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.\"	@(#)msync.2	8.2 (Berkeley) 6/21/94
.\"
.Dd October 10, 1997
.Dt MSYNC 2
.Os
.Sh NAME
.Nm msync
.Nd synchronize a mapped region
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <sys/mman.h>
.Ft int
.Fn msync "void *addr" "size_t len" "int flags"
.Sh DESCRIPTION
The
.Fn msync
system call writes all pages with shared modifications
in the specified
region of the process's address space back to permanent
storage, and, if requested, invalidates cached data mapped
in the region.
If
.Fa len
is 0, all modified pages within the region containing
.Fa addr
will be flushed;
if
.Fa len
is non-zero, only modified pages containing
.Fa addr
and
.Fa len
succeeding locations will be flushed.
Any required synchronization of memory caches
will also take place at this time.
Filesystem operations on a file that is mapped for shared modifications
are unpredictable except after an
.Fn msync .
.Pp
The
.Fa flags
argument is formed by
.Em or Ns 'ing
the following values
.Pp
.Bl -column -offset indent -compact MS_INVALIDATE
.It Dv MS_ASYNC Ta Perform asynchronous writes.
.It Dv MS_SYNC Ta Perform synchronous writes.
.It Dv MS_INVALIDATE Ta Invalidate cached data after writing.
.El
.Sh ERRORS
The following errors may be reported:
.Bl -tag -width Er
.It Bq Er EBUSY
The
.Dv MS_INVALIDATE
flag was specified and a portion of the specified region
was locked with
.Xr mlock 2 .
.It Bq Er EINVAL
The specified
.Fa flags
argument was invalid.
.It Bq Er EINVAL
The
.Fa addr
parameter was not page aligned.
.It Bq Er EINVAL
The
.Fa addr
parameter did not specify an address part of a mapped region.
.It Bq Er EINVAL
The
.Fa len
parameter was negative.
.It Bq Er EIO
An I/O error occurred while writing to the file system.
.It Bq Er ENOMEM
Addresses in the specified region are outside the range allowed
for the address space of the process, or specify one or more pages
which are unmapped.
.El
.Sh SEE ALSO
.Xr mlock 2 ,
.Xr mmap 2 ,
.Xr munlock 2
.Sh HISTORY
The
.Fn msync
function first appeared in
.Bx 4.4 .
It was modified to conform to
.St -p1003.1b-93
in
.Nx 1.3 .
.Sh BUGS
Writes are currently done synchronously even if the
.Dv MS_ASYNC
flag is specified.