msync - synchronize a file with a memory map


       #include <unistd.h>
       #include <sys/mman.h>

       #ifdef _POSIX_MAPPED_FILES

       int msync(const void *start, 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 start 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 val­
       ues just written).


       On success, zero is returned.  On error, -1  is  returned,
       and errno is set appropriately.


       EINVAL start  is  not  a  multiple of PAGESIZE, or any bit
              other than MS_ASYNC | MS_INVALIDATE  |  MS_SYNC  is
              set in flags.

       EFAULT The  indicated  memory  (or  part  of  it)  was not


       POSIX.1b (formerly POSIX.4)


       mmap(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp.  128-129
       and 389-391.