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 disk. 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 may have the bits MS_ASYNC, MS_SYNC, and MS_INVALIDATE set, but not both MS_ASYNC and MS_SYNC. MS_ASYNC specifies that an update be scheduled, but the call returns immediately. MS_SYNC asks for 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).)
B.O. Gallmeister, POSIX.4, OReilly, pp. 128-129 and 389-391.
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