epoll_ctl(2)

1EPOLL_CTL(2)               Linux Programmer's Manual              EPOLL_CTL(2)
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NAME

6       epoll_ctl - control interface for an epoll descriptor
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SYNOPSIS

9       #include <sys/epoll.h>
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11       int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
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DESCRIPTION

14       This  system  call  performs  control  operations on the epoll instance
15       referred to by the file descriptor epfd.  It requests that  the  opera‐
16       tion op be performed for the target file descriptor, fd.
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18       Valid values for the op argument are :
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20       EPOLL_CTL_ADD
21              Register  the  target  file  descriptor fd on the epoll instance
22              referred to by the file descriptor epfd and associate the  event
23              event with the internal file linked to fd.
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25       EPOLL_CTL_MOD
26              Change  the event event associated with the target file descrip‐
27              tor fd.
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29       EPOLL_CTL_DEL
30              Remove (deregister) the target file descriptor fd from the epoll
31              instance  referred  to by epfd.  The event is ignored and can be
32              NULL (but see BUGS below).
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34       The event argument describes the object linked to the  file  descriptor
35       fd.  The struct epoll_event is defined as :
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37           typedef union epoll_data {
38               void        *ptr;
39               int          fd;
40               __uint32_t   u32;
41               __uint64_t   u64;
42           } epoll_data_t;
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44           struct epoll_event {
45               __uint32_t   events;      /* Epoll events */
46               epoll_data_t data;        /* User data variable */
47           };
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49       The  events  member is a bit set composed using the following available
50       event types:
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52       EPOLLIN
53              The associated file is available for read(2) operations.
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55       EPOLLOUT
56              The associated file is available for write(2) operations.
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58       EPOLLRDHUP (since Linux 2.6.17)
59              Stream socket peer closed connection, or shut down writing  half
60              of connection.  (This flag is especially useful for writing sim‐
61              ple code to detect peer shutdown when using Edge Triggered moni‐
62              toring.)
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64       EPOLLPRI
65              There is urgent data available for read(2) operations.
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67       EPOLLERR
68              Error  condition  happened  on  the  associated file descriptor.
69              epoll_wait(2) will always wait for this event; it is not  neces‐
70              sary to set it in events.
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72       EPOLLHUP
73              Hang   up   happened   on   the   associated   file  descriptor.
74              epoll_wait(2) will always wait for this event; it is not  neces‐
75              sary to set it in events.
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77       EPOLLET
78              Sets  the  Edge  Triggered  behavior  for  the  associated  file
79              descriptor.  The default behavior for epoll is Level  Triggered.
80              See  epoll(7) for more detailed information about Edge and Level
81              Triggered event distribution architectures.
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83       EPOLLONESHOT (since Linux 2.6.2)
84              Sets the one-shot behavior for the associated  file  descriptor.
85              This  means that after an event is pulled out with epoll_wait(2)
86              the associated file descriptor is  internally  disabled  and  no
87              other  events will be reported by the epoll interface.  The user
88              must call epoll_ctl()  with  EPOLL_CTL_MOD  to  rearm  the  file
89              descriptor with a new event mask.
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RETURN VALUE

92       When  successful,  epoll_ctl()  returns  zero.   When  an error occurs,
93       epoll_ctl() returns -1 and errno is set appropriately.
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ERRORS

96       EBADF  epfd or fd is not a valid file descriptor.
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98       EEXIST op was EPOLL_CTL_ADD, and the supplied  file  descriptor  fd  is
99              already registered with this epoll instance.
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101       EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd,
102              or the requested operation op is not supported  by  this  inter‐
103              face.
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105       ENOENT op  was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered
106              with this epoll instance.
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108       ENOMEM There was insufficient memory to handle the requested op control
109              operation.
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111       ENOSPC The  limit  imposed  by  /proc/sys/fs/epoll/max_user_watches was
112              encountered while trying to register (EPOLL_CTL_ADD) a new  file
113              descriptor  on  an  epoll  instance.   See  epoll(7) for further
114              details.
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116       EPERM  The target file fd does not support epoll.
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CONFORMING TO

119       epoll_ctl() is Linux-specific, and was introduced in kernel 2.5.44.
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NOTES

122       The epoll interface supports all file descriptors that support poll(2).
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BUGS

125       In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a
126       non-NULL pointer in event, even though this argument is ignored.  Since
127       Linux 2.6.9, event can be specified as NULL when  using  EPOLL_CTL_DEL.
128       Applications  that  need  to be portable to kernels before 2.6.9 should
129       specify a non-NULL pointer in event.
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COLOPHON

135       This page is part of release 3.25 of the Linux  man-pages  project.   A
136       description  of  the project, and information about reporting bugs, can
137       be found at http://www.kernel.org/doc/man-pages/.
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141Linux                             2009-01-17                      EPOLL_CTL(2)