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34 .\" @(#)lseek.2 8.3 (Berkeley) 4/19/94
41 .Nd reposition read/write file offset
43 .Fd #include <unistd.h>
53 function repositions the offset of the file descriptor
58 according to the directive
65 repositions the file pointer
68 .Bl -item -offset indent
82 the offset is set to its current location plus
90 the offset is set to the size of the
99 the offset is set to the start of the next hole greater than or equal
102 The definition of a hole is provided below.
108 the offset is set to the start of the next non-hole file region greater
109 than or equal to the supplied
115 function allows the file offset to be set beyond the end
116 of the existing end-of-file of the file. If data is later written
117 at this point, subsequent reads of the data in the gap return
118 bytes of zeros (until data is actually written into the gap).
120 Some devices are incapable of seeking. The value of the pointer
121 associated with such a device is undefined.
125 is defined as a contiguous range of bytes in a file, all having the value of
126 zero, but not all zeros in a file are guaranteed to be represented as holes
129 File systems are allowed to expose ranges of zeros with
134 to optimise their behavior for ranges of zeros, but must not depend on it to
135 find all such ranges in a file.
136 Each file is presented as having a zero-size virtual hole at the very
138 The existence of a hole at the end of every data region allows for easy
139 programming and also provides compatibility to the original implementation
141 It also causes the current file size (i.e., end-of-file offset) to be returned
142 to indicate that there are no more holes past the supplied
144 Applications should use
145 .Fn fpathconf _PC_MIN_HOLE_SIZE
147 .Fn pathconf _PC_MIN_HOLE_SIZE
148 to determine if a file system supports
153 For file systems that do not supply information about holes, the file will be
154 represented as one entire data region.
156 Upon successful completion,
158 returns the resulting offset location as measured in bytes from the
159 beginning of the file.
161 a value of -1 is returned and
167 will fail and the file pointer will remain unchanged if:
172 is not an open file descriptor.
176 is not a proper value.
179 The seek location (calculated from
188 there are no more data regions past the supplied offset.
189 Due to existence of the hole at the end of the file, for
191 this error is only returned when the
193 already points to the end-of-file position.
195 The seek location is too large to be stored
196 in an object of type off_t.
200 is associated with a pipe, socket, or FIFO.
206 This document's use of
208 is incorrect English, but is maintained for historical reasons.
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