msync — synchronize a file with a memory map
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
should specify exactly one of
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).
On success, zero is returned. On error, −1 is
errno is set
flags, and a memory lock
exists for the specified address range.
addr is not
a multiple of PAGESIZE; or any bit other than
MS_SYNC is set in
flags; or both
MS_ASYNC are set in
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_SYNCHRONIZED_IO are defined in
to a value greater than 0. (See also sysconf(3).)
According to POSIX, either
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
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
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
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Copyright (C) 1996 Andries Brouwer (aebcwi.nl)
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