while getch question

Hi,

I'm trying to write a while loop that is able to recognise and act on keyboard presses without pausing to wait for an input (unlike getchar()).

At the moment I'm just trying to detect any keypress and exit the loop by changing an integer, printing out the recorded keypress on the way out. Later I'd like to recognise the keypress as part of conditional statements and keep looping.

Here is the code that I've got so far, it has been modified from an example. Currently the loop just keeps running indefinately, regardless of what I press.

Some extra information that may be important: this code is a small addition to a large working example (which I am modifying) that runs on a remote embedded PC, I've omitted about 450 lines (including lots of headers) for this snippet. Also, it may be useful to know that I'm accessing the PC via Putty , a telnet / SSH client.

Code:

---

#include <stdio.h>
#include <err.h>       
#include <curses.h> /* for ERR */

int main(){

  char chr;        /* Keyboard character */
  int done = 0;        /* while condition */
 
  while (done != 1){
          printf(". ");    /* show that we are looping */
          usleep(5000); /* wait a moment */
          if ((chr = getch()) != ERR) done = 1; /* if anything has been pressed leave the loop*/
  }

  printf(" chr = %c\n", chr);  /* show which key was pressed */
}

---

Thanks for reading, any help appreciated,

Ad

Are you sure ERR is what you need?

Are you in no-delay mode? What happens if you press enter?

Hi,

The while loop keeps printing '. ' so it isn't blocking. I assume that means no-delay is true (though I haven't set this explicitly I believe that it is the default).

If i press enter I get a line break in the output (i.e. .........[\n]......) but the loop doesn't exit. It's the same with any character, it appears in the console but doesn't affect the loop behaviour.

Thanks,

Ad

Hi,

I used ERR as this seemed to have been used in the code I've been using as an example. I believed that the statement would return ERR if nothing had been pressed. Is this not a good choice?

Btw, I have tried the following without success, the loop still doesn't exit.

Quote:

---

if ((chr = getch()) == 'a') done = 1;

---

Thanks,

Ad

If your char's are unsigned, and ERR is negative (as I suspect it is), then your comparison will never be true. What if you make chr an int instead of a char?

You wait a total of 5 seconds. Your console maynot catch the keyboard hit.

Thanks for the suggestions.

tabstop I've made the chr an integer but I still get the same response.

Sipher I've reduced usleep to 50 and also taken it out completely but the same response continues. Also, I think usleep(5000) is a wait of 50ms (that seems to be how often the dots appear).

Ad

Oh, sorry. I confused usleep with Sleep... :D

Btw, I just checked and ERR is '-1'.

I put this in my while loop

Quote:

---

printf("\n. %d",getch());

---

But even if I hold down a key and make usleep very small, getch = '-1'. Interesting... it seems that my keystrokes aren't being detected by getch?

Could this be something to do with my remote interface? It seems to work on the demo code that shipped with the system, and that used getch for the input.

Aargh, but getchar() detects keys fine! So it's not the remote interface...

You are not using [n]curses correctly.

NCURSES Programming HOWTO

gg

Quote:

---

Originally Posted by Ad_robot

Btw, I just checked and ERR is '-1'.

I put this in my while loop


But even if I hold down a key and make usleep very small, getch = '-1'. Interesting... it seems that my keystrokes aren't being detected by getch?

Could this be something to do with my remote interface? It seems to work on the demo code that shipped with the system, and that used getch for the input.

---

getch may well be polling the keyboard at the other end and not accepting the keys from PuTTY. (I don't remember, if I ever knew, how those two interrelate.) The easy way to check would be to run this on your box and see what happens (or send someone over there to push buttons on that machine, if possible).

tabstop,

Unfortunately the other system doesn't have a keyboard, mouse or display, it's an embedded platform connected to some sensors and actuators.

The strange thing is that getchar() does detect the keypresses from my system, so one would assume that getch() would work the same. It seems not.

I didn't mention that the remote system is running Linux and I'm running Windows. This shouldn't be a problem thought should it?

Back to reading the ncurses guide...

Your code needs some curses requirements and you need to set the mode to cbreak. Here's my version with a few comments:

Code:

---

#include <stdio.h>
#include <unistd.h> // for usleep
#include <err.h>
#include <curses.h>

int main(void)
{
    char chr;

    initscr();  // mandatory curses init
    cbreak();  // unbuffered input
    do {
        chr = getch();
    } while (chr == ERR);

    printw(" chr = %02x\n", chr);
    refresh();  // show the printf stuff
    getch();    // wait for user input

    endwin();  // curses cleanup

    return 0;
}

---

EDIT: I should have used curses' printw function instead of printf.

Thanks anduril, that example certainly works, though curses seems to make the rest of my program behave quite strangely. As I mentioned this is a modification to a large, exisiting bit of code.

I have put your curses example at the begining of my main loop (after removing the 'return 0', of course). I was hoping that afer exiting the while loop and calling endwin(), the rest of my code could run as before, using 'printf' to display strings to the user. However, it seems that I can only do this now by calling refresh, and even then this doesn't always work if a time delay is involved in generating the string (when polling a sensor for example). Worst of all is that my program now hangs occasionally and doesn't respond to Ctrl+C.

So now I have two questions,

1. Is it possible to use curses.h and getch for a while loop, then 'switch it off', so that I can go back to using printf? endwin() doesn't seem to do this...

2. Is there another way to achieve the goal I want (of entering keystrokes while running a while loop) that doesn't mean changing the way I display data on the screen (refreshing doesn't seem to work with time delays). Perhaps something other than getch and curses.h.

Many Thanks,

Ad

Basic unbuffered input:

Code:

---

#include <termios.h>
#include <unistd.h>
#include <sys/select.h>

int my_getch()
{
    termios oldt;
    tcgetattr(STDIN_FILENO, &oldt);
   
    termios newt = oldt;
    newt.c_lflag &= ~(ICANON | ECHO);
    newt.c_cc[VMIN] = 1;
    newt.c_cc[VTIME] = 0;

    tcsetattr(STDIN_FILENO, TCSANOW, &newt);

    unsigned char c;
    ssize_t nread = read(STDIN_FILENO, &c, 1);
   
    tcsetattr(STDIN_FILENO, TCSANOW, &oldt);

    return c;
}

---

gg

Quote:

---

Originally Posted by Ad_robot

Thanks anduril, that example certainly works, though curses seems to make the rest of my program behave quite strangely. As I mentioned this is a modification to a large, exisiting bit of code.

I have put your curses example at the begining of my main loop (after removing the 'return 0', of course). I was hoping that afer exiting the while loop and calling endwin(), the rest of my code could run as before, using 'printf' to display strings to the user. However, it seems that I can only do this now by calling refresh, and even then this doesn't always work if a time delay is involved in generating the string (when polling a sensor for example). Worst of all is that my program now hangs occasionally and doesn't respond to Ctrl+C.

---

Oh, the joys of raw, unbuffered I/O! One of the things that goes along with unbuffered input is that you often get to handle control characters, backspacing, etc yourself.

Quote:

---

1. Is it possible to use curses.h and getch for a while loop, then 'switch it off', so that I can go back to using printf? endwin() doesn't seem to do this...

---

You can use printf in curses. I'm not sure, but I think it's less desirable because it doesn't behave well in regards to curses screens, windows, panes, etc. Also, I'm not sure you can just "turn curses on then back off" for this, because I think curses uses the alternate screen, so it does funky stuff with the display when it goes back to the primary screen.

Quote:

---

2. Is there another way to achieve the goal I want (of entering keystrokes while running a while loop) that doesn't mean changing the way I display data on the screen (refreshing doesn't seem to work with time delays). Perhaps something other than getch and curses.h.

---

If you just wan't unbuffered input (don't wait for a newline to process input), look into the setvbuf function. I've never used it before, so I'm not sure how successful you'll be, but it may do what you need.

>> If you just wan't unbuffered input (don't wait for a newline to process input), look into the setvbuf function.
setvbuf() won't help. Data from the OS to the CRT will still only be available once an entire line is available. setvbuf() only controls buffering at the CRT level.
http://pubs.opengroup.org/onlinepubs...l#tag_11_01_06

gg

Thanks for your help guys. Unfortunately I couldn't get my_getch() working today (some issues with termios.h on the remote platform) which was a shame as it is a nice solution. I'm not back at work until the new year so am not going to find the problem until then. When I start writing my own code for this thing I'll probably end up using curses.h but for now my_getch is a cool solution.

One thing is for sure, this isn't as simple as I expected it to be!

Thanks again,

Ad