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 (consult the correspond-
       ing 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 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 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

       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 Technol-
       ogy -- 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								 MSYNC(3P)