Re: low-level reading of file inside an archive?

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Hello,

Guido De Rosa a =E9crit :
> Hi all,
> =

> is there a way to do something like this:
> =

> read n bytes at offset t of file path/to/my/file.ext inside a DAR
> archive and put the result into a std::string or char* or something?
> =

> I would like to implement a read-only FUSE file system to look inside
> DAR archives, and I need to perform this kind of basic I/O operations
> without extracting real files to temporary  directories (and then make
> the I/O on the file) because this would be highly inefficient.

Yep, I understand the need.

In current development code (which should start its pre-release phase
this year I hope), you have such a possibility:

When you open an archive using the libdar API, you create an "archive"
object, which provide a get_catalogue() method.

The object returned by this archive::get_catalogue() method has several
methods to navigate in the directory tree:
  catalogue::reset_read()
  catalogue::read()
  catalogue::skip_read_to_parent_dir()

and also
  catalogue::direct_read() but which is much less efficient.

the catalogue::read() method will provide you an object of class
"entree" which is the root parent of several classes, among which the
class "file" which corresponds to plain files, you will have to use
dynamic_cast to know whether the returned object is a plain file or not
and to be able to use its specific methods.

The methods you will be able to use are in particular:
file::get_saved_status() which will return the state of the file. If
that state is "saved" then the data of that plain file is present in the
archive and you can get it using the method:

file::get_data()

This method will return you a newly allocated generic_file object (well
a pointer to that object) you will have to destroy using the delete
operator.

This generic_file object has the generic_file::skip() method and the
generic_file::read(char *, size_t) which will provide the data you
requested.



> =

> Thank you very much!
> =



Well, I cannot imagine something more simple, because accessing to one
file's data assumes it is a plain file and it has been saved in the
archive (in other words, its data changed since the archive of reference).

In the hope this will help,

Regards,
Denis.
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