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



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

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 may 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 shall be 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 last data
       modification and last file status change timestamps 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.

       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 msync() function may fail if:

       EINVAL The value of addr is not a multiple of the page size as returned
              by sysconf().

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The msync() function is only supported if the Synchronized Input and
       Output option is 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 POSIX.1-2017, <sys_mman.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                             MSYNC(3)
