MSYNC(3P)                                           POSIX Programmer's Manual                                          MSYNC(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

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 contain-
       ing any part of the address space of the process starting at address addr and continuing for len bytes. If no such  stor-
       age  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  synchro-
       nized  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_PRI-
       VATE 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_INVALI-
              DATE 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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     MSYNC(3P)