This section describes how to implement a new port type in C.
As described in the previous section, a port type object (ptob) is
a structure of type
scm_ptob_descriptor. A ptob is created by
Return a new port type object. The name, fill_input and write parameters are initial values for those port type fields, as described below. The other fields are initialized with default values and can be changed later.
All of the elements of the ptob, apart from
name, are procedures
which collectively implement the port behaviour. Creating a new port
type mostly involves writing these procedures.
A pointer to a NUL terminated string: the name of the port type. This
is the only element of
scm_ptob_descriptor which is not
a procedure. Set via the first argument to
Called during garbage collection to mark any SCM objects that a port
object may contain. It doesn’t need to be set unless the port has
SCM components. Set using
Called when the port is collected during gc. It should free any resources used by the port. Set using
write is called on the port object, to print a
port description. E.g., for an fport it may produce something like:
#<input: /etc/passwd 3>. Set using
The first argument port is the object being printed, the second argument dest_port is where its description should go.
Not used at present. Set using
Called when the port is closed, unless it was collected during gc. It should free any resources used by the port. Set using
Accept data which is to be written using the port. The port implementation
may choose to buffer the data instead of processing it directly.
Set via the third argument to
Complete the processing of buffered output data. Reset the value of
Perform any synchronization required when switching from input to output
on the port. Reset the value of
Read new data into the read buffer and return the first character. It
can be assumed that the read buffer is empty when this procedure is called.
Set via the second argument to
Return a lower bound on the number of bytes that could be read from the
port without blocking. It can be assumed that the current state of
Set the current position of the port. The procedure can not make
any assumptions about the value of
rw_active when it’s
called. It can reset the buffers first if desired by using something
if (pt->rw_active == SCM_PORT_READ) scm_end_input (port); else if (pt->rw_active == SCM_PORT_WRITE) ptob->flush (port);
However note that this will have the side effect of discarding any data
in the unread-char buffer, in addition to any side effects from the
flush ptob procedures. This is undesirable
when seek is called to measure the current position of the port, i.e.,
(seek p 0 SEEK_CUR). The libguile fport and string port
implementations take care to avoid this problem.
The procedure is set using
Truncate the port data to be specified length. It can be assumed that the
current state of