MSYNC(2)                    Linux Programmer's Manual                   MSYNC(2)

       msync - synchronize a file with a memory map

       #include <sys/mman.h>

       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:

              Specifies that an update be scheduled, but the call returns

              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).

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       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.

       POSIX.1-2001, POSIX.1-2008.

       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                              2021-03-22                           MSYNC(2)