lseek(2) — Linux manual page

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

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

NAME         top

       lseek - reposition read/write file offset

SYNOPSIS         top

       #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:

       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         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.

       EOVERFLOW
              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
       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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                       LSEEK(2)

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