LSEEK(2)						     Linux Programmer's Manual							  LSEEK(2)

NAME

       lseek - reposition read/write file offset

SYNOPSIS

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

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

DESCRIPTION

       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:

       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.

       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 ('') 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:

       SEEK_DATA
	      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 offset.

       SEEK_HOLE
	      Adjust  the  file  offset to the next hole in the file greater than or equal to offset.  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  file  system  is not obliged to report holes, so these operations are not a guaranteed mechanism for mapping the storage 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 simplest implementation, a file system 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>.

RETURN VALUE

       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.

ERRORS

       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.

       EOVERFLOW
	      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 the file.

CONFORMING TO

       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).

NOTES

       Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

       On Linux, using lseek() on a terminal device returns ESPIPE.

       When converting old code, substitute values for whence with the following macros:

	old	  new
       0	SEEK_SET
       1	SEEK_CUR
       2	SEEK_END
       L_SET	SEEK_SET
       L_INCR	SEEK_CUR
       L_XTND	SEEK_END

       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.

SEE ALSO

       dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2013-03-27								  LSEEK(2)