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 1997-07-12 "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 SSIZE_MAX, the result is unspecified.
  61 .SH "RETURN VALUE"
  62 On success, the number of bytes read is returned (zero indicates end of
  63 file), and the file position is advanced by this number.
  64 It is not an error if this number is smaller than the number of bytes
  65 requested; this may happen for example because fewer bytes are actually
  66 available right now (maybe because we were close to end-of-file, or
  67 because we are reading from a pipe, or from a terminal), or because
  68 .BR read ()
  69 was interrupted by a signal.
  70 On error, \-1 is returned, and
  71 .I errno
  72 is set appropriately.
  73 In this case it is left unspecified whether
  74 the file position (if any) changes.
  75 .SH ERRORS
  76 .TP
  77 .B EAGAIN
  78 Non-blocking I/O has been selected using
  79 .B O_NONBLOCK
  80 and no data was immediately available for reading.
  81 .TP
  82 .B EBADF
  83 .I fd
  84 is not a valid file descriptor or is not open for reading.
  85 .TP
  86 .B EFAULT
  87 .I buf
  88 is outside your accessible address space.
  89 .TP
  90 .B EINTR
  91 The call was interrupted by a signal before any data was read.
  92 .TP
  93 .B EINVAL
  94 .I fd
  95 is attached to an object which is unsuitable for reading;
  96 or the file was opened with the
  97 .B O_DIRECT
  98 flag, and either the address specified in
  99 .IR buf ,
 100 the value specified in
 101 .IR count ,
 102 or the current file offset is not suitably aligned.
 103 .TP
 104 .B EIO
 105 I/O error.
 106 This will happen for example when the process is in a
 107 background process group, tries to read from its controlling tty,
 108 and either it is ignoring or blocking
 109 .B SIGTTIN
 110 or its process group
 111 is orphaned.
 112 It may also occur when there is a low-level I/O error
 113 while reading from a disk or tape.
 114 .TP
 115 .B EISDIR
 116 .I fd
 117 refers to a directory.
 118 .PP
 119 Other errors may occur, depending on the object connected to
 120 .IR fd .
 121 POSIX allows a
 122 .BR read ()
 123 that is interrupted after reading some data
 124 to return \-1 (with
 125 .I errno
 126 set to EINTR) or to return the number of bytes already read.
 127 .SH "CONFORMING TO"
 128 SVr4, 4.3BSD, POSIX.1-2001.
 129 .SH NOTES
 130 On NFS file systems, reading small amounts of data will only update the
 131 time stamp the first time, subsequent calls may not do so.
 132 This is caused
 133 by client side attribute caching, because most if not all NFS clients
 134 leave st_atime (last file access time)
 135 updates to the server and client side reads satisfied from the
 136 client's cache will not cause st_atime updates on the server as there are no
 137 server side reads.
 138 UNIX semantics can be obtained by disabling client
 139 side attribute caching, but in most situations this will substantially
 140 increase server load and decrease performance.
 141 .PP
 142 Many filesystems and disks were considered to be fast enough that the
 143 implementation of
 144 .B O_NONBLOCK
 145 was deemed unnecessary.
 146 So, O_NONBLOCK may not be available on files
 147 and/or disks.
 148 .SH "SEE ALSO"
 149 .BR close (2),
 150 .BR fcntl (2),
 151 .BR ioctl (2),
 152 .BR lseek (2),
 153 .BR open (2),
 154 .BR pread (2),
 155 .BR readdir (2),
 156 .BR readlink (2),
 157 .BR readv (2),
 158 .BR select (2),
 159 .BR write (2),
 160 .BR fread (3)