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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 file offset of the open file description associated
       with the file descriptor fd to the argument offset according to the  directive  whence  as
       follows:

       SEEK_SET
              The file offset is set to offset bytes.

       SEEK_CUR
              The file offset is set to its current location plus offset bytes.

       SEEK_END
              The file 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 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 filesystem 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 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(5) (since Linux 3.8)

       *  NFS (since Linux 3.18)

       *  FUSE (since Linux 4.5)

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.

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

       EOVERFLOW
              The resulting file offset cannot be represented in an off_t.

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

CONFORMING TO

       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).

NOTES

       See  open(2)  for  a  discussion  of  the relationship between file descriptors, 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 ESPIPE.

SEE ALSO

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

COLOPHON

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