# sd_event_add_io

## NAME



           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



           **#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 ***_ret_**);**
    
           **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 ***_ret_**);**
    
           **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



           **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: **EPOLLIN** ,
           **EPOLLOUT** , **EPOLLRDHUP** , **EPOLLPRI** , and **EPOLLET** , see 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 unnecessary.
    
           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 used.
    
           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 will not 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
           released.
    
           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 as
           parameters. If the event source owned the previous file
           descriptor, that is if **sd_event_source_set_io_fd_own()** had been
           called for the event source with a non-zero value, then the
           previous file descriptor will be closed and the event source will
           also take the ownership of the new file descriptor on success.
    
           **sd_event_source_set_io_fd_own()** controls whether the file
           descriptor of the event source shall take ownership of the file
           descriptor. Takes a boolean parameter _b_. When true (nonzero), the
           file descriptor will be closed automatically when the event source
           is freed or when the file descriptor is replaced by
           **sd_event_source_set_io_fd()**. By default, the descriptor is not
           owned by the event source, and the application has to do close it
           on its own if needed.
    
           **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



           On success, these functions return 0 or a positive integer. On
           failure, they return a negative errno-style error code.
    
       **Errors**
           Returned values may indicate the following problems:
    
           **-ENOMEM**
               Not enough memory to allocate an object.
    
           **-EINVAL**
               An invalid argument has been passed.
    
           **-ESTALE**
               The event loop is already terminated.
    
           **-ECHILD**
               The event loop has been created in a different process,
               library or module instance.
    
           **-EDOM**
               The passed event source is not an I/O event source.
    
           **-EPERM**
               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



           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.
    


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