MSYNC
Section: POSIX Programmer's Manual (3P)
Updated: 2013
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PROLOG
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
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.1-2008,
<sys_mman.h>
COPYRIGHT
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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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