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NAME

       msync - synchronize memory with physical storage

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
       mappings 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
       established 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
       operations are completed as defined for synchronized I/O data integrity
       completion. 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

       Upon successful completion, msync() shall return 0; otherwise, it shall
       return -1 and set errno to indicate the error.

ERRORS

       The msync() function shall fail if:

       EBUSY  Some  or  all of the addresses in the range starting at addr and
              continuing for  len  bytes  are  locked,  and  MS_INVALIDATE  is
              specified.

       EINVAL The value of flags is invalid.

       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.
       Therefore,  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
       permanent  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 .