bsd/man/man2/aio_read.2

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  25 .\" $FreeBSD: src/lib/libc/sys/aio_read.2,v 1.19 2003/01/14 02:37:06 tjr Exp $
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  27 .Dd November 17, 1998
  28 .Dt AIO_READ 2
  29 .Os
  30 .Sh NAME
  31 .Nm aio_read
  32 .Nd asynchronous read from a file (REALTIME)
  33 .Sh LIBRARY
  34 .Lb libc
  35 .Sh SYNOPSIS
  36 .In aio.h
  37 .Ft int
  38 .Fn aio_read "struct aiocb *iocb"
  39 .Sh DESCRIPTION
  40 The
  41 .Fn aio_read
  42 system call allows the calling process to read
  43 .Fa iocb->aio_nbytes
  44 from the descriptor
  45 .Fa iocb->aio_fildes
  46 beginning at the offset
  47 .Fa iocb->aio_offset
  48 into the buffer pointed to by
  49 .Fa iocb->aio_buf .
  50 The call returns immediately after the read request has
  51 been enqueued to the descriptor; the read may or may not have
  52 completed at the time the call returns.
  53 .Pp
  54 If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it,
  55 then the enqueued operation is submitted at a priority equal to that
  56 of the calling process minus
  57 .Fa iocb->aio_reqprio .
  58 .Pp
  59 The
  60 .Fa iocb->aio_lio_opcode
  61 argument
  62 is ignored by the
  63 .Fn aio_read
  64 system call.
  65 .Pp
  66 The
  67 .Fa iocb
  68 pointer may be subsequently used as an argument to
  69 .Fn aio_return
  70 and
  71 .Fn aio_error
  72 in order to determine return or error status for the enqueued operation
  73 while it is in progress.
  74 .Pp
  75 If the request could not be enqueued (generally due to invalid arguments),
  76 then the call returns without having enqueued the request.
  77 .Pp
  78 If the request is successfully enqueued, the value of
  79 .Fa iocb->aio_offset
  80 can be modified during the request as context, so this value must
  81 not be referenced after the request is enqueued.
  82 .Sh RESTRICTIONS
  83 The Asynchronous I/O Control Block structure pointed to by
  84 .Fa iocb
  85 and the buffer that the
  86 .Fa iocb->aio_buf
  87 member of that structure references must remain valid until the
  88 operation has completed.  For this reason, use of auto (stack) variables
  89 for these objects is discouraged.
  90 .Pp
  91 The asynchronous I/O control buffer
  92 .Fa iocb
  93 should be zeroed before the
  94 .Fn aio_read
  95 call to avoid passing bogus context information to the kernel.
  96 .Pp
  97 Modifications of the Asynchronous I/O Control Block structure or the
  98 buffer contents after the request has been enqueued, but before the
  99 request has completed, are not allowed.
 100 .Pp
 101 If the file offset in
 102 .Fa iocb->aio_offset
 103 is past the offset maximum  for
 104 .Fa iocb->aio_fildes ,
 105 no I/O will occur.
 106 .Sh RETURN VALUES
 107 .Rv -std aio_read
 108 .Sh DIAGNOSTICS
 109 None.
 110 .Sh ERRORS
 111 The
 112 .Fn aio_read
 113 system call will fail if:
 114 .Bl -tag -width Er
 115 .It Bq Er EAGAIN
 116 The request was not queued because of system resource limitations.
 117 .It Bq Er ENOSYS
 118 The
 119 .Fn aio_read
 120 system call is not supported.
 121 .El
 122 .Pp
 123 The following conditions may be synchronously detected when the
 124 .Fn aio_read
 125 system call is made, or asynchronously, at any time thereafter.  If they
 126 are detected at call time,
 127 .Fn aio_read
 128 returns -1 and sets
 129 .Va errno
 130 appropriately; otherwise the
 131 .Fn aio_return
 132 system call must be called, and will return -1, and
 133 .Fn aio_error
 134 must be called to determine the actual value that would have been
 135 returned in
 136 .Va errno .
 137 .Pp
 138 .Bl -tag -width Er
 139 .It Bq Er EBADF
 140 The
 141 .Fa iocb->aio_fildes
 142 argument
 143 is invalid.
 144 .It Bq Er EINVAL
 145 The offset
 146 .Fa iocb->aio_offset
 147 is not valid, the priority specified by
 148 .Fa iocb->aio_reqprio
 149 is not a valid priority, or the number of bytes specified by
 150 .Fa iocb->aio_nbytes
 151 is not valid.
 152 .It Bq Er EOVERFLOW
 153 The file is a regular file,
 154 .Fa iocb->aio_nbytes
 155 is greater than zero, the starting offset in
 156 .Fa iocb->aio_offset
 157 is before the end of the file, but is at or beyond the
 158 .Fa iocb->aio_fildes
 159 offset maximum.
 160 .El
 161 .Pp
 162 If the request is successfully enqueued, but subsequently cancelled
 163 or an error occurs, the value returned by the
 164 .Fn aio_return
 165 system call is per the
 166 .Xr read 2
 167 system call, and the value returned by the
 168 .Fn aio_error
 169 system call is either one of the error returns from the
 170 .Xr read 2
 171 system call, or one of:
 172 .Bl -tag -width Er
 173 .It Bq Er EBADF
 174 The
 175 .Fa iocb->aio_fildes
 176 argument
 177 is invalid for reading.
 178 .It Bq Er ECANCELED
 179 The request was explicitly cancelled via a call to
 180 .Fn aio_cancel .
 181 .It Bq Er EINVAL
 182 The offset
 183 .Fa iocb->aio_offset
 184 would be invalid.
 185 .El
 186 .Sh SEE ALSO
 187 .Xr aio_cancel 2 ,
 188 .Xr aio_error 2 ,
 189 .Xr aio_return 2 ,
 190 .Xr aio_suspend 2 ,
 191 .Xr aio_write 2 ,
 192 .Xr aio 4
 193 .Sh STANDARDS
 194 The
 195 .Fn aio_read
 196 system call is expected to conform to the
 197 .St -p1003.1
 198 standard.
 199 .Sh HISTORY
 200 The
 201 .Fn aio_read
 202 system call first appeared in
 203 .Fx 3.0 .
 204 .Sh AUTHORS
 205 This
 206 manual page was written by
 207 .An Terry Lambert Aq terry@whistle.com .
 208 .Sh BUGS
 209 Invalid information in
 210 .Fa iocb->_aiocb_private
 211 may confuse the kernel.