LLSEEK(2) System Calls LLSEEK(2)


llseek - move extended read/write file pointer


#include <sys/types.h>
#include <unistd.h>

offset_t llseek(int fildes, offset_t offset, int whence);


The llseek() function sets the 64-bit extended file pointer associated
with the open file descriptor specified by fildes as follows:

o If whence is SEEK_SET, the pointer is set to offset bytes.

o If whence is SEEK_CUR, the pointer is set to its current
location plus offset.

o If whence is SEEK_END, the pointer is set to the size of the
file plus offset.

o 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 definition of a hole immediately follows this list.

o If whence is SEEK_DATA, the file pointer is set to the start
of the next non-hole file region greater than or equal to the
supplied offset.

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. Filesystems 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.

For filesystems that do not supply information about holes, the file will
be represented as one entire data region.

Although each file has a 64-bit file pointer associated with it, some
existing file system types (such as tmpfs) do not support the full range
of 64-bit offsets. In particular, on such file systems, non-device files
remain limited to offsets of less than two gigabytes. Device drivers may
support offsets of up to 1024 gigabytes for device special files.

Some devices are incapable of seeking. The value of the file pointer
associated with such a device is undefined.


Upon successful completion, llseek() returns the resulting pointer
location as measured in bytes from the beginning of the file. Remote file
descriptors are the only ones that allow negative file pointers.
Otherwise, -1 is returned, the file pointer remains unchanged, and errno
is set to indicate the error.


The llseek() function will fail if:

The fildes argument is not an open file descriptor.

The whence argument is not SEEK_SET, SEEK_CUR, or SEEK_END; the
offset argument is not a valid offset for this file system
type; or the fildes argument is not a remote file descriptor
and the resulting file pointer would be negative.

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.

The fildes argument is associated with a pipe or FIFO.


creat(2), dup(2), fcntl(2), lseek(2), open(2)

April 1, 2005 LLSEEK(2)