printf() hinders getchar() ? (non-blocking)

NeoLux · 09-12-2014, 06:19 PM

so here is a terminal program in C

Code:

#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>

void TTYCook(int) ;
int main() {
	int c ;
	TTYCook( 0 )  ;
	while (1) {
		c = getchar() ;
		if (c != -1) break ;
		printf( "X\n" ) ;
	}
	printf( "done\n" ) ;
	TTYCook( 1 )  ;
	return 0 ;
}
void TTYCook(int yes) {
	struct termios ttystate  ;
	int f				;
	tcgetattr( STDIN_FILENO,&ttystate ) ;
	f = fcntl( STDIN_FILENO,F_GETFL,0 ) ;
	if (yes) {
		ttystate.c_lflag |=  ICANON ;
		ttystate.c_lflag |=  ECHO   ;
		f &= ~O_NONBLOCK ;
	} else {
		ttystate.c_lflag &= ~ICANON ;
		ttystate.c_lflag &= ~ECHO   ;
		f |=  O_NONBLOCK ;
	}
        fcntl( STDIN_FILENO,F_SETFL,f ) ;
	tcsetattr( STDIN_FILENO,TCSANOW,&ttystate ) ;
}

It confuses me that pressing a key either is ignored, or has heavy lag.
I understand it's a busy loop, but input is checked on every single iteration.

My understanding is that every keypress is stored in memory until you take it out with getchar, so a character should be waiting in that buffer after I hit a key.

this program works fine when I replaced the

Code:

printf( "X\n" )

with a call
to a heavy function that takes .3 seconds to compute. All that happened in that case
was that (assuming I pressed a key immediately) the program took either .3 or .6 seconds to stop.

To me that means that there must be something more to the terminal in/out stuff I don't know that would explain this behaviour, I believe that understanding/solving this problem would lead to valuable knowledge for myself.

please let me know if I wasn't clear enough about what I'm asking for help with, thx

smallpond · 09-12-2014, 06:58 PM

O_NONBLOCK means don't block. If you never block, then nothing else can happen on the CPU while your program is running. I would guess your CPU is at 100% while in the loop.

ntubski · 09-12-2014, 08:23 PM

I just tried it here, the printf version seems to stop immediately for me.

NevemTeve · 09-13-2014, 12:37 AM

Here is an example to switch the keyboard into raw mode:

Code:

    struct termios oldtio, newtio;
    int rc;

    rc= tcgetattr (0, &oldtio);
    if (rc) {
        perror ("tcgetattr (0, &oldtio)");
	return 4;
    }
    memcpy (&newtio, &oldtio, sizeof (newtio));
    newtio.c_lflag &= ~(ISIG | ICANON | ECHO);
    newtio.c_iflag &= ~(INLCR | ICRNL | IXON | IXOFF);
    newtio.c_cflag &= ~(PARENB);
    newtio.c_cflag = (newtio.c_cflag & ~CSIZE) | CS8;
    newtio.c_cc[VMIN] = 1;
    rc= tcsetattr (0, TCSAFLUSH, &newtio);
    if (rc) {
	perror ("tcsetattr (0, TCSAFLUSH, &newtio)");
	return 12;
    }

after this, you can use 'read(handle=0)' to read (not getchar, or other buffering function)

NeoLux · 09-13-2014, 07:24 PM

smallpond: I appreciate the feedback, but would you mind explaining more clearly what you're trying to tell me ?

NevemTeve: I tried the code, with the exception of that read function 'read(handle=0)' not sure why you're setting a variable inside of a function I only found one function that was close that is syntax

Code:

 ssize_t read(int,void*,size_t)

and when I tried it out the input was blocking so It didn't get me to the solution I was looking for.

ntubski: I would really love to find out what the differences are for our circumstances. I ran

Code:

gcc test.c

Code:

./a.out

with gcc 4.9.1-1
on
xterm 309-1
ummm this is my output of 'stty'

Code:

# stty
speed 38400 baud; line = 0;
erase = ^H; eof = <undef>;
-brkint -imaxbel iutf8

I would ask for cpuinfo stuff, but I'm really hoping that your circumstances are ones that I can duplicate

I'm not sure what other settings would count, but just to verify, you compiled that code, and when it ran it printed X's in an infinite loop, and when you hit a key it stopped immediately ?

NevemTeve · 09-13-2014, 11:18 PM

Sorry, that part wasn't meant for direct usage, it only meant 'read' from handle #0

The complete program is here: http://dtelnet.sf.net/shkeys.c

ntubski · 09-14-2014, 12:10 PM

I ran

Code:

gcc -Wall -Wextra -Wformat=2 -ggdb -pedantic -std=c99    noblock-term.c   -o noblock-term

Code:

./noblock-term

Code:

% gcc --version
gcc (Debian 4.9.1-4) 4.9.1
% stty
speed 38400 baud; line = 0;
iutf8
% urxvt -help 2>&1 | head -1
rxvt-unicode (urxvt) v9.20 - released: 2014-04-26

When I switch to xterm (310) then I get the delay too. And the scrolling of the Xs flickers a lot more.

stty on xterm is a bit different:

Code:

% stty
speed 38400 baud; line = 0;
-brkint -imaxbel iutf8

But unsetting -brkint and -imaxbel didn't help.

NevemTeve · 09-14-2014, 12:32 PM

@OP: It would be a good idea if you told what did you really wanted to accomplish with this program.

NeoLux · 09-14-2014, 10:35 PM

NevemTeve: i will try it with your read function again once i get the chance God willing. As for the purpose, it was just a test program and this post was to find out why it didnt work the way i expected it to.

ntubski: that is very funny i actually kept thinking "what if hes using a different terminal like urxvt ?"
your post is very helpful, at least i have a chance to find out what might be the cause in difference of behaviour.