des_crypt.3: Minor wording fix in VERSIONS

[thirdparty/man-pages.git] / man2 / epoll_ctl.2

diff --git a/man2/epoll_ctl.2 b/man2/epoll_ctl.2
index 725c445e2b68f16acfb6495371ae06009aa298cf..bc2f1abf7a6a842ca8d21b18740b189f85e81ea9 100644 (file)
--- a/man2/epoll_ctl.2
+++ b/man2/epoll_ctl.2
@@ -1,7 +1,8 @@
-.\"
-.\"  epoll by Davide Libenzi ( efficient event notification retrieval )

 .\"  Copyright (C) 2003  Davide Libenzi
 .\"  Copyright (C) 2003  Davide Libenzi

+.\"  Davide Libenzi <davidel@xmailserver.org>
+.\" and Copyright 2009, 2014, 2016, 2018, 2019 Michael Kerrisk <tk.manpages@gmail.com>

 .\"
 .\"

+.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)

 .\"  This program is free software; you can redistribute it and/or modify
 .\"  it under the terms of the GNU General Public License as published by
 .\"  the Free Software Foundation; either version 2 of the License, or
 .\"  This program is free software; you can redistribute it and/or modify
 .\"  it under the terms of the GNU General Public License as published by
 .\"  the Free Software Foundation; either version 2 of the License, or


@@ -12,61 +13,60 @@
 .\"  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 .\"  GNU General Public License for more details.
 .\"
 .\"  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 .\"  GNU General Public License for more details.
 .\"

-.\"  You should have received a copy of the GNU General Public License
-.\"  along with this program; if not, write to the Free Software
-.\"  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
-.\"
-.\"  Davide Libenzi <davidel@xmailserver.org>
+.\" You should have received a copy of the GNU General Public
+.\" License along with this manual; if not, see
+.\" <http://www.gnu.org/licenses/>.
+.\" %%%LICENSE_END

 .\"
 .\"

-.TH EPOLL_CTL 2 2009-01-17 "Linux" "Linux Programmer's Manual"
+.TH EPOLL_CTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"

 .SH NAME
 .SH NAME

-epoll_ctl \- control interface for an epoll descriptor
+epoll_ctl \- control interface for an epoll file descriptor

 .SH SYNOPSIS
 .B #include <sys/epoll.h>
 .SH SYNOPSIS
 .B #include <sys/epoll.h>

-.sp
+.PP

 .BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
 ", struct epoll_event *" event );
 .SH DESCRIPTION
 .BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
 ", struct epoll_event *" event );
 .SH DESCRIPTION

-This system call performs control operations on the epoll instance
+This system call is used to add, modify, or remove
+entries in the interest list of the
+.BR epoll (7)
+instance

 referred to by the file descriptor
 .IR epfd .
 It requests that the operation
 .I op
 be performed for the target file descriptor,
 .IR fd .
 referred to by the file descriptor
 .IR epfd .
 It requests that the operation
 .I op
 be performed for the target file descriptor,
 .IR fd .

-
+.PP

 Valid values for the
 .I op
 Valid values for the
 .I op

-argument are :
+argument are:

 .TP
 .B EPOLL_CTL_ADD
 .TP
 .B EPOLL_CTL_ADD

-Register the target file descriptor
-.I fd
-on the
-.B epoll
-instance referred to by the file descriptor
-.I epfd
-and associate the event
-.I event
-with the internal file linked to
-.IR fd .
+Add an entry to the interest list of the epoll file descriptor,
+.IR epfd .
+The entry includes the file descriptor,
+.IR fd ,
+a reference to the corresponding open file description (see
+.BR epoll (7)
+and
+.BR open (2)),
+and the settings specified in
+.IR event .

 .TP
 .B EPOLL_CTL_MOD
 .TP
 .B EPOLL_CTL_MOD

-Change the event
-.I event
-associated with the target file descriptor
-.IR fd .
+Change the settings associated with
+.IR fd
+in the interest list to the new settings specified in
+.IR event .

 .TP
 .B EPOLL_CTL_DEL
 Remove (deregister) the target file descriptor
 .I fd
 .TP
 .B EPOLL_CTL_DEL
 Remove (deregister) the target file descriptor
 .I fd

-from the
-.B epoll
-instance referred to by
-.IR epfd .
+from the interest list.

 The
 .I event
 The
 .I event

-is ignored and can be NULL (but see BUGS below).
+argument is ignored and can be NULL (but see BUGS below).

 .PP
 The
 .I event
 .PP
 The
 .I event


@@ -74,28 +74,38 @@ argument describes the object linked to the file descriptor
 .IR fd .
 The
 .I struct epoll_event
 .IR fd .
 The
 .I struct epoll_event

-is defined as :
-.sp
+is defined as:
+.PP

 .in +4n
 .in +4n

-.nf
+.EX

 typedef union epoll_data {
 typedef union epoll_data {

-    void *ptr;
-    int fd;
-    __uint32_t u32;
-    __uint64_t u64;
+    void        *ptr;
+    int          fd;
+    uint32_t     u32;
+    uint64_t     u64;

 } epoll_data_t;
 
 struct epoll_event {
 } epoll_data_t;
 
 struct epoll_event {

-    __uint32_t events;      /* Epoll events */
-    epoll_data_t data;      /* User data variable */
+    uint32_t     events;      /* Epoll events */
+    epoll_data_t data;        /* User data variable */

 };
 };

-.fi
+.EE

 .in
 .in

-
+.PP
+The
+.I data
+member of the
+.I epoll_event
+structure specifies data that the kernel should save and then return (via
+.BR epoll_wait (2))
+when this file descriptor becomes ready.
+.PP

 The
 .I events
 The
 .I events

-member is a bit set composed using the following available event
-types:
+member of the
+.I epoll_event
+structure is a bit mask composed by ORing together zero or more of
+the following available event types:

 .TP
 .B EPOLLIN
 The associated file is available for
 .TP
 .B EPOLLIN
 The associated file is available for


@@ -111,40 +121,62 @@ operations.
 Stream socket peer closed connection,
 or shut down writing half of connection.
 (This flag is especially useful for writing simple code to detect
 Stream socket peer closed connection,
 or shut down writing half of connection.
 (This flag is especially useful for writing simple code to detect

-peer shutdown when using Edge Triggered monitoring.)
+peer shutdown when using edge-triggered monitoring.)

 .TP
 .B EPOLLPRI
 .TP
 .B EPOLLPRI

-There is urgent data available for
-.BR read (2)
-operations.
+There is an exceptional condition on the file descriptor.
+See the discussion of
+.B POLLPRI
+in
+.BR poll (2).

 .TP
 .B EPOLLERR
 Error condition happened on the associated file descriptor.
 .TP
 .B EPOLLERR
 Error condition happened on the associated file descriptor.

+This event is also reported for the write end of a pipe when the read end
+has been closed.
+.IP

 .BR epoll_wait (2)
 .BR epoll_wait (2)

-will always wait for this event; it is not necessary to set it in
-.IR events .
+will always report for this event; it is not necessary to set it in
+.IR events
+when calling
+.BR epoll_ctl ().

 .TP
 .B EPOLLHUP
 Hang up happened on the associated file descriptor.
 .TP
 .B EPOLLHUP
 Hang up happened on the associated file descriptor.

+.IP

 .BR epoll_wait (2)
 will always wait for this event; it is not necessary to set it in
 .BR epoll_wait (2)
 will always wait for this event; it is not necessary to set it in

-.IR events .
+.IR events
+when calling
+.BR epoll_ctl ().
+.IP
+Note that when reading from a channel such as a pipe or a stream socket,
+this event merely indicates that the peer closed its end of the channel.
+Subsequent reads from the channel will return 0 (end of file)
+only after all outstanding data in the channel has been consumed.

 .TP
 .B EPOLLET
 .TP
 .B EPOLLET

-Sets the Edge Triggered behavior for the associated file descriptor.
+Requests edge-triggered notification for the associated file descriptor.

 The default behavior for
 .B epoll
 The default behavior for
 .B epoll

-is Level Triggered.
+is level-triggered.

 See
 .BR epoll (7)
 See
 .BR epoll (7)

-for more detailed information about Edge and Level Triggered event
-distribution architectures.
+for more detailed information about edge-triggered and
+level-triggered notification.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).

 .TP
 .BR EPOLLONESHOT " (since Linux 2.6.2)"
 .TP
 .BR EPOLLONESHOT " (since Linux 2.6.2)"

-Sets the one-shot behavior for the associated file descriptor.
-This means that after an event is pulled out with
-.BR epoll_wait (2)
-the associated file descriptor is internally disabled and no other events
+Requests one-shot notification for the associated file descriptor.
+This means that after an event notified for the file descriptor by
+.BR epoll_wait (2),
+the file descriptor is disabled in the interest list and no other events

 will be reported by the
 .B epoll
 interface.
 will be reported by the
 .B epoll
 interface.


@@ -152,8 +184,129 @@ The user must call
 .BR epoll_ctl ()
 with
 .B EPOLL_CTL_MOD
 .BR epoll_ctl ()
 with
 .B EPOLL_CTL_MOD

-to re-register the file descriptor with a new event mask.
-.SH "RETURN VALUE"
+to rearm the file descriptor with a new event mask.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
+.TP
+.BR EPOLLWAKEUP " (since Linux 3.5)"
+.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
+If
+.B EPOLLONESHOT
+and
+.B EPOLLET
+are clear and the process has the
+.B CAP_BLOCK_SUSPEND
+capability,
+ensure that the system does not enter "suspend" or
+"hibernate" while this event is pending or being processed.
+The event is considered as being "processed" from the time
+when it is returned by a call to
+.BR epoll_wait (2)
+until the next call to
+.BR epoll_wait (2)
+on the same
+.BR epoll (7)
+file descriptor,
+the closure of that file descriptor,
+the removal of the event file descriptor with
+.BR EPOLL_CTL_DEL ,
+or the clearing of
+.B EPOLLWAKEUP
+for the event file descriptor with
+.BR EPOLL_CTL_MOD .
+See also BUGS.
+.IP
+This flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
+.TP
+.BR EPOLLEXCLUSIVE " (since Linux 4.5)"
+Sets an exclusive wakeup mode for the epoll file descriptor that is being
+attached to the target file descriptor,
+.IR fd .
+When a wakeup event occurs and multiple epoll file descriptors
+are attached to the same target file using
+.BR EPOLLEXCLUSIVE ,
+one or more of the epoll file descriptors will receive an event with
+.BR epoll_wait (2).
+The default in this scenario (when
+.BR EPOLLEXCLUSIVE
+is not set) is for all epoll file descriptors to receive an event.
+.BR EPOLLEXCLUSIVE
+is thus useful for avoiding thundering herd problems in certain scenarios.
+.IP
+If the same file descriptor is in multiple epoll instances,
+some with the
+.BR EPOLLEXCLUSIVE
+flag, and others without, then events will be provided to all epoll
+instances that did not specify
+.BR EPOLLEXCLUSIVE ,
+and at least one of the epoll instances that did specify
+.BR EPOLLEXCLUSIVE .
+.IP
+The following values may be specified in conjunction with
+.BR EPOLLEXCLUSIVE :
+.BR EPOLLIN ,
+.BR EPOLLOUT ,
+.BR EPOLLWAKEUP ,
+and
+.BR EPOLLET .
+.BR EPOLLHUP
+and
+.BR EPOLLERR
+can also be specified, but this is not required:
+as usual, these events are always reported if they occur,
+regardless of whether they are specified in
+.IR events .
+Attempts to specify other values in
+.I events
+yield the error
+.BR EINVAL .
+.IP
+.B EPOLLEXCLUSIVE
+may be used only in an
+.B EPOLL_CTL_ADD
+operation; attempts to employ it with
+.B EPOLL_CTL_MOD
+yield an error.
+If
+.B EPOLLEXCLUSIVE
+has been set using
+.BR epoll_ctl (),
+then a subsequent
+.B EPOLL_CTL_MOD
+on the same
+.IR epfd ",\ " fd
+pair yields an error.
+A call to
+.BR epoll_ctl ()
+that specifies
+.B EPOLLEXCLUSIVE
+in
+.I events
+and specifies the target file descriptor
+.I fd
+as an epoll instance will likewise fail.
+The error in all of these cases is
+.BR EINVAL .
+.IP
+The
+.BR EPOLLEXCLUSIVE
+flag is an input flag for the
+.I event.events
+field when calling
+.BR epoll_ctl ();
+it is never returned by
+.BR epoll_wait (2).
+.SH RETURN VALUE

 When successful,
 .BR epoll_ctl ()
 returns zero.
 When successful,
 .BR epoll_ctl ()
 returns zero.


@@ -191,6 +344,46 @@ or the requested operation
 .I op
 is not supported by this interface.
 .TP
 .I op
 is not supported by this interface.
 .TP

+.B EINVAL
+An invalid event type was specified along with
+.B EPOLLEXCLUSIVE
+in
+.IR events .
+.TP
+.B EINVAL
+.I op
+was
+.B EPOLL_CTL_MOD
+and
+.IR events
+included
+.BR EPOLLEXCLUSIVE .
+.TP
+.B EINVAL
+.I op
+was
+.B EPOLL_CTL_MOD
+and the
+.BR EPOLLEXCLUSIVE
+flag has previously been applied to this
+.IR epfd ",\ " fd
+pair.
+.TP
+.B EINVAL
+.BR EPOLLEXCLUSIVE
+was specified in
+.IR event
+and
+.I fd
+refers to an epoll instance.
+.TP
+.B ELOOP
+.I fd
+refers to an epoll instance and this
+.B EPOLL_CTL_ADD
+operation would result in a circular loop of epoll instances
+monitoring one another.
+.TP

 .B ENOENT
 .I op
 was
 .B ENOENT
 .I op
 was


@@ -199,13 +392,14 @@ or
 .BR EPOLL_CTL_DEL ,
 and
 .I fd
 .BR EPOLL_CTL_DEL ,
 and
 .I fd

-is not registered on with this epoll instance.
+is not registered with this epoll instance.

 .TP
 .B ENOMEM
 There was insufficient memory to handle the requested
 .I op
 control operation.
 .TP
 .B ENOMEM
 There was insufficient memory to handle the requested
 .I op
 control operation.

-.TP ENOSPC
+.TP
+.B ENOSPC

 The limit imposed by
 .I /proc/sys/fs/epoll/max_user_watches
 was encountered while trying to register
 The limit imposed by
 .I /proc/sys/fs/epoll/max_user_watches
 was encountered while trying to register


@@ -220,10 +414,18 @@ The target file
 .I fd
 does not support
 .BR epoll .
 .I fd
 does not support
 .BR epoll .

-.SH CONFORMING TO
+This error can occur if
+.I fd
+refers to, for example, a regular file or a directory.
+.SH VERSIONS

 .BR epoll_ctl ()
 .BR epoll_ctl ()

-is Linux-specific, and was introduced in kernel 2.5.44.
+was added to the kernel in version 2.6.
+.\" To be precise: kernel 2.5.44.

 .\" The interface should be finalized by Linux kernel 2.5.66.
 .\" The interface should be finalized by Linux kernel 2.5.66.

+.SH CONFORMING TO
+.BR epoll_ctl ()
+is Linux-specific.
+Library support is provided in glibc starting with version 2.3.2.

 .SH NOTES
 The
 .B epoll
 .SH NOTES
 The
 .B epoll


@@ -232,7 +434,7 @@ interface supports all file descriptors that support
 .SH BUGS
 In kernel versions before 2.6.9, the
 .B EPOLL_CTL_DEL
 .SH BUGS
 In kernel versions before 2.6.9, the
 .B EPOLL_CTL_DEL

-operation required a non-NULL pointer in
+operation required a non-null pointer in

 .IR event ,
 even though this argument is ignored.
 Since Linux 2.6.9,
 .IR event ,
 even though this argument is ignored.
 Since Linux 2.6.9,


@@ -241,9 +443,36 @@ can be specified as NULL
 when using
 .BR EPOLL_CTL_DEL .
 Applications that need to be portable to kernels before 2.6.9
 when using
 .BR EPOLL_CTL_DEL .
 Applications that need to be portable to kernels before 2.6.9

-should specify a non-NULL pointer in
+should specify a non-null pointer in

 .IR event .
 .IR event .

-.SH "SEE ALSO"
+.PP
+If
+.B EPOLLWAKEUP
+is specified in
+.IR flags ,
+but the caller does not have the
+.BR CAP_BLOCK_SUSPEND
+capability, then the
+.B EPOLLWAKEUP
+flag is
+.IR "silently ignored" .
+This unfortunate behavior is necessary because no validity
+checks were performed on the
+.IR flags
+argument in the original implementation, and the addition of the
+.B EPOLLWAKEUP
+with a check that caused the call to fail if the caller did not have the
+.B CAP_BLOCK_SUSPEND
+capability caused a breakage in at least one existing user-space
+application that happened to randomly (and uselessly) specify this bit.
+.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
+.\" https://lwn.net/Articles/520198/
+A robust application should therefore double check that it has the
+.B CAP_BLOCK_SUSPEND
+capability if attempting to use the
+.B EPOLLWAKEUP
+flag.
+.SH SEE ALSO

 .BR epoll_create (2),
 .BR epoll_wait (2),
 .BR poll (2),
 .BR epoll_create (2),
 .BR epoll_wait (2),
 .BR poll (2),