LLSEEK(2) System Calls LLSEEK(2)

NAME

llseek - move extended read/write file pointer

SYNOPSIS

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

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

DESCRIPTION

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.

RETURN VALUES

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.

ERRORS

The llseek() function will fail if:

EBADF
The fildes argument is not an open file descriptor.


EINVAL
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.


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.


ESPIPE
The fildes argument is associated with a pipe or FIFO.

SEE ALSO

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

April 1, 2005 LLSEEK(2)