HP-UX Reference (11i v2 03/08) - 2 System Calls (vol 5)

m
msync(2) msync(2)
NAME
msync - synchronize the memory of a mapped file with physical storage
SYNOPSIS
#include <sys/mman.h>
int msync(void *addr, size_t len, int flags);
DESCRIPTION
The msync() function writes all modified copies of pages over the range [addr, addr+len] to the under-
lying hardware, or invalidates any copies so that further references to the pages will be obtained by the
system from their permanent storage locations.
The flags argument is one of the following:
MS_ASYNC perform asynchronous writes
MS_SYNC perform synchronous writes
MS_INVALIDATE invalidate mappings
If flags is
MS_ASYNC or MS_SYNC, the function synchronizes the file contents to match the current con-
tents of the memory region.
• All write references to the memory region made prior to the call are visible by subsequent read
operations on the file.
• It is unspecified whether writes to the same portion of the file prior to the call are visible by read
references to the memory region.
• It is unspecified whether unmodified pages in the specified range are also written to the underly-
ing hardware.
If flags is
MS_ASYNC, the function may return immediately once all write operations are scheduled; if
flags is MS_SYNC, the function does not return until all write operations are completed.
If flags is
MS_INVALIDATE
, the function synchronizes the contents of the memory region to match the
current file contents.
• All writes to the mapped portion of the file made prior to the call are visible by subsequent read
references to the mapped memory region.
• It is unspecified whether write references prior to the call, by any process, to memory regions
mapped to the same portion of the file using
MAP_SHARED, are visible by read references to the
region.
addr and len specify the region to be synchronized. If these are not the address and length of a region
created by a previous successful call to
mmap(), msync() returns an error. The behavior of
msync()
upon a region created with the MAP_ANONYMOUS
or MAP_PRIVATE flags is undefined.
If
msync() causes any write to the file, then the file’s st_ctime and st_mtime fields are marked for
update.
Performance Considerations
The following performance considerations only apply when using the
MS_INVALIDATE
option with
msync(). These performance constraints do not apply when either MS_ASYNC or MS_SYNC are
exclusively used with msync().
Direct read/write references to portions of a mapped memory region currently undergoing an
msync()
operation (with MS_INVALIDATE specified), may be blocked until all scheduled write operations are
completed. This is especially true when performing an msync() operation across a relatively large
address range that requires many individual write operations to be scheduled out to the underlying
hardware. HP-UX will schedule a separate write operation for each contiguous group of modified pages
on disk. As more write operations are queued out to the device, the overall suspension time of direct
read/write references to the same portions of the memory region will generally increase.
The suspension times of direct read/write references can be reduced by issuing
msync() requests over
smaller portions of the memory region, but issuing them more frequently than a corresponding larger
synchronization request. This will serve to more evenly distribute I/O activity across the mapped file,
while reducing the number of write operations per msync().
Section 2−−196 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: August 2003