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28 .\" @(#)lseek.2 8.3 (Berkeley) 4/19/94
36 .Nd reposition read/write file offset
42 .Fn lseek "int fildes" "off_t offset" "int whence"
46 system call repositions the offset of the file descriptor
51 according to the directive
60 repositions the file position pointer associated with the file
64 .Bl -item -offset indent
78 the offset is set to its current location plus
86 the offset is set to the size of the
95 the offset is set to the start of the next hole greater than or equal
98 The definition of a hole is provided below.
104 the offset is set to the start of the next non-hole file region greater
105 than or equal to the supplied
111 system call allows the file offset to be set beyond the end
112 of the existing end-of-file of the file.
113 If data is later written
114 at this point, subsequent reads of the data in the gap return
115 bytes of zeros (until data is actually written into the gap).
117 Some devices are incapable of seeking.
118 The value of the pointer
119 associated with such a device is undefined.
123 is defined as a contiguous range of bytes in a file, all having the value of
124 zero, but not all zeros in a file are guaranteed to be represented as holes
127 File systems are allowed to expose ranges of zeros with
132 to optimise their behavior for ranges of zeros, but must not depend on it to
133 find all such ranges in a file.
134 Each file is presented as having a zero-size virtual hole at the very
136 The existence of a hole at the end of every data region allows for easy
137 programming and also provides compatibility to the original implementation
139 It also causes the current file size (i.e., end-of-file offset) to be returned
140 to indicate that there are no more holes past the supplied
142 Applications should use
143 .Fn fpathconf _PC_MIN_HOLE_SIZE
145 .Fn pathconf _PC_MIN_HOLE_SIZE
146 to determine if a file system supports
151 For file systems that do not supply information about holes, the file will be
152 represented as one entire data region.
154 Upon successful completion,
156 returns the resulting offset location as measured in bytes from the
157 beginning of the file.
159 a value of -1 is returned and
167 will fail and the file position pointer will remain unchanged if:
173 is not an open file descriptor.
178 is not a proper value
179 or the resulting file offset would
180 be negative for a non-character special file.
184 there are no more data regions past the supplied offset.
185 Due to existence of the hole at the end of the file, for
187 this error is only returned when the
189 already points to the end-of-file position.
191 The resulting file offset would be a value which cannot be represented
192 correctly in an object of type
198 is associated with a pipe, socket, or FIFO.
207 system call is expected to conform to
215 This document's use of
217 is incorrect English, but is maintained for historical reasons.