MSYNC(2) Linux Programmer's Manual MSYNC(2)
msync - synchronize a file with a memory map
int msync(void *addr, size_t length, int flags);
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:
MS_ASYNC Specifies that an update be scheduled, but the call returns immediately.
MS_SYNC Requests an update and waits for it to complete.
MS_INVALIDATE Asks to invalidate other mappings of the same file (so that they can be updated with the fresh values just written).
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EBUSY MS_INVALIDATE was specified in flags, and a memory lock exists for the specified address range.
EINVAL addr is not a multiple of PAGESIZE; or any bit other than MS_ASYNC | MS_INVALIDATE | MS_SYNC is set in flags; or both MS_SYNC and MS_ASYNC are set in flags.
ENOMEM The indicated memory (or part of it) was not mapped.
This call was introduced in Linux 1.3.21, and then used EFAULT instead of ENOMEM. In Linux 2.4.19, this was changed to the POSIX value ENOMEM.
On POSIX systems on which msync() is available, both _POSIX_MAPPED_FILES and _POSIX_SYNCHRONIZED_IO are defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)
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.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
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Linux 2017-09-15 MSYNC(2)