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MSYNC(2) System Calls Manual MSYNC(2)

msyncsynchronize a mapped region

#include <sys/mman.h>

msync(void *addr, size_t len, int flags);

The () 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 len is 0, all modified pages within the region containing addr will be flushed; if len is non-zero, only modified pages containing addr and len-1 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 msync().

The flags argument is the bitwise OR of zero or more of the following values:

MS_ASYNC	Perform asynchronous writes.
MS_SYNC		Perform synchronous writes.
MS_INVALIDATE	Invalidate cached data after writing.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The following errors may be reported:

The MS_INVALIDATE flag was specified and a portion of the specified region was locked with mlock(2).
The specified flags argument was invalid.
The addr parameter was not page aligned or addr and size specify a region that would extend beyond the end of the address space.
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.
An I/O error occurred while writing.

madvise(2), mincore(2), minherit(2), mprotect(2), munmap(2)

The msync() function first appeared in 4.4BSD. It was modified to conform to IEEE Std 1003.1b-1993 (“POSIX.1b”)

Writes are currently done synchronously even if the MS_ASYNC flag is specified.

March 13, 2015 OpenBSD-5.8