NAME

       msync - synchronize a file with a memory map

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/mman.h>

       int msync(void addr[.length], size_t length, int flags);

DESCRIPTION

       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).

RETURN VALUE

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

ERRORS

       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.

VERSIONS

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

       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).)

SEE ALSO

       mmap(2)

       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.

Linux man-pages 6.7                                2023-10-31                                           msync(2)