lseek(2) — Linux manual page


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

NAME         top

       lseek - reposition read/write file offset

SYNOPSIS         top

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

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

DESCRIPTION         top

       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:

              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

       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

              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

       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

       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         top

       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         top

       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.

              The resulting file offset cannot be represented in an off_t.

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

CONFORMING TO         top

       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         top

       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

       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

SEE ALSO         top

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

COLOPHON         top

       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

Linux                            2020-08-13                         LSEEK(2)

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