[plt-scheme] FFI + errno

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Wed Dec 9 13:07:26 EST 2009

Yes, that's true.

I almost suggested wrapping the call to the function that sets `errno'
and the access of `errno' with `scheme_start_atomic()' and
`scheme_end_atomic()' (which you can access them with the FFI). But
that's not good enough, because the runtime system occasionally uses
`errno'-setting function internally, even when not switching Scheme

If the `scheme/tcp', `scheme/udp', or other bindings don't suffice,
then a wrapper function that captures and stores `errno' looks like the
only option right now.

I'll think more about how to solve this in the FFI, though. Most
problems like this can be solved with atomic mode, but `errno' is a
special case. Probably the FFI should provide more direct support for
getting the right `errno' value.

At Mon, 07 Dec 2009 15:48:55 -0800, Ben Goetter wrote:
> If the PLT runtime itself calls any C runtime function between the time 
> that your call to socket() sets errno and the time that you can access 
> your thunk to errno, then errno will no longer contain the value set by 
> socket().
> Hence you cannot use errno usefully through the FFI.  Mechanically, you 
> could access the contents of that variable through make-c-parameter or 
> the like.
> I would favor the existing scheme/tcp or scheme/udp bindings, if at all 
> possible.
> Steve Huffman wrote:
> > Hi All,
> >
> > I'm using OS X and trying to access the raw socket api using PLT's
> > FFI. For example:
> >
> > (define _socket
> >   (get-ffi-obj "socket" #f (_fun _int _int _int -> _int)))
> >
> > (define _connect
> >   (get-ffi-obj "connect" #f (_fun _int _sockaddr_in-pointer _int -> _int)))
> >
> > Things are mostly working, except I'm having trouble accessing the
> > global errno value to see error codes. I've tried wrapping the
> > __error() function, but am not receive relevant error codes.
> >
> > I gather from searching this list that PLT itself might be using errno
> > for its own purposes before I get a chance to see it. Is that
> > accurate?
> >
> > Should I write my own wrapper functions in C around the socket
> > functions and capture/store errno someplace else?
> >
> > Thanks,
> >
> > Steve
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> >   
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