads before this function is called (using sigprocmask(2) or pthread_sigmask(3)).

By default, the event source is enabled permanently (SD_EVENT_ON), 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 if it is still referenced, disable the event source using sd_event_source_set_enabled(3) with SD_EVENT_OFF.

If the second parameter of sd_event_add_signal() is 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.

If the handler parameter to sd_event_add_signal() is NULL, and the event source fires, this will be considered a request to exit the event loop. In this case, the userdata parameter, cast to an integer, is passed as the exit code parameter to sd_event_exit(3).

sd_event_source_get_signal() returns the configured signal number of an event source created previously with sd_event_add_signal(). It takes the event source object as the source parameter.

Return Value

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.


A handler is already installed for this signal or the signal was not blocked previously.


The event loop is already terminated.


The event loop has been created in a different process.


The passed event source is not a signal event source.


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

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_child(3), sd_event_add_inotify(3), sd_event_add_defer(3), sd_event_source_set_enabled(3), sd_event_source_set_description(3), sd_event_source_set_userdata(3), sd_event_source_set_floating(3), signal(7), signalfd(2), sigprocmask(2), pthread_sigmask(3)