sd_event_add_io(3) — Linux manual page


SD_EVENT_ADD_IO(3)           sd_event_add_io          SD_EVENT_ADD_IO(3)

NAME         top

       sd_event_add_io, sd_event_source_get_io_events,
       sd_event_source_set_io_events, sd_event_source_get_io_revents,
       sd_event_source_get_io_fd, sd_event_source_set_io_fd,
       sd_event_source_get_io_fd_own, sd_event_source_set_io_fd_own,
       sd_event_source, sd_event_io_handler_t - Add an I/O 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_io_handler_t)(sd_event_source *s, int fd,
                                            uint32_t revents,
                                            void *userdata);

       int sd_event_add_io(sd_event *event, sd_event_source **source,
                           int fd, uint32_t events,
                           sd_event_io_handler_t handler,
                           void *userdata);

       int sd_event_source_get_io_events(sd_event_source *source,
                                         uint32_t *events);

       int sd_event_source_set_io_events(sd_event_source *source,
                                         uint32_t events);

       int sd_event_source_get_io_revents(sd_event_source *source,
                                          uint32_t *revents);

       int sd_event_source_get_io_fd(sd_event_source *source);

       int sd_event_source_set_io_fd(sd_event_source *source, int fd);

       int sd_event_source_get_io_fd_own(sd_event_source *source);

       int sd_event_source_set_io_fd_own(sd_event_source *source,
                                         int b);

DESCRIPTION         top

       sd_event_add_io() adds a new I/O 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 fd
       parameter takes the UNIX file descriptor to watch, which may
       refer to a socket, a FIFO, a message queue, a serial connection,
       a character device, or any other file descriptor compatible with
       Linux epoll(7). The events parameter takes a bit mask of events
       to watch for, a combination of the following event flags:
       epoll_ctl(2) for details. Note that not all file descriptors are
       compatible with epoll, for example regular file or directories
       are not. If this function is called with a file descriptor that
       does not support epoll, -EPERM is returned (also see below). In
       most cases such file descriptors may be treated as
       always-readable or always-writable, so that IO event watching is

       The handler is a function to call when the event source is
       triggered or NULL. The userdata pointer will be passed to the
       handler function, and may be chosen freely by the caller. The
       handler will also be passed the file descriptor the event was
       seen on, as well as the actual event flags. It's generally a
       subset of the events watched, however may additionally include
       EPOLLERR and EPOLLHUP. 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

       By default, an event source will stay enabled continuously
       (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). Note that an event source
       set to SD_EVENT_ON will fire continuously unless data is read
       from or written to the file descriptor to reset the mask of
       events seen.

       Setting the I/O event mask to watch for to 0 does not mean that
       the event source won't be triggered anymore, as EPOLLHUP and
       EPOLLERR may be triggered even with a zero event mask. To
       temporarily disable an I/O event source use
       sd_event_source_set_enabled(3) with SD_EVENT_OFF instead.

       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_io() 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 to sd_event_add_io() 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).

       Note that this call does not take possession of the file
       descriptor passed in, ownership (and thus the duty to close it
       when it is no longer needed) remains with the caller. However,
       with the sd_event_source_set_io_fd_own() call (see below) the
       event source may optionally take ownership of the file descriptor
       after the event source has been created. In that case the file
       descriptor is closed automatically as soon as the event source is

       It is recommended to use sd_event_add_io() only in conjunction
       with file descriptors that have O_NONBLOCK set, to ensure that
       all I/O operations from invoked handlers are properly
       asynchronous and non-blocking. Using file descriptors without
       O_NONBLOCK might result in unexpected starvation of other event
       sources. See fcntl(2) for details on enabling O_NONBLOCK mode.

       sd_event_source_get_io_events() retrieves the configured mask of
       watched I/O events of an event source created previously with
       sd_event_add_io(). It takes the event source object and a pointer
       to a variable to store the mask in.

       sd_event_source_set_io_events() configures the mask of watched
       I/O events of an event source created previously with
       sd_event_add_io(). It takes the event source object and the new
       event mask.

       sd_event_source_get_io_revents() retrieves the I/O event mask of
       currently seen but undispatched events from an event source
       created previously with sd_event_add_io(). It takes the event
       source object and a pointer to a variable to store the event mask
       in. When called from a handler function on the handler's event
       source object this will return the same mask as passed to the
       handler's revents parameter. This call is primarily useful to
       check for undispatched events of an event source from the handler
       of an unrelated (possibly higher priority) event source. Note the
       relation between sd_event_source_get_pending() and
       sd_event_source_get_io_revents(): both functions will report
       non-zero results when there's an event pending for the event
       source, but the former applies to all event source types, the
       latter only to I/O event sources.

       sd_event_source_get_io_fd() retrieves the UNIX file descriptor of
       an event source created previously with sd_event_add_io(). It
       takes the event source object and returns the non-negative file
       descriptor or a negative error number on error (see below).

       sd_event_source_set_io_fd() changes the UNIX file descriptor of
       an I/O event source created previously with sd_event_add_io(). It
       takes the event source object and the new file descriptor.

       sd_event_source_set_io_fd_own() controls whether the file
       descriptor of the event source shall be closed automatically when
       the event source is freed, i.e. whether it shall be considered
       'owned' by the event source object. By default it is not closed
       automatically, and the application has to do this on its own. The
       b parameter is a boolean parameter: if zero, the file descriptor
       is not closed automatically when the event source is freed,
       otherwise it is closed.

       sd_event_source_get_io_fd_own() may be used to query the current
       setting of the file descriptor ownership boolean flag as set with
       sd_event_source_set_io_fd_own(). It returns positive if the file
       descriptor is closed automatically when the event source is
       destroyed, zero if not, and negative on error.

RETURN VALUE         top

       On success, these functions return 0 or a positive integer. On
       failure, they return a negative errno-style error code.

       Returned values may indicate the following problems:

           Not enough memory to allocate an object.

           An invalid argument has been passed.

           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 an I/O event source.

           The passed file descriptor does not support the epoll(7) API,
           for example because it is a regular file or directory. See
           epoll_ctl(2) for details.

           Added in version 255.

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_io_handler_t(), sd_event_add_io(),
       sd_event_source_get_io_events(), sd_event_source_set_io_events(),
       sd_event_source_get_io_revents(), sd_event_source_get_io_fd(),
       and sd_event_source_set_io_fd() were added in version 229.

       sd_event_source_get_io_fd_own() and
       sd_event_source_set_io_fd_own() were added in version 239.

SEE ALSO         top

       systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3),
       sd_event_add_time(3), sd_event_add_signal(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_priority(3), sd_event_source_set_userdata(3),
       sd_event_source_get_pending(3), sd_event_source_set_floating(3),
       epoll_ctl(2), epoll(7)

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_IO(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_memory_pressure(3)sd_event_add_signal(3)sd_event_add_time(3)sd_event_exit(3)sd_event_new(3)sd_event_run(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)