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(guile.info)Port Implementation

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Port Implementation
-------------------

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 calling
`scm_make_port_type'.

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.

`scm_make_port_type' initialises three elements of the structure
(`name', `fill_input' and `write') from its arguments.  The remaining
elements are initialised with default values and can be set later if
required.

`name'
     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 `scm_make_port_type'.

`mark'
     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 `scm_set_port_mark'.

`free'
     Called when the port is collected during gc.  It should free any
     resources used by the port.  Set using `scm_set_port_free'.

`print'
     Called when `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 `scm_set_port_print'.

`equalp'
     Not used at present.  Set using `scm_set_port_equalp'.

`close'
     Called when the port is closed, unless it was collected during gc.
     It should free any resources used by the port.  Set using
     `scm_set_port_close'.

`write'
     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 `scm_make_port_type'.

`flush'
     Complete the processing of buffered output data.  Reset the value
     of `rw_active' to `SCM_PORT_NEITHER'.  Set using
     `scm_set_port_flush'.

`end_input'
     Perform any synchronisation required when switching from input to
     output on the port.  Reset the value of `rw_active' to
     `SCM_PORT_NEITHER'.  Set using `scm_set_port_end_input'.

`fill_input'
     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
     `scm_make_port_type'.

`input_waiting'
     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 `rw_active' is `SCM_PORT_NEITHER'.  Set using
     `scm_set_port_input_waiting'.

`seek'
     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 like:

                if (pt->rw_active == SCM_PORT_READ)
          	scm_end_input (object);
                else if (pt->rw_active == SCM_PORT_WRITE)
          	ptob->flush (object);

     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 `end_input' and `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 `scm_set_port_seek'.

`truncate'
     Truncate the port data to be specified length.  It can be assumed
     that the current state of `rw_active' is `SCM_PORT_NEITHER'.  Set
     using `scm_set_port_truncate'.