'\" et
.TH MSYNC "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
msync
\(em synchronize memory with physical storage
.SH SYNOPSIS
.LP
.nf
#include <sys/mman.h>
.P
int msync(void *\fIaddr\fP, size_t \fIlen\fP, int \fIflags\fP);
.fi
.SH DESCRIPTION
The
\fImsync\fR()
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
.IR addr
and continuing for
.IR len
bytes. If no such storage exists,
\fImsync\fR()
need not have any effect. If requested, the
\fImsync\fR()
function shall then invalidate cached copies of data.
.P
The implementation may require that
.IR addr
be a multiple of the page size as returned by
\fIsysconf\fR().
.P
For mappings to files, the
\fImsync\fR()
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
\fImsync\fR()
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
\fImsync\fR()
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
\fImmap\fR().
.P
The
.IR flags
argument is constructed from the bitwise-inclusive OR of one or more of
the following flags defined in the
.IR <sys/mman.h> 
header:
.TS
center box tab(!);
cB | cB
lw(1.5i) | lw(2i).
Symbolic Constant!Description
_
MS_ASYNC!Perform asynchronous writes.
MS_SYNC!Perform synchronous writes.
MS_INVALIDATE!Invalidate cached data.
.TE
.P
When MS_ASYNC is specified,
\fImsync\fR()
shall return immediately once all the write operations are initiated or
queued for servicing; when MS_SYNC is specified,
\fImsync\fR()
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.
.P
When MS_INVALIDATE is specified,
\fImsync\fR()
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
\fImsync\fR()
and the first subsequent memory reference to the data.
.P
If
\fImsync\fR()
causes any write to a file, the file's last data modification and
last file status change timestamps shall be marked for update.
.SH "RETURN VALUE"
Upon successful completion,
\fImsync\fR()
shall return 0; otherwise, it shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fImsync\fR()
function shall fail if:
.TP
.BR EBUSY
Some or all of the addresses in the range starting at
.IR addr
and continuing for
.IR len
bytes are locked, and MS_INVALIDATE is specified.
.TP
.BR EINVAL
The value of
.IR flags
is invalid.
.TP
.BR ENOMEM
The addresses in the range starting at
.IR addr
and continuing for
.IR len
bytes are outside the range allowed for the address space of a process
or specify one or more pages that are not mapped.
.P
The
\fImsync\fR()
function may fail if:
.TP
.BR EINVAL
The value of
.IR addr
is not a multiple of the page size as returned by
\fIsysconf\fR().
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The
\fImsync\fR()
function is only supported if the Synchronized Input and Output
option is supported, and thus need not be available on all implementations.
.P
The
\fImsync\fR()
function should be used by programs that require a memory object to be
in a known state; for example, in building transaction facilities.
.P
Normal system activity can cause pages to be written to disk.
Therefore, there are no guarantees that
\fImsync\fR()
is the only control over when pages are or are not written to disk.
.SH RATIONALE
The
\fImsync\fR()
function writes out data in a mapped region to the permanent
storage for the underlying object. The call to
\fImsync\fR()
ensures data integrity of the file.
.P
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.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fImmap\fR\^(\|)",
.IR "\fIsysconf\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_mman.h>\fP"
.\"
.SH 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 .
.PP
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 .