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6.15.11.2 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.

Function: scm_t_bits scm_make_port_type (char *name, int (*fill_input) (SCM port), void (*write) (SCM port, const void *data, size_t size))

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.

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

Function: void scm_set_port_mark (scm_t_bits tc, SCM (*mark) (SCM port))
free

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

Function: void scm_set_port_free (scm_t_bits tc, size_t (*free) (SCM port))
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

Function: void scm_set_port_print (scm_t_bits tc, int (*print) (SCM port, SCM dest_port, scm_print_state *pstate))

The first argument port is the object being printed, the second argument dest_port is where its description should go.

equalp

Not used at present. Set using

Function: void scm_set_port_equalp (scm_t_bits tc, SCM (*equalp) (SCM, SCM))
close

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

Function: void scm_set_port_close (scm_t_bits tc, int (*close) (SCM port))
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

Function: void scm_set_port_flush (scm_t_bits tc, void (*flush) (SCM port))
end_input

Perform any synchronization required when switching from input to output on the port. Reset the value of rw_active to SCM_PORT_NEITHER. Set using

Function: void scm_set_port_end_input (scm_t_bits tc, void (*end_input) (SCM port, int offset))
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

Function: void scm_set_port_input_waiting (scm_t_bits tc, int (*input_waiting) (SCM port))
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 (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 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

Function: void scm_set_port_seek (scm_t_bits tc, scm_t_off (*seek) (SCM port, scm_t_off offset, int whence))
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

Function: void scm_set_port_truncate (scm_t_bits tc, void (*truncate) (SCM port, scm_t_off length))

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