msync - synchronize memory with physical storage
int msync(void *addr, size_t len, int flags);
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 refer-
ences to the pages will be obtained by the system from their
permanent storage locations. The permanent storage for a
modified MAP_SHARED mapping is the file the page is mapped
to; the permanent storage for a modified MAP_PRIVATE mapping
is its swap area.
The flags argument is a bit pattern built from the following
perform asynchronous writes
perform synchronous writes
If flags is MS_ASYNC or MS_SYNC, the function synchronizes
the file contents to match the current contents of the
o All write references to the memory region made prior
to the call are visible by subsequent read operations
on the file.
o All writes to the same portion of the file prior to
the call may or may not be visible by read references
to the memory region.
o Unmodified pages in the specified range are not writ-
ten to the underlying 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 opera-
tions are completed.
If flags is MS_INVALIDATE, the function synchronizes the
contents of the memory region to match the current file
o 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.
o All write references prior to the call, by any pro-
cess, to memory regions mapped to the same portion of
the file using MAP_SHARED, are visible by read refer-
ences to the region.
If msync() causes any write to the file, then the file's
st_ctime and st_mtime fields are marked for update.
Upon successful completion, msync() returns 0; otherwise, it
returns -1 and sets errno to indicate the error.
The msync() function will fail if:
EBUSY Some or all of the addresses in the range [addr, addr
+ len) are locked and MS_SYNC with the MS_INVALIDATE
option is specified.
Some or all pages in the range [addr, addr + len)
are locked for I/O.
The addr argument is not a multiple of the page size
as returned by sysconf(3C).
The flags argument is not some combination of MS_ASYNC
EIO An I/O error occurred while reading from or writing to
the file system.
Addresses in the range [addr, addr + len) are outside
the valid range for the address space of a process, or
specify one or more pages that are not mapped.
EPERM MS_INVALIDATE was specified and one or more of the
pages is locked in memory.
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
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
memcntl(2), mmap(2), sysconf(3C), attributes(5), stan-
Man(1) output converted with