MSYNC(3C) Standard C Library Functions MSYNC(3C)

NAME

msync - synchronize memory 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 underlying hardware, or invalidates any copies
so that further references 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 values:

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 contents of the memory region.

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 written 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 operations are completed.


If flags is MS_INVALIDATE, the function synchronizes the contents of the
memory region to match the current file contents.

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 process, to
memory regions mapped to the same portion of the file using
MAP_SHARED, are visible by read references 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.

RETURN VALUES

Upon successful completion, msync() returns 0; otherwise, it returns -1
and sets errno to indicate the error.

ERRORS

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.


EAGAIN
Some or all pages in the range [addr, addr + len) are locked
for I/O.


EINVAL
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 and
MS_INVALIDATE.


EIO
An I/O error occurred while reading from or writing to the file
system.


ENOMEM
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.

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.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

memcntl(2), mmap(2), sysconf(3C), attributes(7), standards(7)

July 24, 2002 MSYNC(3C)