|LSEEK(2)||System Calls Manual||LSEEK(2)|
lseek — reposition
read/write file offset
off_t offset, int whence);
function 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.
lseek() repositions the file pointer
fildes as follows:
SEEK_SET, the offset is set to offset bytes.
SEEK_CUR, the offset is set to its current location plus offset bytes.
SEEK_END, the offset is set to the size of the file plus offset bytes.
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.
SEEK_DATA, the offset is set to the start of the next non-hole file region greater than or equal to the supplied offset.
function 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
to determine if a file system supports
For file systems that do not supply information about holes, the file will be represented as one entire data region.
Upon successful completion,
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.
lseek() will fail and the file pointer
will remain unchanged if:
SEEK_DATA, there are no more data regions past the supplied offset. Due to existence of the hole at the end of the file, for
SEEK_HOLEthis error is only returned when the offset already points to the end-of-file position.
This document's use of whence is incorrect English, but is maintained for historical reasons.
lseek() function conforms to
IEEE Std 1003.1-1988 (“POSIX.1”).
|April 19, 1994||BSD 4|