Immediate Detection of Character Input

[Alex Sramek]

I'm writing a game in C, on a linux box (working on recent RedHat and
Mandrake systems).

I want to get input from the keyboard immediately into the program,
without having to wait for an end of line to empty the buffer. Just
whenever a key is pressed, I'd like to hear about it and be able to tell
what key(s) was (were) pressed, preferably in a non-blocking manner (the
code can go on if nothing is received).

I would prefer not to use curses.h if possible, but if I must, I must.

I think ioctl might be able to do it, but I'm unfamiliar with the
syntax, and the manpage for it isn't at all helpful.

Any suggestions/comments are appreciated.

Thanks!

-Alex

[Floyd Davidson]

Here is a program that demonstrates the functionality you seek.

/*
* kbhit(), a keyboard lookahead monitor
* getch(), a blocking single character input from stdin
*
* Plus a demo main() to illustrate usage.
*/

#include <stdio.h>
#include <unistd.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>

int getch(void);
int kbhit(void);

#define CMIN 1

#ifdef CTIME
#undef CTIME
#endif

#define CTIME 1

/*
* kbhit() -- a keyboard lookahead monitor
*
* returns the number of characters available to read.
*/
int
kbhit(void)
{
int cnt = 0;
int error;
static struct termios Otty, Ntty;

tcgetattr(0, &Otty);
Ntty = Otty;

Ntty.c_iflag = 0; /* input mode */
Ntty.c_oflag = 0; /* output mode */
Ntty.c_lflag &= ~ICANON; /* raw mode */
Ntty.c_cc[VMIN] = CMIN; /* minimum chars to wait for */
Ntty.c_cc[VTIME] = CTIME; /* minimum wait time */

if (0 == (error = tcsetattr(0, TCSANOW, &Ntty))) {
struct timeval tv;
error += ioctl(0, FIONREAD, &cnt);
error += tcsetattr(0, TCSANOW, &Otty);
tv.tv_sec = 0;
tv.tv_usec = 100; /* insert a minimal delay */
select(1, NULL, NULL, NULL, &tv);
}

return (error == 0 ? cnt : -1 );
}

/*
* getch() -- a blocking single character input from stdin
*
* Returns a character, or -1 if an input error occurs.
*
* Conditionals allow compiling with or without echoing of
* the input characters, and with or without flushing pre-existing
* existing buffered input before blocking.
*
*/
int
getch(void)
{
char ch;
int error;
static struct termios Otty, Ntty;

fflush(stdout);
tcgetattr(0, &Otty);
Ntty = Otty;

Ntty.c_iflag = 0; /* input mode */
Ntty.c_oflag = 0; /* output mode */
Ntty.c_lflag &= ~ICANON; /* line settings */

#if 1
/* disable echoing the char as it is typed */
Ntty.c_lflag &= ~ECHO; /* disable echo */
#else
/* enable echoing the char as it is typed */
Ntty.c_lflag |= ECHO; /* enable echo */
#endif

Ntty.c_cc[VMIN] = CMIN; /* minimum chars to wait for */
Ntty.c_cc[VTIME] = CTIME; /* minimum wait time */

#if 1
/*
* use this to flush the input buffer before blocking for new input
*/
#define FLAG TCSAFLUSH
#else
/*
* use this to return a char from the current input buffer, or block if
* no input is waiting.
*/
#define FLAG TCSANOW

#endif

if (0 == (error = tcsetattr(0, FLAG, &Ntty))) {
error = read(0, &ch, 1 ); /* get char from stdin */
error += tcsetattr(0, FLAG, &Otty); /* restore old settings */
}

return (error == 1 ? (int) ch : -1 );
}

/*
* a cutsie main() to demo how getch() and kbhit() work.
*/

#include <ctype.h>

int
main(void)
{
int ch, cnt;
static struct termios Otty, Ntty;

tcgetattr(0, &Otty);
Ntty = Otty;
Ntty.c_lflag &= ~ECHO;

printf("You must enter 10 characters to get\nthis program to continue: ");
fflush(stdout);
/* collect 10 characters */
while (1) {
if (10 <= (cnt = kbhit())) {
break;
}
}

tcsetattr(0, TCSANOW, &Ntty); /* disable echoing of further input */
printf("\nSTOP!");
fflush(stdout);
printf("\nNow type <Enter> to continue!");
fflush(stdout);
ch = getchar();
tcsetattr(0, TCSANOW, &Otty); /* enable echoing of further input */
printf("\n\nThe first five characters are: \"");
/*
* print a few chars, note that calling getch() will flush
* remaining buffered input.
*/
cnt = 4;
do {
printf("%c", ch);
ch = getchar();
} while (--cnt);
printf("%c\"\n\n", ch);

printf("\n\n *** Demo Menu ***\n\n");
printf(" Option Action\n");
printf(" A Exit and print an A word\n");
printf(" B Exit and print a B word\n");
printf(" C Exit and print a C word\n");
printf("\n Enter your choice: ?\b");
fflush(stdout);

while(1) {
switch (ch = getch()) {
case 'a': case 'A':
printf("%c\n\nThat is an *awesome* function!\n", toupper(ch));
break;
case 'b': case 'B':
printf("%c\n\nThat is a *beneficial* function!\n", toupper(ch));
break;
case 'c': case 'C':
printf("%c\n\nThat is a *critical* function!\n", toupper(ch));
break;
default:
continue;
}
break;
}
return 0;
}

--
Floyd L. Davidson <http://www.ptialaska.net/~floyd>
Ukpeagvik (Barrow, Alaska) fl...@barrow.com

[Alex Sramek]

Thanks a bunch! This is very helpful, especially in context.

With the handful of replies I've gotten, I should be able to hammer
something out. Once I'm done, I'll probably post the game and source
code to my website.

-Alex