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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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MSYNC(3P) POSIX Programmer's Manual MSYNC(3P)
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.
msync — synchronize memory with physical storage
#include <sys/mman.h>
int msync(void *addr, size_t len, int flags);
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.
Upon successful completion, msync() shall return 0; otherwise, it
shall return -1 and set errno to indicate the error.
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.
None.
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.
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.
None.
mmap(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, sys_mman.h(0p)
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 .
IEEE/The Open Group 2017 MSYNC(3P)
Pages that refer to this page: sys_mman.h(0p), mmap(3p), posix_typed_mem_open(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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