NAME

       lseek - move the read/write file offset

SYNOPSIS

       #include <unistd.h>

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

DESCRIPTION

       The  lseek()  function  shall  set the file offset for the open file description associated with the file
       descriptor fildes, as follows:

        * If whence is SEEK_SET, the file offset shall be set to offset bytes.

        * If whence is SEEK_CUR, the file offset shall be set to its current location plus offset.

        * If whence is SEEK_END, the file offset shall be set to the size of the file plus offset.

       The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in <unistd.h>.

       The behavior of lseek() on devices which are incapable of seeking is implementation-defined. The value of
       the file offset associated with such a device is undefined.

       The lseek() function shall allow the file offset to be set beyond the end of the  existing  data  in  the
       file. If data is later written at this point, subsequent reads of data in the gap shall return bytes with
       the value 0 until data is actually written into the gap.

       The lseek() function shall not, by itself, extend the size of a file.

       If fildes refers to a shared memory object, the result of the lseek() function is unspecified.

       If fildes refers to a typed memory object, the result of the lseek() function is unspecified.

RETURN VALUE

       Upon  successful  completion,  the resulting offset, as measured in bytes from the beginning of the file,
       shall be returned.  Otherwise, (off_t)-1 shall be returned, errno shall be set to indicate the error, and
       the file offset shall remain unchanged.

ERRORS

       The lseek() function shall fail 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
              regular file, block special file, or directory.

       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, FIFO, or socket.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       The ISO C standard includes the functions fgetpos() and fsetpos(), which work on very large files by  use
       of a special positioning type.

       Although  lseek()  may position the file offset beyond the end of the file, this function does not itself
       extend the size of the file. While the only function in IEEE Std 1003.1-2001 that may directly extend the
       size of the file is write(), truncate(), and ftruncate(), several functions originally derived  from  the
       ISO C standard, such as fwrite(), fprintf(), and so on, may do so (by causing calls on write()).

       An  invalid  file  offset that would cause [EINVAL] to be returned may be both implementation-defined and
       device-dependent (for example, memory may have few invalid values). A negative file offset may  be  valid
       for some devices in some implementations.

       The  POSIX.1-1990  standard  did  not  specifically  prohibit  lseek()  from returning a negative offset.
       Therefore, an application was required to clear errno prior to the call and check errno  upon  return  to
       determine  whether  a  return  value  of  (  off_t)-1  is  a negative offset or an indication of an error
       condition. The standard developers did not wish to require this  action  on  the  part  of  a  conforming
       application,  and  chose to require that errno be set to [EINVAL] when the resulting file offset would be
       negative for a regular file, block special file, or directory.

FUTURE DIRECTIONS

       None.

SEE ALSO

       open() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc  and  The  Open Group. In the event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                              LSEEK(P)