msync() flushes changes made to the in-core copy of a file that was mapped into memory using mmap(2) back to the filesystem. Without use of this call, there is no guarantee that changes are written back before munmap(2) is called. To be more precise, the part of the file that corresponds to the memory area starting at addr and having length length is updated.
The flags argument should specify exactly one of MS_ASYNC and MS_SYNC, and may additionally include the MS_INVALIDATE bit. These bits have the following meanings:
Specifies that an update be scheduled, but the call returns immediately.
Requests an update and waits for it to complete.
Asks to invalidate other mappings of the same file (so that they can be updated with the fresh values just written).
According to POSIX, either MS_SYNC or MS_ASYNC must be specified in flags, and indeed failure to include one of these flags will cause msync() to fail on some systems. However, Linux permits a call to msync() that specifies neither of these flags, with semantics that are (currently) equivalent to specifying MS_ASYNC. (Since Linux 2.6.19, MS_ASYNC is in fact a no-op, since the kernel properly tracks dirty pages and flushes them to storage as necessary.) Notwithstanding the Linux behavior, portable, future-proof applications should ensure that they specify either MS_SYNC or MS_ASYNC in flags.