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MSYNC
Section: POSIX Programmer's Manual (3P)
Updated: 2003
Index
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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 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:
| 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
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.
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.
- EINVAL
-
The value of addr is not a multiple of the page size {PAGESIZE}.
- 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 following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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 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
IEEE Std 1003.1-2001, <sys/mman.h>
COPYRIGHT
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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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