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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 of the start of the next hole greater than or equal to the supplied
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 The existence of a hole at the end of every data region allows for easy
135 programming and implies that a virtual hole exists at the end of the file.
136 Applications should use
137 .Fn fpathconf _PC_MIN_HOLE_SIZE
139 .Fn pathconf _PC_MIN_HOLE_SIZE
140 to determine if a file system supports
145 For file systems that do not supply information about holes, the file will be
146 represented as one entire data region.
148 Upon successful completion,
150 returns the resulting offset location as measured in bytes from the
151 beginning of the file.
153 a value of -1 is returned and
161 will fail and the file position pointer will remain unchanged if:
167 is not an open file descriptor.
172 is not a proper value
173 or the resulting file offset would
174 be negative for a non-character special file.
178 there are no more data regions past the supplied offset.
181 there are no more holes past the supplied offset.
183 The resulting file offset would be a value which cannot be represented
184 correctly in an object of type
190 is associated with a pipe, socket, or FIFO.
199 system call is expected to conform to
207 This document's use of
209 is incorrect English, but is maintained for historical reasons.