Re: [Boost-users] [Interprocess] [file_mapping] is there any way to avoid writing a modified file to disc?

Subject: Re: [Boost-users] [Interprocess] [file_mapping] is there any way to avoid writing a modified file to disc?
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2011-12-07 10:21:56

Den 07-12-2011 16:13, Ion Gaztañaga skrev:
> El 07/12/2011 14:44, Dean Michael Berris escribió:
>> Actually, I just checked the docs: it seems you want the
>> 'copy_on_write' mode which combines PROT_WRITE|PROT_READ and
>> MAP_PRIVATE. This allows you to make changes in place to the memory
>> region without having the changes reflected to the original file.
>
> Yes, copy_on_write should work. When a page is modified the OS creates a
> new page for you but other process sees your changes (well, I guess they
> can end in the page file).

Well, at least with 1.45 I cannot get it to work.

I also speculate if its actually any faster, since I need to change 
every single byte in the memory region (I'm decrypting).

My understanding of these OS features are weak, but my mental model of 
it is that

             //Create a file mapping
             file_mapping m_file( file.c_str(), 
boost::interprocess::read_only );

             //Map the whole file with read-write permissions in this 
process
             mapped_region region( m_file, boost::interprocess::read_only );

             //Get the address of the mapped region
             const char* addr  = static_cast<char*>(region.get_address());
             const size_t size = static_cast<int>(region.get_size());

makes the whole file avaiable as one big memory segment.

It seems to me that the OS must do something to write changed data to 
disc (that is, there is an implicit flush() in one of the destructors).
Hence I assumed that it would be possible to avoid this flush, leaving 
the file intact.

-Thorsten