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MSYNC(2) FreeBSD System Calls Manual MSYNC(2)

synchronize a mapped region

Standard C Library (libc, -lc)

#include <sys/mman.h>
msync(void *addr, size_t len, int flags);

The msync() system call writes any modified pages back to the file system and updates the file modification time. If len is 0, all modified pages within the region containing addr will be flushed; if len is non-zero, only those pages containing addr and len-1 succeeding locations will be examined. The flags argument may be specified as follows:
Return immediately
Perform synchronous writes
Invalidate all cached data

The msync() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The msync() system call will fail if:
Some or all of the pages in the specified region are locked and MS_INVALIDATE is specified.
The addr argument is not a multiple of the hardware page size.
The addresses in the range starting at addr and continuing for len bytes are outside the range allowed for the address space of a process or specify one or more pages that are not mapped.
The flags argument was both MS_ASYNC and MS_INVALIDATE. Only one of these flags is allowed.
An error occurred while writing at least one of the pages in the specified region.

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

The msync() system call first appeared in 4.4BSD.

The msync() system call is usually not needed since BSD implements a coherent file system buffer cache. However, it may be used to associate dirty VM pages with file system buffers and thus cause them to be flushed to physical media sooner rather than later.
March 18, 2012 FreeBSD 12.0-RELEASE

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