1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
|
.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)read.2 8.4 (Berkeley) 2/26/94
.\" $FreeBSD$
.\"
.Dd February 26, 1994
.Dt READ 2
.Os
.Sh NAME
.Nm read ,
.Nm readv ,
.Nm pread
.Nd read input
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/uio.h
.In unistd.h
.Ft ssize_t
.Fn read "int d" "void *buf" "size_t nbytes"
.Ft ssize_t
.Fn readv "int d" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
.Sh DESCRIPTION
The
.Fn read
system call
attempts to read
.Fa nbytes
of data from the object referenced by the descriptor
.Fa d
into the buffer pointed to by
.Fa buf .
The
.Fn readv
system call
performs the same action, but scatters the input data
into the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
The
.Fn pread
system call
performs the same function, but reads from the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn readv ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
void *iov_base; /* Base address. */
size_t iov_len; /* Length. */
};
.Ed
.Pp
Each
.Fa iovec
entry specifies the base address and length of an area
in memory where data should be placed.
The
.Fn readv
system call
will always fill an area completely before proceeding
to the next.
.Pp
On objects capable of seeking, the
.Fn read
starts at a position
given by the pointer associated with
.Fa d
(see
.Xr lseek 2 ) .
Upon return from
.Fn read ,
the pointer is incremented by the number of bytes actually read.
.Pp
Objects that are not capable of seeking always read from the current
position. The value of the pointer associated with such an
object is undefined.
.Pp
Upon successful completion,
.Fn read ,
.Fn readv ,
and
.Fn pread
return the number of bytes actually read and placed in the buffer.
The system guarantees to read the number of bytes requested if
the descriptor references a normal file that has that many bytes left
before the end-of-file, but in no other case.
.Sh RETURN VALUES
If successful, the
number of bytes actually read is returned.
Upon reading end-of-file,
zero is returned.
Otherwise, a -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn read ,
.Fn readv ,
and
.Fn pread
system calls
will succeed unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa d
argument
is not a valid file or socket descriptor open for reading.
.It Bq Er EFAULT
The
.Fa buf
argument
points outside the allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er EINTR
A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
.It Bq Er EINVAL
The pointer associated with
.Fa d
was negative.
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data were ready to be read.
.It Bq Er EPERM
The file descriptor is associated with a non regular file
(e.g. a directory) mounted over NFS.
.El
.Pp
In addition,
.Fn readv
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa iovcnt
argument
was less than or equal to 0, or greater than 16.
.It Bq Er EINVAL
One of the
.Fa iov_len
values in the
.Fa iov
array was negative.
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.It Bq Er EFAULT
Part of the
.Fa iov
points outside the process's allocated address space.
.El
.Pp
The
.Fn pread
system call may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The specified file offset is invalid.
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr fcntl 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
The
.Fn read
system call is expected to conform to
.St -p1003.1-90 .
The
.Fn readv
and
.Fn pread
system calls are expected to conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn pread
function appeared in
.At V.4 .
The
.Fn readv
system call appeared in
.Bx 4.2 .
The
.Fn read
function appeared in
.At v6 .
|