# msync

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## 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 may 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 shall be
           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 last data
           modification and last file status change timestamps 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.
    
           **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 _msync_() function may fail if:
    
           **EINVAL** The value of _addr_ is not a multiple of the page size as
                  returned by _sysconf_().
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           The _msync_() function is only supported if the Synchronized Input
           and Output option is 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.
    


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