man2/epoll_create.2

   1 .\"  Copyright (C) 2003  Davide Libenzi
   2 .\" and Copyright 2008, 2009, 2012 Michael Kerrisk <tk.manpages@gmail.com>
   3 .\"  Davide Libenzi <davidel@xmailserver.org>
   4 .\"
   5 .\" SPDX-License-Identifier: GPL-2.0-or-later
   6 .\"
   7 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
   8 .\" Modified 2005-04-04 by Marko Kohtala <marko.kohtala@gmail.com>
   9 .\" 2008-10-10, mtk: add description of epoll_create1()
  10 .\"
  11 .TH EPOLL_CREATE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  12 .SH NAME
  13 epoll_create, epoll_create1 \- open an epoll file descriptor
  14 .SH LIBRARY
  15 Standard C library
  16 .RI ( libc ", " \-lc )
  17 .SH SYNOPSIS
  18 .nf
  19 .B #include <sys/epoll.h>
  20 .PP
  21 .BI "int epoll_create(int " size );
  22 .BI "int epoll_create1(int " flags );
  23 .fi
  24 .SH DESCRIPTION
  25 .BR epoll_create ()
  26 creates a new
  27 .BR epoll (7)
  28 instance.
  29 Since Linux 2.6.8, the
  30 .I size
  31 argument is ignored, but must be greater than zero; see NOTES.
  32 .PP
  33 .BR epoll_create ()
  34 returns a file descriptor referring to the new epoll instance.
  35 This file descriptor is used for all the subsequent calls to the
  36 .B epoll
  37 interface.
  38 When no longer required, the file descriptor returned by
  39 .BR epoll_create ()
  40 should be closed by using
  41 .BR close (2).
  42 When all file descriptors referring to an epoll instance have been closed,
  43 the kernel destroys the instance
  44 and releases the associated resources for reuse.
  45 .SS epoll_create1()
  46 If
  47 .I flags
  48 is 0, then, other than the fact that the obsolete
  49 .I size
  50 argument is dropped,
  51 .BR epoll_create1 ()
  52 is the same as
  53 .BR epoll_create ().
  54 The following value can be included in
  55 .I flags
  56 to obtain different behavior:
  57 .TP
  58 .B EPOLL_CLOEXEC
  59 Set the close-on-exec
  60 .RB ( FD_CLOEXEC )
  61 flag on the new file descriptor.
  62 See the description of the
  63 .B O_CLOEXEC
  64 flag in
  65 .BR open (2)
  66 for reasons why this may be useful.
  67 .SH RETURN VALUE
  68 On success,
  69 these system calls
  70 return a file descriptor (a nonnegative integer).
  71 On error, \-1 is returned, and
  72 .I errno
  73 is set to indicate the error.
  74 .SH ERRORS
  75 .TP
  76 .B EINVAL
  77 .I size
  78 is not positive.
  79 .TP
  80 .B EINVAL
  81 .RB ( epoll_create1 ())
  82 Invalid value specified in
  83 .IR flags .
  84 .TP
  85 .B EMFILE
  86 The per-user limit on the number of epoll instances imposed by
  87 .I /proc/sys/fs/epoll/max_user_instances
  88 was encountered.
  89 See
  90 .BR epoll (7)
  91 for further details.
  92 .TP
  93 .B EMFILE
  94 The per-process limit on the number of open file descriptors has been reached.
  95 .TP
  96 .B ENFILE
  97 The system-wide limit on the total number of open files has been reached.
  98 .TP
  99 .B ENOMEM
 100 There was insufficient memory to create the kernel object.
 101 .SH VERSIONS
 102 .BR epoll_create ()
 103 was added to the kernel in version 2.6.
 104 Library support is provided in glibc starting with version 2.3.2.
 105 .PP
 106 .\" To be precise: kernel 2.5.44.
 107 .\" The interface should be finalized by Linux kernel 2.5.66.
 108 .BR epoll_create1 ()
 109 was added to the kernel in version 2.6.27.
 110 Library support is provided in glibc starting with version 2.9.
 111 .SH CONFORMING TO
 112 .BR epoll_create ()
 113 and
 114 .BR epoll_create1 ()
 115 are Linux-specific.
 116 .SH NOTES
 117 In the initial
 118 .BR epoll_create ()
 119 implementation, the
 120 .I size
 121 argument informed the kernel of the number of file descriptors
 122 that the caller expected to add to the
 123 .B epoll
 124 instance.
 125 The kernel used this information as a hint for the amount of
 126 space to initially allocate in internal data structures describing events.
 127 (If necessary, the kernel would allocate more space
 128 if the caller's usage exceeded the hint given in
 129 .IR size .)
 130 Nowadays,
 131 this hint is no longer required
 132 (the kernel dynamically sizes the required data structures
 133 without needing the hint), but
 134 .I size
 135 must still be greater than zero,
 136 in order to ensure backward compatibility when new
 137 .B epoll
 138 applications are run on older kernels.
 139 .SH SEE ALSO
 140 .BR close (2),
 141 .BR epoll_ctl (2),
 142 .BR epoll_wait (2),
 143 .BR epoll (7)