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3 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
4 .\" 1993 Michael Haardt, Ian Jackson.
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26 .\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
27 .\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
28 .\" <michael@cantor.informatik.rwth-aachen.de>
29 .\" Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aeb@cwi.nl>
30 .\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
31 .\" Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esr@thyrsus.com>
32 .\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
33 .\" <michael@cantor.informatik.rwth-aachen.de>
35 .TH READ 2 2009-02-23 "Linux" "Linux Programmer's Manual"
37 read \- read from a file descriptor
40 .B #include <unistd.h>
42 .BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
46 attempts to read up to
48 bytes from file descriptor
50 into the buffer starting at
57 returns zero and has no other results.
62 the result is unspecified.
64 On success, the number of bytes read is returned (zero indicates end of
65 file), and the file position is advanced by this number.
66 It is not an error if this number is smaller than the number of bytes
67 requested; this may happen for example because fewer bytes are actually
68 available right now (maybe because we were close to end-of-file, or
69 because we are reading from a pipe, or from a terminal), or because
71 was interrupted by a signal.
72 On error, \-1 is returned, and
75 In this case it is left unspecified whether
76 the file position (if any) changes.
82 refers to a file other than a socket and has been marked nonblocking
84 and the read would block.
86 .BR EAGAIN " or " EWOULDBLOCK
87 .\" Actually EAGAIN on Linux
90 refers to a socket and has been marked nonblocking
92 and the read would block.
93 POSIX.1-2001 allows either error to be returned for this case,
94 and does not require these constants to have the same value,
95 so a portable application should check for both possibilities.
99 is not a valid file descriptor or is not open for reading.
103 is outside your accessible address space.
106 The call was interrupted by a signal before any data was read; see
111 is attached to an object which is unsuitable for reading;
112 or the file was opened with the
114 flag, and either the address specified in
116 the value specified in
118 or the current file offset is not suitably aligned.
122 was created via a call to
123 .BR timerfd_create (2)
124 and the wrong size buffer was given to
127 .BR timerfd_create (2)
128 for further information.
132 This will happen for example when the process is in a
133 background process group, tries to read from its controlling tty,
134 and either it is ignoring or blocking
138 It may also occur when there is a low-level I/O error
139 while reading from a disk or tape.
143 refers to a directory.
145 Other errors may occur, depending on the object connected to
149 that is interrupted after reading some data
154 or to return the number of bytes already read.
156 SVr4, 4.3BSD, POSIX.1-2001.
158 On NFS file systems, reading small amounts of data will only update the
159 timestamp the first time, subsequent calls may not do so.
161 by client side attribute caching, because most if not all NFS clients
162 leave st_atime (last file access time)
163 updates to the server and client side reads satisfied from the
164 client's cache will not cause st_atime updates on the server as there are no
166 Unix semantics can be obtained by disabling client
167 side attribute caching, but in most situations this will substantially
168 increase server load and decrease performance.
170 Many file systems and disks were considered to be fast enough that the
173 was deemed unnecessary.
176 may not be available on files