[gambit-list] Some high level questions about open-tcp-client

Jeff Read bitwize at gmail.com
Mon Jul 7 18:24:24 EDT 2014

On Jul 7, 2014 5:54 PM, "Rob Kellock" <sales at creditscore.co.nz> wrote:
> Hi,
> Quite some time ago I managed to successfully interface with a C library
> using FFI.  Now I'm having another go, but this time I want to connect
> to it using native Scheme code.  The C library is the interface to a
> select based TCP socket server, so I figure I should be able to connect
> to it directly using open-tcp-client and forgo the whole FFI thing.
> The way the C library works is that a C struct is sent to the receiving
> socket first so that it knows the length of the message buffer that will
> follow and some other information specific to the socket server's
> operation. This is immediately followed up by the message buffer which
> is in binary format.
> So far... I think I've managed to work out that I should be using
> u8vector data structure for both the C struct and the message buffer,
> although I don't yet understand what kind of encoding is going on in
> there. That will come after I've looked at some examples.
> My plan is to extend the existing web-server.scm example to produce a
> web app that talks to it's relational database using native Scheme. I'm
> going down this route because I want to extend web-server.scm using the
> GSI.  If I use the FFI I'm restricted to the GSC which prevents the
> whole interactive development scenario I want where I can keep the
> browser running, the database running and just stop and start the GSI as
> I hack code.
> What I want to know first though is if I use open-tcp-client inside
> Gambit's threads will the native Scheme calls out to the external socket
> server block?  If they don't then am I on the right track to use u8vector?

Yes, using u8vector is the way to go imho.

I find it strange -- and a misfeature -- that the TCP server accepts a C
struct, because depending on what architecture the server is running on and
even what compiler was used to build it, the length, endianness, and
padding of the fields in the struct can vary. So you will have to take
those into account when packing a u8vector full of bytes to send to this

And you can certainly use FFI functions from within gsi. Simply compile
just the FFI wrapper with gsc into a loadable module, then load it with
LOAD into gsi while you develop the upper level bits in Scheme.
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