msync

SYNOPSIS
       #include <sys/mman.h>

       int msync(void *addr, size_t len, int flags);


DESCRIPTION
       The msync() function shall write all modified data to permanent storage
       locations, if any, in those whole pages  containing  any  part  of  the
       address  space  of  the process starting at address addr and continuing
       for len bytes. If no such storage exists, msync()  need  not  have  any
       effect. If requested, the msync() function shall then invalidate cached
       copies of data.

       The implementation shall require that addr be a multiple  of  the  page
       size as returned by sysconf().

       For mappings to files, the msync() function shall ensure that all write
       operations are completed as defined for synchronized I/O data integrity
       completion.  It  is  unspecified whether the implementation also writes
       out other file attributes.  When the  msync()  function  is  called  on
       MAP_PRIVATE  mappings,  any  modified  data shall not be written to the
       underlying object and shall not cause such data to be made  visible  to
       other  processes.   It  is unspecified whether data in MAP_PRIVATE map-
       pings has any permanent storage locations.    The effect of msync()  on
       a  shared  memory  object or a typed memory object is unspecified.  The
       behavior of this function is unspecified if the mapping was not  estab-
       lished by a call to mmap().

       The  flags argument is constructed from the bitwise-inclusive OR of one
       or more of the following flags defined in the <sys/mman.h> header:

                   Symbolic Constant  Description
                   MS_ASYNC           Perform asynchronous writes.
                   MS_SYNC            Perform synchronous writes.
                   MS_INVALIDATE      Invalidate cached data.

       When MS_ASYNC is specified, msync() shall return immediately  once  all
       the  write  operations  are  initiated  or  queued  for servicing; when
       MS_SYNC is specified, msync() shall not return until all  write  opera-
       tions are completed as defined for synchronized I/O data integrity com-
       pletion. Either MS_ASYNC or MS_SYNC is specified, but not both.

       When MS_INVALIDATE is specified, msync() shall  invalidate  all  cached
       copies  of mapped data that are inconsistent with the permanent storage
       locations such that subsequent references shall obtain  data  that  was
       consistent  with  the  permanent storage locations sometime between the
       call to msync() and the first subsequent memory reference to the data.

       If msync() causes any write to a file, the file's st_ctime and st_mtime
       fields shall be marked for update.

RETURN VALUE
       EINVAL The value of addr is not a multiple of the page size {PAGESIZE}.

       ENOMEM The addresses in the range starting at addr and  continuing  for
              len bytes are outside the range allowed for the address space of
              a process or specify one or more pages that are not mapped.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The msync() function is only  supported  if  the  Memory  Mapped  Files
       option  and the Synchronized Input and Output option are supported, and
       thus need not be available on all implementations.

       The msync() function should be used by programs that require  a  memory
       object  to  be  in  a known state; for example, in building transaction
       facilities.

       Normal system activity can cause pages to be written  to  disk.  There-
       fore,  there  are  no  guarantees that msync() is the only control over
       when pages are or are not written to disk.

RATIONALE
       The msync() function writes out data in a mapped region to  the  perma-
       nent  storage  for  the  underlying object. The call to msync() ensures
       data integrity of the file.

       After the data is written out, any cached data may  be  invalidated  if
       the MS_INVALIDATE flag was specified. This is useful on systems that do
       not support read/write consistency.

FUTURE DIRECTIONS
       None.

SEE ALSO
       mmap()   ,   sysconf()   ,   the    Base    Definitions    volume    of
       IEEE Std 1003.1-2001, <sys/mman.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 .