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sd_event_add_io(3) — Linux manual page

NAME \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|NOTES \|HISTORY \|SEE ALSO \|COLOPHON

SD_EVENT_ADD_IO(3)           sd_event_add_ioSD_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, intfd,uint32_trevents,void *userdata);int sd_event_add_io(sd_event *event, sd_event_source **source,intfd, uint32_tevents,sd_event_io_handler_thandler,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_tevents);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, intfd);int sd_event_source_get_io_fd_own(sd_event_source *source);int sd_event_source_set_io_fd_own(sd_event_source *source, intb);

DESCRIPTION top

sd_event_add_io()adds a new I/O event source to an event loop. The event loop object is specified in theevent parameter, the event source object is returned in thesource parameter. Thefd 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 Linuxepoll(7). Theevents parameter takes a bit mask of events to watch for, a combination of the following event flags:EPOLLIN,EPOLLOUT,EPOLLRDHUP,EPOLLPRI, andEPOLLET, seeepoll_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,-EPERMis 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. Thehandler is a function to call when the event source is triggered orNULL. Theuserdata 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 includeEPOLLERRandEPOLLHUP. The handler may return negative to signal an error (see below), other return values are ignored. Ifhandler isNULL, a default handler that callssd_event_exit(3) will be used. By default, an event source will stay enabled continuously (SD_EVENT_ON), but this may be changed withsd_event_source_set_enabled(3). If the handler function returns a negative error code, it will either be disabled after the invocation, even if theSD_EVENT_ONmode was requested before, or it will cause the loop to terminate, seesd_event_source_set_exit_on_failure(3). Note that an event source set toSD_EVENT_ONwill 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, asEPOLLHUPandEPOLLERRmay be triggered even with a zero event mask. To temporarily disable an I/O event source usesd_event_source_set_enabled(3) withSD_EVENT_OFFinstead. To destroy an event source object usesd_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 usingsd_event_source_set_enabled(3) withSD_EVENT_OFF. If the second parameter ofsd_event_add_io()isNULLno 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 thehandler tosd_event_add_io()isNULL, and the event source fires, this will be considered a request to exit the event loop. In this case, theuserdata parameter, cast to an integer, is passed as the exit code parameter tosd_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 thesd_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 usesd_event_add_io()only in conjunction with file descriptors that haveO_NONBLOCKset, to ensure that all I/O operations from invoked handlers are properly asynchronous and non-blocking. Using file descriptors withoutO_NONBLOCKmight result in unexpected starvation of other event sources. Seefcntl(2) for details on enablingO_NONBLOCKmode.sd_event_source_get_io_events()retrieves the configured mask of watched I/O events of an event source created previously withsd_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 withsd_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 withsd_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'srevents 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 betweensd_event_source_get_pending()andsd_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 withsd_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 withsd_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 ifsd_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 parameterb. When true (nonzero), the file descriptor will be closed automatically when the event source is freed or when the file descriptor is replaced bysd_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 withsd_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.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 theepoll(7) API,           for example because it is a regular file or directory. Seeepoll_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 thelibsystemd pkg-config(1) file.       The code described here usesgetenv(3), which is declared to be       not multi-thread-safe. This means that the code calling the       functions described here must not callsetenv(3) from a parallel       thread. It is recommended to only do calls tosetenv()from an       early phase of the program when no other threads have been       started.

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(), andsd_event_source_set_io_fd()were added in version 229.sd_event_source_get_io_fd_own()andsd_event_source_set_io_fd_own()were added in version 239.

COLOPHON top

       This page is part of thesystemd (systemd system and service       manager) project.  Information about the project can be found at       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a       bug report for this manual page, see       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.       This page was obtained from the project's upstream Git repository       ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11.  (At that       time, the date of the most recent commit that was found in the       repository was 2025-08-11.)  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 isnot part of the original manual page), send a mail to       man-pages@man7.orgsystemd 258~rc2SD_EVENT_ADD_IO(3)

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