bsd/man/man2/poll.2

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  51 .Dd February 27, 2005
  52 .Dt POLL 2
  53 .Os
  54 .Sh NAME
  55 .Nm poll
  56 .Nd synchronous I/O multiplexing
  57 .Sh SYNOPSIS
  58 .In poll.h
  59 .Ft int
  60 .Fo poll
  61 .Fa "struct pollfd fds[]"
  62 .Fa "nfds_t nfds"
  63 .Fa "int timeout"
  64 .Fc
  65 .Sh DESCRIPTION
  66 .Fn Poll
  67 examines a set of file descriptors
  68 to see if some of them are ready for I/O
  69 or if certain events have occurred on them.
  70 The
  71 .Fa fds
  72 argument is a pointer to an array of pollfd structures,
  73 as defined in
  74 .Aq Pa poll.h
  75 (shown below).  The
  76 .Fa nfds
  77 argument specifies the size of the
  78 .Fa fds
  79 array.
  80 .Bd -literal
  81 struct pollfd {
  82     int    fd;       /* file descriptor */
  83     short  events;   /* events to look for */
  84     short  revents;  /* events returned */
  85 };
  86 .Ed
  87 .Pp
  88 The fields of
  89 .Fa struct pollfd
  90 are as follows:
  91 .Bl -tag -width XXXPOLLWRNORM
  92 .It fd
  93 File descriptor to poll.
  94 .It events
  95 Events to poll for.  (See below.)
  96 .It revents
  97 Events which may occur or have occurred.  (See below.)
  98 .El
  99 .Pp
 100 The event bitmasks in
 101 .Fa events
 102 and
 103 .Fa revents
 104 have the following bits:
 105 .Bl -tag -width XXXPOLLWRNORM
 106 .\" ===========
 107 .It POLLERR
 108 An exceptional condition has occurred on the device or socket.
 109 This flag is output only, and ignored if present in the input
 110 .Fa events
 111 bitmask.
 112 .\" ===========
 113 .It POLLHUP
 114 The device or socket has been disconnected.
 115 This flag is output only,
 116 and ignored if present in the input
 117 .Fa events
 118 bitmask.
 119 Note that POLLHUP and POLLOUT
 120 are mutually exclusive and should never be present in the
 121 .Fa revents
 122 bitmask at the same time.
 123 .\" ===========
 124 .It POLLIN
 125 Data other than high priority data may be read without blocking.
 126 This is equivalent to ( POLLRDNORM | POLLRDBAND ).
 127 .\" ===========
 128 .It POLLNVAL
 129 The file descriptor is not open.
 130 This flag is output only, and ignored if present in the input
 131 .Fa events
 132 bitmask.
 133 .\" ===========
 134 .It POLLOUT
 135 Normal data may be written without blocking.
 136 This is equivalent to POLLWRNORM.
 137 .\" ===========
 138 .It POLLPRI
 139 High priority data may be read without blocking.
 140 .\" ===========
 141 .It POLLRDBAND
 142 Priority data may be read without blocking.
 143 .\" ===========
 144 .It POLLRDNORM
 145 Normal data may be read without blocking.
 146 .\" ===========
 147 .It POLLWRBAND
 148 Priority data may be written without blocking.
 149 .\" ===========
 150 .It POLLWRNORM
 151 Normal data may be written without blocking.
 152 .El
 153 .Pp
 154 The distinction between normal, priority, and high-priority data
 155 is specific to particular file types or devices.
 156 .Pp
 157 If
 158 .Fa timeout
 159 is greater than zero,
 160 it specifies a maximum interval (in milliseconds)
 161 to wait for any file descriptor to become ready.
 162 If
 163 .Fa timeout
 164 is zero, then
 165 .Fn poll
 166 will return without blocking. If the value of
 167 .Fa timeout
 168 is -1, the poll blocks indefinitely.
 169 .Sh RETURN VALUES
 170 .Fn Poll
 171 returns the number of descriptors that are ready for I/O,
 172 or -1 if an error occurred.
 173 If the time limit expires,
 174 .Fn poll
 175 returns 0.
 176 If
 177 .Fn poll
 178 returns with an error,
 179 including one due to an interrupted call,
 180 the
 181 .Fa fds
 182 array will be unmodified and the global variable
 183 .Va errno
 184 will be set to indicate the error.
 185 .Sh ERRORS
 186 .Fn Poll
 187 will fail if:
 188 .Bl -tag -width Er
 189 .\" ===========
 190 .It Bq Er EAGAIN
 191 Allocation of internal data structures fails.
 192 A subsequent request may succeed.
 193 .\" ===========
 194 .It Bq Er EFAULT
 195 .Fa Fds
 196 points outside the process's allocated address space.
 197 .\" ===========
 198 .It Bq Er EINTR
 199 A signal is delivered before the time limit expires
 200 and before any of the selected events occurs.
 201 .\" ===========
 202 .It Bq Er EINVAL
 203 The
 204 .Fa nfds
 205 argument is greater than OPEN_MAX or the
 206 .Fa timeout
 207 argument is less than -1.
 208 .El
 209 .Sh BUGS
 210 The
 211 .Fn poll
 212 system call currently does not support devices.
 213 .Sh SEE ALSO
 214 .Xr accept 2 ,
 215 .Xr connect 2 ,
 216 .Xr kevent 2 ,
 217 .Xr read 2 ,
 218 .Xr recv 2 ,
 219 .Xr select 2 ,
 220 .Xr send 2 ,
 221 .Xr write 2
 222 .Sh HISTORY
 223 The
 224 .Fn poll
 225 function call appeared in
 226 .At V .