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6.15.2 Reading

[Generic procedures for reading from ports.]

These procedures pertain to reading characters and strings from ports. To read general S-expressions from ports, See Scheme Read.

Scheme Procedure: eof-object? x
C Function: scm_eof_object_p (x)

Return #t if x is an end-of-file object; otherwise return #f.

Scheme Procedure: char-ready? [port]
C Function: scm_char_ready_p (port)

Return #t if a character is ready on input port and return #f otherwise. If char-ready? returns #t then the next read-char operation on port is guaranteed not to hang. If port is a file port at end of file then char-ready? returns #t.

char-ready? exists to make it possible for a program to accept characters from interactive ports without getting stuck waiting for input. Any input editors associated with such ports must make sure that characters whose existence has been asserted by char-ready? cannot be rubbed out. If char-ready? were to return #f at end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.

Scheme Procedure: read-char [port]
C Function: scm_read_char (port)

Return the next character available from port, updating port to point to the following character. If no more characters are available, the end-of-file object is returned.

When port’s data cannot be decoded according to its character encoding, a decoding-error is raised and port points past the erroneous byte sequence.

C Function: size_t scm_c_read (SCM port, void *buffer, size_t size)

Read up to size bytes from port and store them in buffer. The return value is the number of bytes actually read, which can be less than size if end-of-file has been reached.

Note that this function does not update port-line and port-column below.

Scheme Procedure: peek-char [port]
C Function: scm_peek_char (port)

Return the next character available from port, without updating port to point to the following character. If no more characters are available, the end-of-file object is returned.

The value returned by a call to peek-char is the same as the value that would have been returned by a call to read-char on the same port. The only difference is that the very next call to read-char or peek-char on that port will return the value returned by the preceding call to peek-char. In particular, a call to peek-char on an interactive port will hang waiting for input whenever a call to read-char would have hung.

As for read-char, a decoding-error may be raised if such a situation occurs. However, unlike with read-char, port still points at the beginning of the erroneous byte sequence when the error is raised.

Scheme Procedure: unread-char cobj [port]
C Function: scm_unread_char (cobj, port)

Place character cobj in port so that it will be read by the next read operation. If called multiple times, the unread characters will be read again in last-in first-out order. If port is not supplied, the current input port is used.

Scheme Procedure: unread-string str port
C Function: scm_unread_string (str, port)

Place the string str in port so that its characters will be read from left-to-right as the next characters from port during subsequent read operations. If called multiple times, the unread characters will be read again in last-in first-out order. If port is not supplied, the current-input-port is used.

Scheme Procedure: drain-input port
C Function: scm_drain_input (port)

This procedure clears a port’s input buffers, similar to the way that force-output clears the output buffer. The contents of the buffers are returned as a single string, e.g.,

(define p (open-input-file ...))
(drain-input p) => empty string, nothing buffered yet.
(unread-char (read-char p) p)
(drain-input p) => initial chars from p, up to the buffer size.

Draining the buffers may be useful for cleanly finishing buffered I/O so that the file descriptor can be used directly for further input.

Scheme Procedure: port-column port
Scheme Procedure: port-line port
C Function: scm_port_column (port)
C Function: scm_port_line (port)

Return the current column number or line number of port. If the number is unknown, the result is #f. Otherwise, the result is a 0-origin integer - i.e. the first character of the first line is line 0, column 0. (However, when you display a file position, for example in an error message, we recommend you add 1 to get 1-origin integers. This is because lines and column numbers traditionally start with 1, and that is what non-programmers will find most natural.)

Scheme Procedure: set-port-column! port column
Scheme Procedure: set-port-line! port line
C Function: scm_set_port_column_x (port, column)
C Function: scm_set_port_line_x (port, line)

Set the current column or line number of port.


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