LSEEK(2)                    BSD System Calls Manual                   LSEEK(2)

NAME
     lseek -- reposition read/write file offset

SYNOPSIS
     #include <unistd.h>

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

DESCRIPTION
     The lseek() function 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.  lseek() repositions the file
     pointer 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() function 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).

     Some devices are incapable of seeking.  The value of the pointer associ-
     ated with such a device is undefined.

     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 rep-
     resented 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 implementa-
     tion 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
     lseek() will fail and the file pointer will remain unchanged if:

     [EBADF]            Fildes is not an open file descriptor.

     [EINVAL]           Whence is not a proper value.

     [EINVAL]           The seek location (calculated from offset and whence)
                        is negative.

     [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 seek location is too large to be stored in an
                        object of type off_t.

     [ESPIPE]           Fildes is associated with a pipe, socket, or FIFO.

SEE ALSO
     dup(2), open(2)

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

STANDARDS
     The lseek() function conforms to IEEE Std 1003.1-1988 (``POSIX.1'').

4th Berkeley Distribution       April 19, 1994       4th Berkeley Distribution