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