LSEEK(2) Linux Programmer's Manual LSEEK(2)
lseek - reposition read/write file offset
off_t lseek(int fd, off_t offset, int whence);
The lseek() function repositions the offset of the open file associated with the file
descriptor fd to the argument offset according to the directive whence as follows:
The offset is set to offset bytes.
The offset is set to its current location plus offset bytes.
The offset is set to the size of the file plus offset bytes.
The lseek() function allows the file offset to be set beyond the end of the file (but this
does not change the size of the file). If data is later written at this point, subsequent
reads of the data in the gap (a "hole") return null bytes ('\0') until data is actually
written into the gap.
Seeking file data and holes
Since version 3.1, Linux supports the following additional values for whence:
Adjust the file offset to the next location in the file greater than or equal to
offset containing data. If offset points to data, then the file offset is set to
Adjust the file offset to the next hole in the file greater than or equal to off-
set. If offset points into the middle of a hole, then the file offset is set to
offset. If there is no hole past offset, then the file offset is adjusted to the
end of the file (i.e., there is an implicit hole at the end of any file).
In both of the above cases, lseek() fails if offset points past the end of the file.
These operations allow applications to map holes in a sparsely allocated file. This can
be useful for applications such as file backup tools, which can save space when creating
backups and preserve holes, if they have a mechanism for discovering holes.
For the purposes of these operations, a hole is a sequence of zeros that (normally) has
not been allocated in the underlying file storage. However, a filesystem is not obliged
to report holes, so these operations are not a guaranteed mechanism for mapping the stor-
age space actually allocated to a file. (Furthermore, a sequence of zeros that actually
has been written to the underlying storage may not be reported as a hole.) In the sim-
plest implementation, a filesystem can support the operations by making SEEK_HOLE always
return the offset of the end of the file, and making SEEK_DATA always return offset (i.e.,
even if the location referred to by offset is a hole, it can be considered to consist of
data that is a sequence of zeros).
The _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of
SEEK_DATA and SEEK_HOLE from <unistd.h>.
Upon successful completion, lseek() returns the resulting offset location as measured in
bytes from the beginning of the file. On error, the value (off_t) -1 is returned and
errno is set to indicate the error.
EBADF fd is not an open file descriptor.
EINVAL whence is not valid. Or: the resulting file offset would be negative, or beyond
the end of a seekable device.
The resulting file offset cannot be represented in an off_t.
ESPIPE fd is associated with a pipe, socket, or FIFO.
ENXIO whence is SEEK_DATA or SEEK_HOLE, and the current file offset is beyond the end of
SVr4, 4.3BSD, POSIX.1-2001.
SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in Solaris, FreeBSD, and
DragonFly BSD; they are proposed for inclusion in the next POSIX revision (Issue 8).
Some devices are incapable of seeking and POSIX does not specify which devices must sup-
On Linux, using lseek() on a terminal device returns ESPIPE.
When converting old code, substitute values for whence with the following macros:
Note that file descriptors created by dup(2) or fork(2) share the current file position
pointer, so seeking on such files may be subject to race conditions.
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
This page is part of release 3.55 of the Linux man-pages project. A description of the
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Linux 2013-03-27 LSEEK(2)