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:
Perform asynchronous writes.
Perform synchronous writes.
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.
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
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.
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.
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