[boost] Re: [iostreams] Reading a part of file

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2005-03-16 11:16:42

Vladimir Prus wrote:
> Hello,
> I wonder if the iostreams library allows me to get an istream that
> will read a part of a file. That is: I specify a file, start offset
> and end offset. The resulting istream begins reading at start offset
> and returns EOF when it arrives at end offset.

There wil be an offset filter very soon. I first need to implement in-place
filters; otherwise the offset filter will make an unnecessary copy of all the
data.

> I haven't found anything like this in the docs and I also don't know
> if such functionality makes sense for the library. I just need to
> read individual members of Unix static archives (.a files) and wanted
> to know if the library can help here. If not, I'll just read the data
> into strstream.

> Also: the POSIX mmap call allows one to specify start and end offset.
> However, the mapped_file_source does not allow to specify that, and
> documentation does not clearly explain why one would want to use that
> class at all.

It looks like the docs are over-simplified. Looking at the source I see the
following constructors:

    mapped_file_source( const std::string& path,
                        size_type length = max_length,
                        boost::intmax_t offset = 0 );

    mapped_file_sink( const std::string& path,
                      size_type length = max_length,
                      boost::intmax_t offset = 0 ) ;

    mapped_file( const std::string& path,
                 ios::openmode mode =
                    ios::in | ios::out,
                 size_type length = max_length,
                 boost::intmax_t offset = 0 ) ;

Also, someone sent me a version of mapped_file with a lot more functionality,
but I haven't intergrated it yet.

Best Regards,
Jonathan