lseek

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
       lseek() 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.

       lseek() 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)

       *  GFS2 (since Linux 4.15)

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 offset is beyond the end of
              the file, or whence is SEEK_DATA and offset is within a hole at
              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
       This page is part of release 5.09 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-08-13                           LSEEK(2)