PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

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

NAME         top

       msync — synchronize memory with physical storage

SYNOPSIS         top

       #include <sys/mman.h>

       int msync(void *addr, size_t len, int flags);

DESCRIPTION         top

       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         top

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

ERRORS         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       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         top

       None.

SEE ALSO         top

       mmap(3p), sysconf(3p)

       The Base Definitions volume of POSIX.1‐2008, sys_mman.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                           MSYNC(3P)