Index · Directives systemd 254


sd_event_add_signal, sd_event_source_get_signal, sd_event_signal_handler_t, SD_EVENT_SIGNAL_PROCMASK — 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 is a function to call when the signal is received or 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. The handler may return negative to signal an error (see below), other return values are ignored. If handler is NULL, a default handler that calls sd_event_exit(3) will be used.

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)). For convenience, if the special flag SD_EVENT_SIGNAL_PROCMASK is ORed into the specified signal the signal will be automatically masked as necessary, for the calling thread. Note that this only works reliably if the signal is already masked in all other threads of the process, or if there are no other threads at the moment of invocation.

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 either be disabled after the invocation, even if the SD_EVENT_ON mode was requested before, or it will cause the loop to terminate, see sd_event_source_set_exit_on_failure(3).

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, library or module instance.


The passed event source is not a signal event source.


Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.

The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.

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)