epoll_ctl.2: Document ELOOP error for circular monitoring loops

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

.\"  Copyright (C) 2003  Davide Libenzi
.\"  Davide Libenzi <davidel@xmailserver.org>
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.TH EPOLL_CTL 2 2015-05-07 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_ctl \- control interface for an epoll file descriptor
.SH SYNOPSIS
.B #include <sys/epoll.h>
.sp
.BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
", struct epoll_event *" event );
.SH DESCRIPTION
This system call performs control operations on 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 .

Valid values for the
.I op
argument are:
.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 .
.TP
.B EPOLL_CTL_MOD
Change the event
.I event
associated with the target file descriptor
.IR fd .
.TP
.B EPOLL_CTL_DEL
Remove (deregister) the target file descriptor
.I fd
from the
.B epoll
instance referred to by
.IR epfd .
The
.I event
is ignored and can be NULL (but see BUGS below).
.PP
The
.I event
argument describes the object linked to the file descriptor
.IR fd .
The
.I struct epoll_event
is defined as:
.sp
.in +4n
.nf
typedef union epoll_data {
    void        *ptr;
    int          fd;
    uint32_t     u32;
    uint64_t     u64;
} epoll_data_t;

struct epoll_event {
    uint32_t     events;      /* Epoll events */
    epoll_data_t data;        /* User data variable */
};
.fi
.in

The
.I events
member is a bit mask composed using the following available event
types:
.TP
.B EPOLLIN
The associated file is available for
.BR read (2)
operations.
.TP
.B EPOLLOUT
The associated file is available for
.BR write (2)
operations.
.TP
.BR EPOLLRDHUP " (since Linux 2.6.17)"
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.)
.TP
.B EPOLLPRI
There is urgent data available for
.BR read (2)
operations.
.TP
.B EPOLLERR
Error condition happened on the associated file descriptor.
.BR epoll_wait (2)
will always wait for this event; it is not necessary to set it in
.IR events .
.TP
.B EPOLLHUP
Hang up happened on the associated file descriptor.
.BR epoll_wait (2)
will always wait for this event; it is not necessary to set it in
.IR events .
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
Sets the Edge Triggered behavior for the associated file descriptor.
The default behavior for
.B epoll
is Level Triggered.
See
.BR epoll (7)
for more detailed information about Edge and Level Triggered event
distribution architectures.
.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
will be reported by the
.B epoll
interface.
The user must call
.BR epoll_ctl ()
with
.B EPOLL_CTL_MOD
to rearm the file descriptor with a new event mask.
.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.
.SH RETURN VALUE
When successful,
.BR epoll_ctl ()
returns zero.
When an error occurs,
.BR epoll_ctl ()
returns \-1 and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I epfd
or
.I fd
is not a valid file descriptor.
.TP
.B EEXIST
.I op
was
.BR EPOLL_CTL_ADD ,
and the supplied file descriptor
.I fd
is already registered with this epoll instance.
.TP
.B EINVAL
.I epfd
is not an
.B epoll
file descriptor,
or
.I fd
is the same as
.IR epfd ,
or the requested operation
.I op
is not supported by this interface.
.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 EPOLL_CTL_MOD
or
.BR EPOLL_CTL_DEL ,
and
.I fd