sd_event_add_signal(3) — Linux manual page


SD_EVENT_ADD_SIGNAL(3)     sd_event_add_signal    SD_EVENT_ADD_SIGNAL(3)

NAME         top

       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

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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

       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_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         top

       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.

NOTES         top

       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

HISTORY         top

       sd_event_add_signal(), sd_event_signal_handler_t(), and
       sd_event_source_get_signal() were added in version 217.

SEE ALSO         top

       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_userdata(3), sd_event_source_set_floating(3),
       signal(7), signalfd(2), sigprocmask(2), pthread_sigmask(3)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 255                                       SD_EVENT_ADD_SIGNAL(3)

Pages that refer to this page: sd-event(3)sd_event_add_child(3)sd_event_add_defer(3)sd_event_add_inotify(3)sd_event_add_io(3)sd_event_add_time(3)sd_event_exit(3)sd_event_new(3)sd_event_run(3)sd_event_set_signal_exit(3)sd_event_set_watchdog(3)sd_event_source_get_event(3)sd_event_source_get_pending(3)sd_event_source_set_description(3)sd_event_source_set_destroy_callback(3)sd_event_source_set_enabled(3)sd_event_source_set_exit_on_failure(3)sd_event_source_set_floating(3)sd_event_source_set_prepare(3)sd_event_source_set_priority(3)sd_event_source_set_ratelimit(3)sd_event_source_set_userdata(3)sd_event_source_unref(3)sd_event_wait(3)systemd.directives(7)systemd.index(7)