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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.1‐2017, <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 .