Re: [Boost-users] memory mapped io and boost

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2008-05-02 04:14:04

peter_foelsche_at_[hidden] wrote:
> Dear All,
> 
> from what it looks like the memory mapped io classes in 
> boost::interprocess cannot deal with readonly files.

Which memory mapped classes? Managed or not?

For basic operations like obtain a file mapping:

file_mapping(const char *filename, mode_t mode);

And map it:

    template<class MemoryMappable>
    mapped_region(const MemoryMappable& mapping
                 ,mode_t mode
                 ,offset_t offset = 0
                 ,std::size_t size = 0
                 ,const void *address = 0);

you can specify mode_t.

For managed_mapped_file you can't use read-only, because there are 
mutexes built in the mapped file that have to be locked in case one 
process opens creates new elements.

So I guess you are referring to managed classes, right?
a)The problem with allowing read-only is that another process can open 
it for writting and you would have problems.

b)The internal structure should be changed to avoid using mutexes and 
that can be difficult because internal structures and user-defined 
algorithms and indexes should avoid that (and also user-defined objects 
that write in some read-operations).

You also say that disk-space is being allocated when growing a file. 
That's the idea. Otherwise, it would be the internal "offset_ptr" 
pointers would become useless unless we map all file portions 
contiguously and try to manage several mapped regions (which might be a 
bit tricky). Anyway, I don't see why growing a file should be slower 
than creating a new one. The OS looks for free disk sectors so file 
contents are not physically contiguous although you get a single file. 
Why is making the file sparse an advantage?

Regards,

Ion

> 
> This is a big problem.
> 
> I think opening readonly files should be possible but all write 
> operations to the file should fail.
> 
> Of course mapping a readonly file should be done with the matching flags 
> so that the mapping can be shared and that the matching address space is 
> readonly.
> 
> Also another drawback is that at least on Windows disk-space is being 
> allocated when growing a file.
> 
> This is slow and it can be avoided by making the file sparse.
> 
> On UNIXs this is the default.
> 
>  
> 
> Thanks
> 
> Peter
> 
> 
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