Node:Reading, Next:, Previous:Ports, Up:Input and Output


27.2 Reading

[Generic procedures for reading from ports.]

eof-object? x Scheme Procedure
scm_eof_object_p (x) C Function
Return #t if x is an end-of-file object; otherwise return #f.

char-ready? [port] Scheme Procedure
scm_char_ready_p (port) C Function
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. 1

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

peek-char [port] Scheme Procedure
scm_peek_char (port) C Function
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.2

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

unread-string str port Scheme Procedure
scm_unread_string (str, port) C Function
Place the string str in port so that its characters will be read in 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.

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

port-column port Scheme Procedure
port-line port Scheme Procedure
scm_port_column (port) C Function
scm_port_line (port) C Function
Return the current column number or line number of port, using the current input port if none is specified. 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.)

set-port-column! port column Scheme Procedure
set-port-line! port line Scheme Procedure
scm_set_port_column_x (port, column) C Function
scm_set_port_line_x (port, line) C Function
Set the current column or line number of port, using the current input port if none is specified.

Footnotes

  1. 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.

  2. 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.