NAME

       msync - synchronize memory with physical storage

SYNOPSIS

       #include <sys/mman.h>

       int msync(void *addr, size_t len, int flags);

DESCRIPTION

       The msync() function shall write all modified data to permanent storage locations, if any,
       in those whole pages containing any part of the address space of the process  starting  at
       address  addr  and  continuing  for len bytes. If no such storage exists, msync() need not
       have any effect. If requested, the msync() function shall then invalidate cached copies of
       data.

       The  implementation  shall require that addr be a multiple of the page size as returned by
       sysconf().

       For mappings to files, the msync() function shall ensure that  all  write  operations  are
       completed  as  defined  for  synchronized I/O data integrity completion. It is unspecified
       whether the implementation also writes  out  other  file  attributes.   When  the  msync()
       function  is called on MAP_PRIVATE mappings, any modified data shall not be written to the
       underlying object and shall not cause such data to be made visible to other processes.  It
       is  unspecified  whether data in MAP_PRIVATE mappings has any permanent storage locations.
        The effect of msync() on a shared memory object or a typed memory object is  unspecified.
       The  behavior of this function is unspecified if the mapping was not established by a call
       to mmap().

       The flags argument is constructed from the bitwise-inclusive OR of  one  or  more  of  the
       following flags defined in the <sys/mman.h> header:

                            Symbolic Constant  Description
                            MS_ASYNC           Perform asynchronous writes.
                            MS_SYNC            Perform synchronous writes.
                            MS_INVALIDATE      Invalidate cached data.

       When MS_ASYNC is specified, msync() shall return immediately once all the write operations
       are initiated or queued for servicing; when MS_SYNC is specified, msync() shall not return
       until  all  write  operations are completed as defined for synchronized I/O data integrity
       completion. Either MS_ASYNC or MS_SYNC is specified, but not both.

       When MS_INVALIDATE is specified, msync() shall invalidate all cached copies of mapped data
       that are inconsistent with the permanent storage locations such that subsequent references
       shall obtain data that was  consistent  with  the  permanent  storage  locations  sometime
       between the call to msync() and the first subsequent memory reference to the data.

       If  msync()  causes  any write to a file, the file's st_ctime and st_mtime fields shall be
       marked for update.

RETURN VALUE

       Upon successful completion, msync() shall return 0; otherwise, it shall return -1 and  set
       errno to indicate the error.

ERRORS

       The msync() function shall fail if:

       EBUSY  Some  or  all of the addresses in the range starting at addr and continuing for len
              bytes are locked, and MS_INVALIDATE is specified.

       EINVAL The value of flags is invalid.

       EINVAL The value of addr is not a multiple of the page size {PAGESIZE}.

       ENOMEM The addresses in the range starting at  addr  and  continuing  for  len  bytes  are
              outside the range allowed for the address space of a process or specify one or more
              pages that are not mapped.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The msync() function is  only  supported  if  the  Memory  Mapped  Files  option  and  the
       Synchronized  Input and Output option are supported, and thus need not be available on all
       implementations.

       The msync() function should be used by programs that require a memory object to  be  in  a
       known state; for example, in building transaction facilities.

       Normal  system  activity  can  cause  pages to be written to disk. Therefore, there are no
       guarantees that msync() is the only control over when pages are  or  are  not  written  to
       disk.

RATIONALE

       The  msync()  function writes out data in a mapped region to the permanent storage for the
       underlying object. The call to msync() ensures data integrity of the file.

       After the data is written out, any cached data may be  invalidated  if  the  MS_INVALIDATE
       flag was specified. This is useful on systems that do not support read/write consistency.

FUTURE DIRECTIONS

       None.

SEE ALSO

       mmap() , sysconf() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/mman.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .