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.\"	@(#)msync.2	8.2 (Berkeley) 6/21/94
.\" $FreeBSD: src/lib/libc/sys/msync.2,v 1.17 2000/04/21 09:41:53 phantom Exp $
.\"
.Dd June 21, 1994
.Dt MSYNC 2
.Os
.Sh NAME
.Nm msync
.Nd synchronize a mapped region
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Ft int
.Fn msync "void *addr" "size_t len" "int flags"
.Sh DESCRIPTION
The
.Fn msync
system call
writes any modified pages back to the filesystem and updates
the file modification time.
If
.Fa len
is 0, all modified pages within the region containing
.Fa addr
will be flushed;
if
.Fa len
is non-zero, only those pages containing 
.Fa addr
and
.Fa len-1
succeeding locations will be examined.
The
.Fa flags
argument may be specified as follows:
.Bd -literal
MS_ASYNC	Return immediately
MS_SYNC		Perform synchronous writes
MS_INVALIDATE	Invalidate all cached data
.Ed
.Sh RETURN VALUES
If any errors occur, -1 is returned and errno is set to indicate the
error.
Otherwise, a 0 value is returned.
.Sh ERRORS
.Fn msync
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa addr
is not a multiple of the hardware page size.
.It Bq Er EINVAL
.Fa len
is too large or negative.
.It Bq Er EINVAL
.Fa flags
was both MS_ASYNC and MS_INVALIDATE.
Only one of these flags is allowed.
.It Bq Er EIO
An I/O error occurred while writing to the file system.
.Sh SEE ALSO
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mprotect 2 ,
.Xr munmap 2
.Sh HISTORY
The
.Fn msync
function first appeared in
.Bx 4.4 .