LSEEK(2) BSD System Calls Manual LSEEK(2)NAMElseek — reposition read/write file offset
Standard C Library (libc, -lc)
lseek(int fildes, off_t offset, int whence);
The lseek() system call repositions the offset of the file descriptor
fildes to the argument offset according to the directive whence. The
argument fildes must be an open file descriptor. The lseek() system call
repositions the file position pointer associated with the file descriptor
fildes as follows:
If whence is SEEK_SET, the offset is set to offset bytes.
If whence is SEEK_CUR, the offset is set to its current location
plus offset bytes.
If whence is SEEK_END, the offset is set to the size of the file
plus offset bytes.
If whence is SEEK_HOLE, the offset of the start of the next hole
greater than or equal to the supplied offset is returned. The def‐
inition of a hole is provided below.
If whence is SEEK_DATA, the offset is set to the start of the next
non-hole file region greater than or equal to the supplied offset.
The lseek() system call allows the file offset to be set beyond the end
of the existing end-of-file of the file. If data is later written at
this point, subsequent reads of the data in the gap return bytes of zeros
(until data is actually written into the gap).
Some devices are incapable of seeking. The value of the pointer associ‐
ated with such a device is undefined.
A "hole" is defined as a contiguous range of bytes in a file, all having
the value of zero, but not all zeros in a file are guaranteed to be rep‐
resented as holes returned with SEEK_HOLE. File systems are allowed to
expose ranges of zeros with SEEK_HOLE, but not required to. Applications
can use SEEK_HOLE to optimise their behavior for ranges of zeros, but
must not depend on it to find all such ranges in a file. The existence
of a hole at the end of every data region allows for easy programming and
implies that a virtual hole exists at the end of the file. Applications
should use fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to
determine if a file system supports SEEK_HOLE. See pathconf(2).
For file systems that do not supply information about holes, the file
will be represented as one entire data region.
Upon successful completion, lseek() returns the resulting offset location
as measured in bytes from the beginning of the file. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
The lseek() system call will fail and the file position pointer will
remain unchanged if:
[EBADF] The fildes argument is not an open file descriptor.
[EINVAL] The whence argument is not a proper value or the
resulting file offset would be negative for a non-
character special file.
[ENXIO] For SEEK_DATA, there are no more data regions past the
supplied offset. For SEEK_HOLE, there are no more
holes past the supplied offset.
[EOVERFLOW] The resulting file offset would be a value which can‐
not be represented correctly in an object of type
[ESPIPE] The fildes argument is associated with a pipe, socket,
SEE ALSOdup(2), open(2), pathconf(2)STANDARDS
The lseek() system call is expected to conform to ISO/IEC 9945-1:1990
The lseek() function appeared in Version 7 AT&T UNIX.
This document's use of whence is incorrect English, but is maintained for
BSD April 5, 2007 BSD