Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
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 di-
       rective 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 ('\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:

       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 allo-
       cated 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	opera-
       tions  are not a	guaranteed mechanism for mapping the storage space ac-
       tually allocated	to a file.  (Furthermore, a sequence of	zeros that ac-
       tually  has  been written to the	underlying storage may not be reported
       as a hole.)  In the simplest 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_.

       The  SEEK_HOLE and SEEK_DATA operations are supported for the following
       filesystems:

       *  Btrfs	(since Linux 3.1)

       *  OCFS (since Linux 3.2)

       *  XFS (since Linux 3.5)

       *  ext4 (since Linux 3.8)

       *  tmpfs	(since Linux 3.8)

RETURN VALUE
       Upon successful completion, lseek() returns the resulting offset	 loca-
       tion  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  er-
       ror.

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 So-
       laris, FreeBSD, and DragonFly BSD; they are proposed for	 inclusion  in
       the next	POSIX revision (Issue 8).

NOTES
       See  open(2) for	a discussion of	the relationship between file descrip-
       tors, open file descriptions, and files.

       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 follow-
       ing 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  cur-
       rent  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 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-06-13			      LSEEK(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=lseek&sektion=2&manpath=Debian+8.1.0>

home | help