Node:C Port Interface, Next:Port Implementation, Up:I/O Extensions
This section describes how to use Scheme ports from C.
There are two main data structures. A port type object (ptob) is of
scm_ptob_descriptor. A port instance is of type
scm_port. Given an
SCM variable which points to a port,
the corresponding C port object can be obtained using the
SCM_PTAB_ENTRY macro. The ptob can be obtained by using
SCM_PTOBNUM to give an index into the
An input port always has a read buffer and an output port always has a
write buffer. However the size of these buffers is not guaranteed to be
more than one byte (e.g., the
shortbuf field in
which is used when no other buffer is allocated). The way in which the
buffers are allocated depends on the implementation of the ptob. For
example in the case of an fport, buffers may be allocated with malloc
when the port is created, but in the case of an strport the underlying
string is used as the buffer.
Special treatment is required for ports which can be seeked at random.
Before various operations, such as seeking the port or changing from
input to output on a bidirectional port or vice versa, the port
implementation must be given a chance to update its state. The write
buffer is updated by calling the
flush ptob procedure and the
input buffer is updated by calling the
end_input ptob procedure.
In the case of an fport,
flush causes buffered output to be
written to the file descriptor, while
end_input causes the
descriptor position to be adjusted to account for buffered input which
was never read.
The special treatment must be performed if the
rw_random flag in
the port is non-zero.
rw_active variable in the port is only used if
rw_random is set. It's defined as an enum with the following
To read from a port, it's possible to either call existing libguile
procedures such as
scm_read_line or to read
data from the read buffer directly. Reading from the buffer involves
the following steps:
To write data to a port, calling
scm_lfwrite should be sufficient for
most purposes. This takes care of the following steps:
writeptob procedure. The advantage of using the ptob
writeinstead of manipulating the write buffer directly is that it allows the data to be written in one operation even if the port is using the single-byte