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.
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>
│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
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
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.
The msync() function is only supported if the Synchronized Input and
Output option is supported, and thus need not be available on all
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, 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
IEEE/The Open Group 2013 MSYNC(3P)