man2/read.2

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   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>
  34 .\"
  35 .TH READ 2 2007-11-15 "Linux" "Linux Programmer's Manual"
  36 .SH NAME
  37 read \- read from a file descriptor
  38 .SH SYNOPSIS
  39 .nf
  40 .B #include <unistd.h>
  41 .sp
  42 .BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
  43 .fi
  44 .SH DESCRIPTION
  45 .BR read ()
  46 attempts to read up to
  47 .I count
  48 bytes from file descriptor
  49 .I fd
  50 into the buffer starting at
  51 .IR buf .
  52 .PP
  53 If
  54 .I count
  55 is zero,
  56 .BR read ()
  57 returns zero and has no other results.
  58 If
  59 .I count
  60 is greater than
  61 .BR SSIZE_MAX ,
  62 the result is unspecified.
  63 .SH "RETURN VALUE"
  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
  70 .BR read ()
  71 was interrupted by a signal.
  72 On error, \-1 is returned, and
  73 .I errno
  74 is set appropriately.
  75 In this case it is left unspecified whether
  76 the file position (if any) changes.
  77 .SH ERRORS
  78 .TP
  79 .B EAGAIN
  80 Non-blocking I/O has been selected using
  81 .B O_NONBLOCK
  82 and no data was immediately available for reading.
  83 .TP
  84 .B EBADF
  85 .I fd
  86 is not a valid file descriptor or is not open for reading.
  87 .TP
  88 .B EFAULT
  89 .I buf
  90 is outside your accessible address space.
  91 .TP
  92 .B EINTR
  93 The call was interrupted by a signal before any data was read.
  94 .TP
  95 .B EINVAL
  96 .I fd
  97 is attached to an object which is unsuitable for reading;
  98 or the file was opened with the
  99 .B O_DIRECT
 100 flag, and either the address specified in
 101 .IR buf ,
 102 the value specified in
 103 .IR count ,
 104 or the current file offset is not suitably aligned.
 105 .TP
 106 .B EINVAL
 107 .I fd
 108 was created via a call to
 109 .BR timerfd_create (2)
 110 and the wrong size buffer was given to
 111 .BR read ();
 112 see
 113 .BR timerfd_create (2)
 114 for further information.
 115 .TP
 116 .B EIO
 117 I/O error.
 118 This will happen for example when the process is in a
 119 background process group, tries to read from its controlling tty,
 120 and either it is ignoring or blocking
 121 .B SIGTTIN
 122 or its process group
 123 is orphaned.
 124 It may also occur when there is a low-level I/O error
 125 while reading from a disk or tape.
 126 .TP
 127 .B EISDIR
 128 .I fd
 129 refers to a directory.
 130 .PP
 131 Other errors may occur, depending on the object connected to
 132 .IR fd .
 133 POSIX allows a
 134 .BR read ()
 135 that is interrupted after reading some data
 136 to return \-1 (with
 137 .I errno
 138 set to
 139 .BR EINTR )
 140 or to return the number of bytes already read.
 141 .SH "CONFORMING TO"
 142 SVr4, 4.3BSD, POSIX.1-2001.
 143 .SH NOTES
 144 On NFS file systems, reading small amounts of data will only update the
 145 timestamp the first time, subsequent calls may not do so.
 146 This is caused
 147 by client side attribute caching, because most if not all NFS clients
 148 leave st_atime (last file access time)
 149 updates to the server and client side reads satisfied from the
 150 client's cache will not cause st_atime updates on the server as there are no
 151 server side reads.
 152 UNIX semantics can be obtained by disabling client
 153 side attribute caching, but in most situations this will substantially
 154 increase server load and decrease performance.
 155 .PP
 156 Many filesystems and disks were considered to be fast enough that the
 157 implementation of
 158 .B O_NONBLOCK
 159 was deemed unnecessary.
 160 So,
 161 .B O_NONBLOCK
 162 may not be available on files
 163 and/or disks.
 164 .SH "SEE ALSO"
 165 .BR close (2),
 166 .BR fcntl (2),
 167 .BR ioctl (2),
 168 .BR lseek (2),
 169 .BR open (2),
 170 .BR pread (2),
 171 .BR readdir (2),
 172 .BR readlink (2),
 173 .BR readv (2),
 174 .BR select (2),
 175 .BR write (2),
 176 .BR fread (3)