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
); 

Parameters

addr

With len, specifies the region to be synchronized.

len

With addr, specifies the region to be synchronized.

flags

Specifies one of the following values:

MS_ASYNC

Performs asynchronous writes. The function synchronizes the file contents to match the current contents of the memory region. If flags is MS_ASYNC, the function may return immediately once all write operations have been scheduled.

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 underlying hardware.

MS_SYNC

Performs synchronous writes. The function synchronizes the file contents to match the current contents of the memory region. If flags is MS_SYNC, the function does not return until all write operations are completed.

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 underlying hardware.

MS_INVALIDATE

Invalidates mappings. 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.

DESCRIPTION

The msync() function writes all modified copies of pages over the range [addr, addr+len] to the underlying hardware, or invalidates any copies so that further references to the pages will be obtained by the system from their permanent storage locations.

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().

RETURN VALUE

msync() returns the following values:

0: Successful completion.
-1: Failure. errno is set to indicate the error.

ERRORS

The msync() function will fail if:

EAGAIN: A transient error occurred while reading from or writing to the file system.
EBUSY: Some or all of the addresses in the range starting at addr and continuing for len bytes are locked, and MS_INVALIDATE is specified.
EINVAL: The addr argument is not a multiple of the page size as returned by sysconf(_SC_PAGE_SIZE).
EINVAL: The address range specified by addr and len was not created by a successful call to mmap().
EINVAL: The value of flags is invalid.
EIO: An I/O error occurred while reading from or writing to the file system.
ENOMEM: Some or all the addresses in the range [addr, addr+len] are invalid for the address space of the process. Or, pages that are not mapped are specified.

APPLICATION USAGE

The msync() function should be used by programs that require a memory object to be in a known state. For example, in building transaction facilities.

Normal system activity can cause pages to be written to disk. Therefore, there are no guarantees that msync() is the only control over when pages are or are not written to disk.

AUTHOR

msync() was developed by HP, AT&T, and OSF.

STANDARDS CONFORMANCE

msync(): AES, SVID3

CHANGE HISTORY

First released in Issue 4, Version 2.