.\" Copyright (c) 2006, 2010, 2020, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright (C) 2009 Petr Baudis <pasky@suse.cz>
.\" Copyright (c) 2020-2023, Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH sigevent 3type 2023-10-31 "Linux man-pages 6.7"
.SH NAME
sigevent, sigval \- structure for notification from asynchronous routines
.SH SYNOPSIS
.EX
.B #include <signal.h>
.P
.B struct sigevent {
.BR "    int             sigev_notify;" "  /* Notification type */"
.BR "    int             sigev_signo;" "   /* Signal number */"
.BR "    union sigval    sigev_value;" "   /* Data passed with notification */"
\&
.B  "    void          (*sigev_notify_function)(union sigval);"
.BR "                                " "   /* Notification function"
.BR "                                " "      (SIGEV_THREAD) */"
.B  "    pthread_attr_t *sigev_notify_attributes;"
.BR "                                " "   /* Notification attributes */"
\&
.BR "    " "/* Linux only: */"
.B  "    pid_t           sigev_notify_thread_id;"
.BR "                                " "   /* ID of thread to signal"
.BR "                                " "      (SIGEV_THREAD_ID) */"
.B };
.P
.BR "union sigval {" "                     /* Data passed with notification */"
.BR "    int             sival_int;" "     /* Integer value */"
.BR "    void           *sival_ptr;" "     /* Pointer value */"
.B };
.EE
.SH DESCRIPTION
.SS sigevent
The
.I sigevent
structure is used by various APIs
to describe the way a process is to be notified about an event
(e.g., completion of an asynchronous request, expiration of a timer,
or the arrival of a message).
.P
The definition shown in the SYNOPSIS is approximate:
some of the fields in the
.I sigevent
structure may be defined as part of a union.
Programs should employ only those fields relevant
to the value specified in
.IR sigev_notify .
.P
The
.I sigev_notify
field specifies how notification is to be performed.
This field can have one of the following values:
.TP
.B SIGEV_NONE
A "null" notification: don't do anything when the event occurs.
.TP
.B SIGEV_SIGNAL
Notify the process by sending the signal specified in
.IR sigev_signo .
.IP
If the signal is caught with a signal handler that was registered using the
.BR sigaction (2)
.B SA_SIGINFO
flag, then the following fields are set in the
.I siginfo_t
structure that is passed as the second argument of the handler:
.RS
.TP 10
.I si_code
This field is set to a value that depends on the API
delivering the notification.
.TP
.I si_signo
This field is set to the signal number (i.e., the same value as in
.IR sigev_signo ).
.TP
.I si_value
This field is set to the value specified in
.IR sigev_value .
.RE
.IP
Depending on the API, other fields may also be set in the
.I siginfo_t
structure.
.IP
The same information is also available if the signal is accepted using
.BR sigwaitinfo (2).
.TP
.B SIGEV_THREAD
Notify the process by invoking
.I sigev_notify_function
"as if" it were the start function of a new thread.
(Among the implementation possibilities here are that
each timer notification could result in the creation of a new thread,
or that a single thread is created to receive all notifications.)
The function is invoked with
.I sigev_value
as its sole argument.
If
.I sigev_notify_attributes
is not NULL, it should point to a
.I pthread_attr_t
structure that defines attributes for the new thread (see
.BR pthread_attr_init (3)).
.TP
.BR SIGEV_THREAD_ID " (Linux-specific)"
.\" | SIGEV_SIGNAL vs not?
Currently used only by POSIX timers; see
.BR timer_create (2).
.SS sigval
Data passed with a signal.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.P
.I <aio.h>
and
.I <time.h>
define
.I sigevent
since POSIX.1-2008.
.SH NOTES
The following headers also provide
.IR sigevent :
.IR <aio.h> ,
.IR <mqueue.h> ,
and
.IR <time.h> .
.SH SEE ALSO
.BR timer_create (2),
.BR getaddrinfo_a (3),
.BR lio_listio (3),
.BR mq_notify (3),
.BR pthread_sigqueue (3),
.BR sigqueue (3),
.BR aiocb (3type),
.BR siginfo_t (3type)