bsd/man/man2/lseek.2

   1 .\"     $NetBSD: lseek.2,v 1.6 1995/02/27 12:34:09 cgd Exp $
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  34 .\"     @(#)lseek.2     8.3 (Berkeley) 4/19/94
  35 .\"
  36 .Dd April 19, 1994
  37 .Dt LSEEK 2
  38 .Os BSD 4
  39 .Sh NAME
  40 .Nm lseek
  41 .Nd reposition read/write file offset
  42 .Sh SYNOPSIS
  43 .Fd #include <unistd.h>
  44 .Ft off_t
  45 .Fo lseek
  46 .Fa "int fildes"
  47 .Fa "off_t offset"
  48 .Fa "int whence"
  49 .Fc
  50 .Sh DESCRIPTION
  51 The
  52 .Fn lseek
  53 function repositions the offset of the file descriptor
  54 .Fa fildes
  55 to the
  56 argument
  57 .Fa offset ,
  58 according to the directive
  59 .Fa whence .
  60 The argument
  61 .Fa fildes
  62 must be an open
  63 file descriptor.
  64 .Fn Lseek
  65 repositions the file pointer
  66 .Fa fildes
  67 as follows:
  68 .Bl -item -offset indent
  69 .It
  70 If
  71 .Fa whence
  72 is
  73 .Dv SEEK_SET ,
  74 the offset is set to
  75 .Fa offset
  76 bytes.
  77 .It
  78 If
  79 .Fa whence
  80 is
  81 .Dv SEEK_CUR ,
  82 the offset is set to its current location plus
  83 .Fa offset
  84 bytes.
  85 .It
  86 If
  87 .Fa whence
  88 is
  89 .Dv SEEK_END ,
  90 the offset is set to the size of the
  91 file plus
  92 .Fa offset
  93 bytes.
  94 .El
  95 .Pp
  96 The
  97 .Fn lseek
  98 function allows the file offset to be set beyond the end
  99 of the existing end-of-file of the file. If data is later written
 100 at this point, subsequent reads of the data in the gap return
 101 bytes of zeros (until data is actually written into the gap).
 102 .Pp
 103 Some devices are incapable of seeking.  The value of the pointer
 104 associated with such a device is undefined.
 105 .Sh RETURN VALUES
 106 Upon successful completion,
 107 .Fn lseek
 108 returns the resulting offset location as measured in bytes from the
 109 beginning of the file.
 110 Otherwise,
 111 a value of -1 is returned and
 112 .Va errno
 113 is set to indicate
 114 the error.
 115 .Sh ERRORS
 116 .Fn Lseek
 117 will fail and the file pointer will remain unchanged if:
 118 .Bl -tag -width Er
 119 .\" ==========
 120 .It Bq Er EBADF
 121 .Em Fildes
 122 is not an open file descriptor.
 123 .\" ==========
 124 .It Bq Er EINVAL
 125 .Fa Whence
 126 is not a proper value.
 127 .\" ==========
 128 .It Bq Er EINVAL
 129 The seek location (calculated from
 130 .Fa offset
 131 and
 132 .Fa whence )
 133 is negative.
 134 .\" ==========
 135 .It Bq Er EOVERFLOW
 136 The seek location is too large to be stored
 137 in an object of type off_t.
 138 .\" ==========
 139 .It Bq Er ESPIPE
 140 .Em Fildes
 141 is associated with a pipe, socket, or FIFO.
 142 .El
 143 .Sh SEE ALSO
 144 .Xr dup 2 ,
 145 .Xr open 2
 146 .Sh BUGS
 147 This document's use of
 148 .Fa whence
 149 is incorrect English, but is maintained for historical reasons.
 150 .Sh STANDARDS
 151 The
 152 .Fn lseek
 153 function conforms to
 154 .St -p1003.1-88 .