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User Interfaces
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15.6. Reading from the Keyboard

Problem

You want to read a single character from the keyboard. For instance, you've displayed a menu of one-character options, and you don't want to require the user to press Enter to make their selection.

Solution

Use the CPAN module Term::ReadKey to put the terminal into cbreak mode, read characters from STDIN, and then put the terminal back into its normal mode:

use Term::ReadKey;

ReadMode 'cbreak';
$key = ReadKey(0);
ReadMode 'normal';

Discussion

Term::ReadKey can put the terminal into many modes  - cbreak is just one of them. cbreak mode makes each character available to your program as it is typed (see Example 15.1). It also echoes the characters to the screen; see Recipe 15.10 for an example of a mode that does not echo.

Example 15.1: sascii

#!/usr/bin/perl -w
# sascii - Show ASCII values for keypresses

use Term::ReadKey;
ReadMode('cbreak');
print "Press keys to see their ASCII values.  Use Ctrl-C to quit.\n";

while (1) {
    $char = ReadKey(0);
    last unless defined $char;
    printf(" Decimal: %d\tHex: %x\n", ord($char), ord($char));
}

ReadMode('normal');

Using cbreak mode doesn't prevent the terminal's device driver from interpreting end-of-file and flow-control characters. If you want to be able to read a real Ctrl-C (which normally sends a SIGINT to your process) or a Ctrl-D (which indicates end-of-file under Unix), you want to use raw mode.

An argument of 0 to ReadKey indicates that we want a normal read using getc. If no input is available, the program will pause until there is some. We can also pass -1 to indicate a non-blocking read, or a number greater than 0 to indicate the number of seconds to wait for input to become available; fractional seconds are allowed. Non-blocking reads and timed-out reads return either undef when no input is available or a zero-length string on end of file.

Recent versions of Term::ReadKey also include limited support for non-Unix systems.

See Also

The getc and sysread functions in Chapter 3 of Programming Perl, and in perlfunc (1); the documentation for the Term::ReadKey module from CPAN; Recipe 15.8; Recipe 15.9


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