14.12.3 Textual Input Port Operations

This section describes the standard operations on textual input ports. Following that, some useful custom operations are described.

operation on textual input port: read-char port

Removes the next character available from port and returns it. If port has no more characters and will never have any (e.g. at the end of an input file), this operation returns an end-of-file object. If port has no more characters but will eventually have some more (e.g. a terminal where nothing has been typed recently), and it is in non-blocking mode, #f is returned; otherwise the operation hangs until input is available.

operation on textual input port: peek-char port

Reads the next character available from port and returns it. The character is not removed from port, and a subsequent attempt to read from the port will get that character again. In other respects this operation behaves like read-char.

operation on textual input port: char-ready? port k

char-ready? returns #t if at least one character is available to be read from port. If no characters are available, the operation waits up to k milliseconds before returning #f, returning immediately if any characters become available while it is waiting.

operation on textual input port: read-string port char-set

operation on textual input port: discard-chars port char-set

These operations are like read-char, except that they read or discard multiple characters at once. All characters up to, but excluding, the first character in char-set (or end of file) are read from port. read-string returns these characters as a newly allocated string, while discard-chars discards them and returns an unspecified value. These operations hang until sufficient input is available, even if port is in non-blocking mode. If end of file is encountered before any input characters, read-string returns an end-of-file object.

operation on textual input port: read-substring port string start end

Reads characters from port into the substring defined by string, start, and end until either the substring has been filled or there are no more characters available. Returns the number of characters written to the substring.

If port is an interactive port, and at least one character is immediately available, the available characters are written to the substring and this operation returns immediately. If no characters are available, and port is in blocking mode, the operation blocks until at least one character is available. Otherwise, the operation returns #f immediately.

This is an extremely fast way to read characters from a port.

procedure: input-port/read-char textual-input-port

procedure: input-port/peek-char textual-input-port

procedure: input-port/char-ready? textual-input-port k

procedure: input-port/read-string textual-input-port char-set

procedure: input-port/discard-chars textual-input-port char-set

procedure: input-port/read-substring textual-input-port string start end

Each of these procedures invokes the respective operation on textual-input-port. For example, the following are equivalent:

(input-port/read-char textual-input-port)
((textual-port-operation textual-input-port 'read-char)
 textual-input-port)

The following custom operations are implemented for input ports to files, and will also work with some other kinds of input ports:

operation on textual input port: eof? port

Returns #t if port is known to be at end of file, otherwise it returns #f.

operation on textual input port: chars-remaining port

Returns an estimate of the number of characters remaining to be read from port. This is useful only when port is a file port in binary mode; in other cases, it returns #f.

operation on textual input port: buffered-input-chars port

Returns the number of unread characters that are stored in port’s buffer. This will always be less than or equal to the buffer’s size.

operation on textual input port: input-buffer-size port

Returns the maximum number of characters that port’s buffer can hold.

operation on textual input port: set-input-buffer-size port size

Resizes port’s buffer so that it can hold at most size characters. Characters in the buffer are discarded. Size must be an exact non-negative integer.