[Generic procedures for reading from ports.]
#tif x is an end-of-file object; otherwise return
#tif a character is ready on input port and return
#tthen the next
read-charoperation on port is guaranteed not to hang. If port is a file port at end of file then
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
#fat end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.
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.
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
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-charis the same as the value that would have been returned by a call to
read-charon the same port. The only difference is that the very next call to
peek-charon that port will return the value returned by the preceding call to
peek-char. In particular, a call to
peek-charon an interactive port will hang waiting for input whenever a call to
read-charwould have hung.
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.
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
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.
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.)