Re: Has anyone implemented their own port types?

[Jim Blandy <jimb@red-bean.com>]

> > Has anyone out there written C code that adds new port types to Guile?
> > This is probably the case iff your code calls scm_newptob.
> 
> Yup, we have a C++ stream wrapper around output ports.  That being said,
> make the changes.  We aren't actively updating to the latest guile
> stuff.

Okay; the change is made.  I was wrong --- the compiler will not warn
you if your code is out of date.

Here's the NEWS entry:

** The semantics of the I/O port functions in scm_ptobfuns have changed.

If you have implemented your own I/O port type, by writing the
functions required by the scm_ptobfuns and then calling scm_newptob,
you will need to change your functions slightly.

The functions in a scm_ptobfuns structure now expect the port itself
as their argument; they used to expect the `stream' member of the
port's scm_port_table structure.  This allows functions in an
scm_ptobfuns structure to easily access the port's cell (and any flags
it its CAR), and the port's scm_port_table structure.

Guile now passes the I/O port itself as the `port' argument in the
following scm_ptobfuns functions:

  int (*free) (SCM port);
  int (*fputc) (int, SCM port);
  int (*fputs) (char *, SCM port);
  scm_sizet (*fwrite) SCM_P ((char *ptr,
			      scm_sizet size,
			      scm_sizet nitems,
			      SCM port));
  int (*fflush) (SCM port);
  int (*fgetc) (SCM port);
  int (*fclose) (SCM port);

The interfaces to the `mark', `print', `equalp', and `fgets' methods
are unchanged.

If you have existing code which defines its own port types, it is easy
to convert your code to the new interface; simply apply SCM_STREAM to
the port argument to yield the value you code used to expect.

Note that since both the port and the stream have the same type in the
C code --- they are both SCM values --- the C compiler will not remind
you if you forget to update your scm_ptobfuns functions.