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.\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH msync 2 2024-05-02 "Linux man-pages 6.8"
.SH NAME
msync \- synchronize a file with a memory map
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.P
.BI "int msync(void " addr [. length "], size_t " length ", int " flags );
.fi
.SH DESCRIPTION
.BR msync ()
flushes changes made to the in-core copy of a file that was mapped
into memory using
.BR mmap (2)
back to the filesystem.
Without use of this call,
there is no guarantee that changes are written back before
.BR munmap (2)
is called.
To be more precise, the part of the file that
corresponds to the memory area starting at
.I addr
and having length
.I length
is updated.
.P
The
.I flags
argument should specify exactly one of
.B MS_ASYNC
and
.BR MS_SYNC ,
and may additionally include the
.B MS_INVALIDATE
bit.
These bits have the following meanings:
.TP
.B MS_ASYNC
Specifies that an update be scheduled, but the call returns immediately.
.TP
.B MS_SYNC
Requests an update and waits for it to complete.
.TP
.B MS_INVALIDATE
.\" Since Linux 2.4, this seems to be a no-op (other than the
.\" EBUSY check for VM_LOCKED).
Asks to invalidate other mappings of the same file
(so that they can be updated with the fresh values just written).
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBUSY
.B MS_INVALIDATE
was specified in
.IR flags ,
and a memory lock exists for the specified address range.
.TP
.B EINVAL
.I addr
is not a multiple of PAGESIZE; or any bit other than
.BR MS_ASYNC " | " MS_INVALIDATE " | " MS_SYNC
is set in
.IR flags ;
or both
.B MS_SYNC
and
.B MS_ASYNC
are set in
.IR flags .
.TP
.B ENOMEM
The indicated memory (or part of it) was not mapped.
.SH VERSIONS
According to POSIX, either
.B MS_SYNC
or
.B MS_ASYNC
must be specified in
.IR flags ,
and indeed failure to include one of these flags will cause
.BR msync ()
to fail on some systems.
However, Linux permits a call to
.BR msync ()
that specifies neither of these flags,
with semantics that are (currently) equivalent to specifying
.BR MS_ASYNC .
(Since Linux 2.6.19,
.\" commit 204ec841fbea3e5138168edbc3a76d46747cc987
.B MS_ASYNC
is in fact a no-op, since the kernel properly tracks dirty
pages and flushes them to storage as necessary.)
Notwithstanding the Linux behavior,
portable, future-proof applications should ensure that they specify either
.B MS_SYNC
or
.B MS_ASYNC
in
.IR flags .
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.P
This call was introduced in Linux 1.3.21, and then used
.B EFAULT
instead of
.BR ENOMEM .
In Linux 2.4.19, this was changed to the POSIX value
.BR ENOMEM .
.P
On POSIX systems on which
.BR msync ()
is available, both
.B _POSIX_MAPPED_FILES
and
.B _POSIX_SYNCHRONIZED_IO
are defined in
.I <unistd.h>
to a value greater than 0.
(See also
.BR sysconf (3).)
.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines them to 1.
.SH SEE ALSO
.BR mmap (2)
.P
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128\[en]129 and 389\[en]391.
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