LSEEK manpage

Search topic Section

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

       lseek - reposition read/write file offset

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

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

       The  lseek()  function  repositions  the	 file  offset of the open file
       description associated with the file descriptor fd to the argument off-
       set according to the directive whence as follows:

	      The file offset is set to offset bytes.

	      The  file	 offset	 is  set  to  its current location plus offset

	      The file offset is set to the  size  of  the  file  plus	offset

       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

	      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.

	      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
       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 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

       *  Btrfs (since Linux 3.1)

       *  OCFS (since Linux 3.2)

       *  XFS (since Linux 3.5)

       *  ext4 (since Linux 3.8)

       *  tmpfs(5) (since Linux 3.8)

       *  NFS (since Linux 3.18)

       *  FUSE (since Linux 4.5)

       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

       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.

       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the file offset is	beyond
	      the end of the file.

	      The resulting file offset cannot be represented in an off_t.

       ESPIPE fd is associated with a pipe, socket, or FIFO.

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       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).

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

       If the O_APPEND file status flag is set on the open  file  description,
       then  a	write(2)  always moves the file offset to the end of the file,
       regardless of the use of lseek().

       The off_t data type is a signed integer data type specified by POSIX.1.

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

       On  Linux,  using  lseek()  on  a  terminal device fails with the error

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

       This page is part of release 4.10 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

Linux				  2017-03-13			      LSEEK(2)