NAME
lseek —
reposition read/write file
offset
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
off_t
lseek(int
fildes, off_t
offset, int
whence);
DESCRIPTION
Thelseek()
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 is set to the start of the next hole greater than or equal to the supplied offset. The definition 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 associated 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 represented 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. Each file is presented as having a zero-size virtual hole
at the very end of the file. The existence of a hole at the end of every
data region allows for easy programming and also provides compatibility to
the original implementation in Solaris. It also causes the current file size
(i.e., end-of-file offset) to be returned to indicate that there are no more
holes past the supplied offset. 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.
RETURN VALUES
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.
ERRORS
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. Due to existence of the hole at the end of the file, forSEEK_HOLEthis error is only returned when the offset already points to the end-of-file position. - [
EOVERFLOW] - The resulting file offset would be a value which cannot be represented correctly in an object of type off_t.
- [
ESPIPE] - The fildes argument is associated with a pipe, socket, or FIFO.
SEE ALSO
STANDARDS
The lseek() system call is expected to
conform to IEEE Std 1003.1-1990
(“POSIX.1”).
HISTORY
The lseek() function appeared in
Version 7 AT&T UNIX.
BUGS
This document's use of whence is incorrect English, but is maintained for historical reasons.