[ruby-talk:21458] Re: Low level file I/O question

On Fri, 21 Sep 2001, Michael Davis wrote:

> I am considering writing my own version of IO.read and IO.write call
> IO.read_direct(buffer, size) and IO.write_direct(buffer, size).  What are
> the risks with me calling fread/fwrite directly?  What are the risks if I
> later call IO.read and IO.write against the same file pointer?  Has anyone
> done anything similar?  How can I pass a buffer pointer (declared as a char
> * in my buffer class) to these functions in Ruby?

A Ruby string is essentially a char* pointer and a length.  Thus, you can
use:
    int len;
    char * s = str2cstr(your_ruby_string, &len);

(BTW, why does rb_str2cstr() take an int* argument instead of a size_t*?
Does it make sense for a string to have a negative length?)

Anyhow, what you can do is allocate a string in Ruby of the given length,
with:

    def string_of_length(len)
      return "%#{len}s" % ''
    end

Then call your C functions read_direct/write_direct:

    f = File.new(...)
    s = string_of_length(100)
    f.read_direct(s)
    f.write_direct(s)

If you really want to be able to read/write to a buffer that is
larger/smaller than the desired read/write length, then you can use an
optional length argument to the two functions (otherwise they could
default to the length of the string).  However, make sure you do the
appropriate bounds checking:

    /* starting after char * s = ... above */
    if(specified_length > len) {
        rb_raise("Uh oh!  You specified a length longer than "
                 "the size of the string");
    }

If you don't do this check, then specifying a bad length could cause your
program to segfault (and that, of course would be bad).

If you REALLY don't want to use a Ruby string (since they can change size
when you call certain methods), then you can create a struct with a single
char* member, then call Data_Wrap_Struct on it.  You would need to write
some simple string operations, though, in order for this to work (or
perhaps a function to convert to a Ruby string).  This, however, probably
involves more work.

Hope this helps!

Paul