Manpage of MSYNC
MSYNCSection: Linux Programmer's Manual (2)
NAMEmsync - synchronize a file with a memory map
DESCRIPTIONmsync() 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 addrand having length lengthis updated.
The flagsargument should specify exactly one of MS_ASYNCand MS_SYNC, and may additionally include the MS_INVALIDATEbit. 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).
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errnois set appropriately.
- MS_INVALIDATEwas specified in flags, and a memory lock exists for the specified address range.
- addris not a multiple of PAGESIZE; or any bit other than MS_ASYNC | MS_INVALIDATE | MS_SYNCis set in flags; or both MS_SYNCand MS_ASYNCare set in flags.
- The indicated memory (or part of it) was not mapped.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008.
AVAILABILITYOn POSIX systems on which msync() is available, both _POSIX_MAPPED_FILESand _POSIX_SYNCHRONIZED_IOare defined in <unistd.h>to a value greater than 0. (See also sysconf(3).)
NOTESAccording to POSIX, either MS_SYNCor MS_ASYNCmust 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_ASYNCis 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_SYNCor MS_ASYNCin flags.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
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