DragonFly On-Line Manual Pages

LSEEK(2)                 DragonFly 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() 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
     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() 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
     associated 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
     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
     Lseek() will fail and the file position pointer will remain unchanged if:

     [EBADF]            Fildes is not an open file descriptor.

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

     [EINVAL]           Whence is not a proper value.

     [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.

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

STANDARDS
     The lseek() function call is expected to conform to IEEE Std 1003.1-1990
     ("POSIX.1").

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

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

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

DragonFly 6.5-DEVELOPMENT       April 19, 1994       DragonFly 6.5-DEVELOPMENT