trusty (3) msync.3posix.gz

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 .