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1 .\" Copyright (c) 1998 Terry Lambert
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25 .\" $FreeBSD: src/lib/libc/sys/aio_read.2,v 1.19 2003/01/14 02:37:06 tjr Exp $
26 .\"
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 .Fo aio_read
39 .Fa "struct aiocb *aiocbp"
40 .Fc
41 .Sh DESCRIPTION
42 The
43 .Fn aio_read
44 system call allows the calling process to read
45 .Fa aiocbp->aio_nbytes
46 from the descriptor
47 .Fa aiocbp->aio_fildes ,
48 beginning at the offset
49 .Fa aiocbp->aio_offset ,
50 into the buffer pointed to by
51 .Fa aiocbp->aio_buf .
52 The call returns immediately after the read request has
53 been enqueued to the descriptor;
54 the read may or may not have completed at the time the call returns.
55 .Pp
56 If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it,
57 then the enqueued operation is submitted at a priority equal to that
58 of the calling process minus
59 .Fa aiocbp->aio_reqprio .
60 .Pp
61 The
62 .Fa aiocbp->aio_lio_opcode
63 argument
64 is ignored by the
65 .Fn aio_read
66 system call.
67 .Pp
68 The
69 .Fa aiocbp
70 pointer may be subsequently used as an argument to
71 .Fn aio_return
72 and
73 .Fn aio_error
74 in order to determine return or error status for the enqueued operation
75 while it is in progress.
76 .Pp
77 If the request could not be enqueued (generally due to invalid arguments),
78 then the call returns without having enqueued the request.
79 .Pp
80 If the request is successfully enqueued, the value of
81 .Fa aiocbp->aio_offset
82 can be modified during the request as context, so this value must
83 not be referenced after the request is enqueued.
84 .Sh RESTRICTIONS
85 The Asynchronous I/O Control Block structure pointed to by
86 .Fa aiocbp
87 and the buffer that the
88 .Fa aiocbp->aio_buf
89 member of that structure references must remain valid until the
90 operation has completed. For this reason, use of auto (stack) variables
91 for these objects is discouraged.
92 .Pp
93 The asynchronous I/O control buffer
94 .Fa aiocbp
95 should be zeroed before the
96 .Fn aio_read
97 call to avoid passing bogus context information to the kernel.
98 .Pp
99 Modifications of the Asynchronous I/O Control Block structure or the
100 buffer contents after the request has been enqueued, but before the
101 request has completed, are not allowed.
102 .Pp
103 If the file offset in
104 .Fa aiocbp->aio_offset
105 is past the offset maximum for
106 .Fa aiocbp->aio_fildes ,
107 no I/O will occur.
108 .Sh RETURN VALUES
109 .Rv -std aio_read
110 .Sh DIAGNOSTICS
111 None.
112 .Sh ERRORS
113 The
114 .Fn aio_read
115 system call will fail if:
116 .Bl -tag -width Er
117 .\" ==========
118 .It Bq Er EAGAIN
119 Because of system resource limitations,
120 the request was not queued.
121 .\" ==========
122 .It Bq Er ENOSYS
123 The
124 .Fn aio_read
125 system call is not supported.
126 .El
127 .Pp
128 The following conditions may be synchronously detected when the
129 .Fn aio_read
130 system call is made, or asynchronously, at any time thereafter.
131 If they are detected at call time,
132 .Fn aio_read
133 returns -1 and sets
134 .Va errno
135 appropriately.
136 Otherwise, the
137 .Fn aio_return
138 system call must be called.
139 It will return -1;
140 .Fn aio_error
141 must then be called to determine the actual value
142 that would have been returned in
143 .Va errno .
144 .Pp
145 .Bl -tag -width Er
146 .\" ==========
147 .It Bq Er EBADF
148 The
149 .Fa aiocbp->aio_fildes
150 argument
151 is invalid.
152 .\" ==========
153 .It Bq Er EINVAL
154 The offset
155 .Fa aiocbp->aio_offset
156 is not valid, the priority specified by
157 .Fa aiocbp->aio_reqprio
158 is not a valid priority, or the number of bytes specified by
159 .Fa aiocbp->aio_nbytes
160 is not valid.
161 .\" ==========
162 .It Bq Er EOVERFLOW
163 The file is a regular file,
164 .Fa aiocbp->aio_nbytes
165 is greater than zero, the starting offset in
166 .Fa aiocbp->aio_offset
167 is before the end of the file, but is at or beyond the
168 .Fa aiocbp->aio_fildes
169 offset maximum.
170 .El
171 .Pp
172 If the request is successfully enqueued, but subsequently cancelled
173 or an error occurs, the value returned by the
174 .Fn aio_return
175 system call is per the
176 .Xr read 2
177 system call, and the value returned by the
178 .Fn aio_error
179 system call is either one of the error returns from the
180 .Xr read 2
181 system call, or one of:
182 .Bl -tag -width Er
183 .\" ==========
184 .It Bq Er EBADF
185 The
186 .Fa aiocbp->aio_fildes
187 argument
188 is invalid for reading.
189 .\" ==========
190 .It Bq Er ECANCELED
191 The request was explicitly cancelled via a call to
192 .Fn aio_cancel .
193 .\" ==========
194 .It Bq Er EINVAL
195 The offset
196 .Fa aiocbp->aio_offset
197 would be invalid.
198 .El
199 .Sh SEE ALSO
200 .Xr aio_cancel 2 ,
201 .Xr aio_error 2 ,
202 .Xr aio_return 2 ,
203 .Xr aio_suspend 2 ,
204 .Xr aio_write 2 ,
205 .Xr aio 4
206 .Sh STANDARDS
207 The
208 .Fn aio_read
209 system call is expected to conform to the
210 .St -p1003.1
211 standard.
212 .Sh HISTORY
213 The
214 .Fn aio_read
215 system call first appeared in
216 .Fx 3.0 .
217 .Sh AUTHORS
218 This
219 manual page was written by
220 .An Terry Lambert Aq terry@whistle.com .
221 .Sh BUGS
222 Invalid information in
223 .Fa aiocbp->_aiocb_private
224 may confuse the kernel.