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33 .Nd reposition read/write file offset
39 .Fn lseek "int fildes" "off_t offset" "int whence"
43 system call repositions the offset of the file descriptor
48 according to the directive
57 repositions the file position pointer associated with the file
61 .Bl -item -offset indent
75 the offset is set to its current location plus
83 the offset is set to the size of the
92 the offset is set to the start of the next hole greater than or equal
95 The definition of a hole is provided below.
101 the offset is set to the start of the next non-hole file region greater
102 than or equal to the supplied
108 system call allows the file offset to be set beyond the end
109 of the existing end-of-file of the file.
110 If data is later written
111 at this point, subsequent reads of the data in the gap return
112 bytes of zeros (until data is actually written into the gap).
115 system call does not, by itself, extend the size of a file.
119 is defined as a contiguous range of bytes in a file, all having the value of
120 zero, but not all zeros in a file are guaranteed to be represented as holes
123 File systems are allowed to expose ranges of zeros with
128 to optimise their behavior for ranges of zeros, but must not depend on it to
129 find all such ranges in a file.
130 Each file is presented as having a zero-size virtual hole at the very
132 The existence of a hole at the end of every data region allows for easy
133 programming and also provides compatibility to the original implementation
135 It also causes the current file size (i.e., end-of-file offset) to be returned
136 to indicate that there are no more holes past the supplied
138 Applications should use
139 .Fn fpathconf _PC_MIN_HOLE_SIZE
141 .Fn pathconf _PC_MIN_HOLE_SIZE
142 to determine if a file system supports
147 For file systems that do not supply information about holes, the file will be
148 represented as one entire data region.
150 Upon successful completion,
152 returns the resulting offset location as measured in bytes from the
153 beginning of the file.
155 a value of -1 is returned and
163 will fail and the file position pointer will remain unchanged if:
169 is not an open file descriptor.
174 is not a proper value
175 or the resulting file offset would
176 be negative for a non-character special file.
180 there are no more data regions past the supplied offset.
181 Due to existence of the hole at the end of the file, for
183 this error is only returned when the
185 already points to the end-of-file position.
187 The resulting file offset would be a value which cannot be represented
188 correctly in an object of type
194 is associated with a pipe, socket, or FIFO.
203 system call is expected to conform to
210 directives, along with the
212 error, are extensions to that specification.
221 system call is operating on a device which is incapable of seeking,
222 it will request the seek operation and return successfully,
223 even though no seek was performed.
226 argument will be stored unconditionally in the file descriptor of that device,
227 there is no way to confirm if the seek operation succeeded or not
231 Device types which are known to be incapable of seeking include
236 system call will not detect whether media are present in changeable
237 media devices such as DVD or Blu-ray devices.
238 A requested seek operation will therefore return sucessfully when no
241 This document's use of
243 is incorrect English, but is maintained for historical reasons.