LSEEK(2)                  FreeBSD System Calls Manual                 LSEEK(2)

NAME
     lseek - reposition read/write file offset

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

DESCRIPTION
     The lseek() system call repositions the offset of the file descriptor
     fildes to the argument offset according to the directive whence.  The
     argument fildes must be an open file descriptor.  The lseek() system call
     repositions the file position pointer associated with the file descriptor
     fildes as follows:

           If whence is SEEK_SET, the offset is set to offset bytes.

           If whence is SEEK_CUR, the offset is set to its current location
           plus offset bytes.

           If whence is SEEK_END, the offset is set to the size of the file
           plus offset bytes.

           If whence is SEEK_HOLE, the offset is set to the start of the next
           hole greater than or equal to the supplied offset.  The definition
           of a hole is provided below.

           If whence is SEEK_DATA, the offset is set to the start of the next
           non-hole file region greater than or equal to the supplied offset.

     The lseek() system call allows the file offset to be set beyond the end
     of the existing end-of-file of the file.  If data is later written at
     this point, subsequent reads of the data in the gap return bytes of zeros
     (until data is actually written into the gap).  However, the lseek()
     system call does not, by itself, extend the size of a file.

     A "hole" is defined as a contiguous range of bytes in a file, all having
     the value of zero, but not all zeros in a file are guaranteed to be
     represented as holes returned with SEEK_HOLE.  File systems are allowed
     to expose ranges of zeros with SEEK_HOLE, but not required to.
     Applications can use SEEK_HOLE to optimise their behavior for ranges of
     zeros, but must not depend on it to find all such ranges in a file.  Each
     file is presented as having a zero-size virtual hole at the very end of
     the file.  The existence of a hole at the end of every data region allows
     for easy programming and also provides compatibility to the original
     implementation in Solaris.  It also causes the current file size (i.e.,
     end-of-file offset) to be returned to indicate that there are no more
     holes past the supplied offset.  Applications should use
     fpathconf(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to determine
     if a file system supports SEEK_HOLE.  See pathconf(2).

     For file systems that do not supply information about holes, the file
     will be represented as one entire data region.

RETURN VALUES
     Upon successful completion, lseek() returns the resulting offset location
     as measured in bytes from the beginning of the file.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.

ERRORS
     The lseek() system call will fail and the file position pointer will
     remain unchanged if:

     [EBADF]            The fildes argument is not an open file descriptor.

     [EINVAL]           The whence argument is not a proper value or the
                        resulting file offset would be negative for a non-
                        character special file.

     [ENXIO]            For SEEK_DATA, there are no more data regions past the
                        supplied offset.  Due to existence of the hole at the
                        end of the file, for SEEK_HOLE this error is only
                        returned when the offset already points to the end-of-
                        file position.

     [EOVERFLOW]        The resulting file offset would be a value which
                        cannot be represented correctly in an object of type
                        off_t.

     [ESPIPE]           The fildes argument is associated with a pipe, socket,
                        or FIFO.

SEE ALSO
     dup(2), open(2), pathconf(2)

STANDARDS
     The lseek() system call is expected to conform to IEEE Std 1003.1-2008
     ("POSIX.1").

     The SEEK_HOLE and SEEK_DATA directives, along with the ENXIO error, are
     extensions to that specification.

HISTORY
     The lseek() function appeared in Version 7 AT&T UNIX.

BUGS
     If the lseek() system call is operating on a device which is incapable of
     seeking, it will request the seek operation and return successfully, even
     though no seek was performed.  Because the offset argument will be stored
     unconditionally in the file descriptor of that device, there is no way to
     confirm if the seek operation succeeded or not (e.g. using the ftell()
     function).  Device types which are known to be incapable of seeking
     include tape drives.

     The lseek() system call will not detect whether media are present in
     changeable media devices such as DVD or Blu-ray devices.  A requested
     seek operation will therefore return sucessfully when no medium is
     present.

     This document's use of whence is incorrect English, but is maintained for
     historical reasons.

FreeBSD 13.1-RELEASE-p6          July 13, 2020         FreeBSD 13.1-RELEASE-p6