bsd/man/man2/lseek.2

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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 .Fn lseek "int fildes" "off_t offset" "int whence"
  46 .Sh DESCRIPTION
  47 The
  48 .Fn lseek
  49 function repositions the offset of the file descriptor
  50 .Fa fildes
  51 to the
  52 argument
  53 .Fa offset
  54 according to the directive
  55 .Fa whence.
  56 The argument
  57 .Fa fildes
  58 must be an open
  59 file descriptor.
  60 .Fn Lseek
  61 repositions the file pointer
  62 .Fa fildes
  63 as follows:
  64 .Bl -item -offset indent
  65 .It
  66 If
  67 .Fa whence
  68 is
  69 .Dv SEEK_SET ,
  70 the offset is set to
  71 .Fa offset
  72 bytes.
  73 .It
  74 If
  75 .Fa whence
  76 is
  77 .Dv SEEK_CUR ,
  78 the offset is set to its current location plus
  79 .Fa offset
  80 bytes.
  81 .It
  82 If
  83 .Fa whence
  84 is
  85 .Dv SEEK_END ,
  86 the offset is set to the size of the
  87 file plus
  88 .Fa offset
  89 bytes.
  90 .El
  91 .Pp
  92 The
  93 .Fn lseek
  94 function allows the file offset to be set beyond the end
  95 of the existing end-of-file of the file. If data is later written
  96 at this point, subsequent reads of the data in the gap return
  97 bytes of zeros (until data is actually written into the gap).
  98 .Pp
  99 Some devices are incapable of seeking.  The value of the pointer
 100 associated with such a device is undefined.
 101 .Sh RETURN VALUES
 102 Upon successful completion,
 103 .Fn lseek
 104 returns the resulting offset location as measured in bytes from the
 105 beginning of the file.
 106 Otherwise,
 107 a value of -1 is returned and
 108 .Va errno
 109 is set to indicate
 110 the error.
 111 .Sh ERRORS
 112 .Fn Lseek
 113 will fail and the file pointer will remain unchanged if:
 114 .Bl -tag -width Er
 115 .It Bq Er EBADF
 116 .Em Fildes
 117 is not an open file descriptor.
 118 .It Bq Er ESPIPE
 119 .Em Fildes
 120 is associated with a pipe, socket, or FIFO.
 121 .It Bq Er EINVAL
 122 .Fa Whence
 123 is not a proper value.
 124 .El
 125 .Sh SEE ALSO
 126 .Xr dup 2 ,
 127 .Xr open 2
 128 .Sh BUGS
 129 This document's use of
 130 .Fa whence
 131 is incorrect English, but is maintained for historical reasons.
 132 .Sh STANDARDS
 133 The
 134 .Fn lseek
 135 function conforms to
 136 .St -p1003.1-88 .