Tribblix: manual page: lseek.2

LSEEK(2) System Calls LSEEK(2)

NAME

lseek - move read/write file pointer

SYNOPSIS

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

off_t lseek(int fildes, off_t offset, int whence);

DESCRIPTION

The lseek() function sets the 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 is provided near the
end of the DESCRIPTION.

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.

The symbolic constants SEEK_SET, SEEK_CUR, SEEK_END, SEEK_HOLE, and
SEEK_DATA are defined in the header <unistd.h>.

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

The lseek() function allows the file pointer to be set beyond the
existing data in the file. If data are later written at this point,
subsequent reads in the gap between the previous end of data and the
newly written data will return bytes of value 0 until data are
written into the gap.

If fildes is a remote file descriptor and offset is negative, lseek()
returns the file pointer even if it is negative. The lseek()
function will not, by itself, extend the size of a file.

If fildes refers to a shared memory object, lseek() behaves as if
fildes referred to a regular file.

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. Applications should use
fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to
determine if a filesystem supports SEEK_HOLE. See fpathconf(2).

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

RETURN VALUES

Upon successful completion, the resulting offset, as measured in
bytes from the beginning of the file, is returned. Otherwise,
(off_t)-1 is returned, the file offset remains unchanged, and errno
is set to indicate the error.

ERRORS

The lseek() 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; 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.

EOVERFLOW
The resulting file offset would be a value which cannot
be represented correctly in an object of type off_t for
regular files.

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

USAGE

The lseek() function has a transitional interface for 64-bit file
offsets. See lf64(7).

In multithreaded applications, using lseek() in conjunction with a
read(2) or write(2) call on a file descriptor shared by more than one
thread is not an atomic operation. To ensure atomicity, use pread()
or pwrite().

ATTRIBUTES