How to break a while loop in C on linux by taking any value from keyboard?

Question

The program I am building runs in an infinite while loop with a switch case in it. I want to insert a while loop under each case and perform few operations in the loop, but the loop should exit as soon as a keyboard input is given. So that after taking the input from keyboard another case is run with the nested while loop in it, and the process continues.

The structure is:

while()//infinite loop
    {
    ...............      //operations
    ...............      //operations
    switch()
        {
        case 1:
        ...............    //operations
        ...............    //operations
        while()//infinite loop
             {
             ..............
             ..............
             exit if there is any input from keyboard
             }
        break;

        case 2:
        ...............    //operations
        ...............    //operations
        while()//infinite loop
             {
             ..............
             ..............
             exit if there is any input from keyboard
             }
        break;


        case n:
        ...............    //operations
        ...............    //operations
        while()//infinite loop
             {
             ..............
             ..............
             exit if there is any input from keyboard
             }
        break;
        }
  }

Is there any way to do it???

Are you asking how to get keyboard input, or how to break nested loops? — ugoren, Jun 05 '13 at 10:24
Are you asking how to do raw input, so that waiting ends as soon as _any_ key that produces a char or control code is pressed, as opposed to being forced to press enter (after optionally inputting some text)? — blubberdiblub, Jun 05 '13 at 10:29
Regardless of the solution to the keyboard problem, this function should be refactored. — unwind, Jun 05 '13 at 10:30
I need to know how to break the nested loop with a keyboard input, so that, that input is fed to the case again as the outer while loop is run. What I am planning to do is, when a key is pressed and the nested while loop breaks, that key is written into a file. Then this key is taken as an input from the file, for the switch when the outer while loop runs again. — Nitish Sahay, Jun 05 '13 at 10:33
Indeed, waiting could be done after the switch, once for all cases, instead of code duplication. — blubberdiblub, Jun 05 '13 at 10:35
You could use the ncurses library and its `getch()` function. There are a few examples available on Stack Overflow. — , Jun 05 '13 at 10:45
Have You considered using threads ? One thread monitors keyboard, second is main loop ? Also You might try using signals, and for example wrap ctrl-c signal, which will set and clear flag in all loops ? — JosiP, Jun 05 '13 at 10:58
On Linux, learning about [poll(2)](http://man7.org/linux/man-pages/man2/poll.2.html), perhaps [ncurses](http://www.gnu.org/software/ncurses/) or [readline](http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html) libraries could help you significantly. You still need to master the C programming language. There are good books for that. — Basile Starynkevitch, Jun 05 '13 at 17:40

Déjà vu · Answer 1 · 2016-07-05T12:11:13.390

Linux keyboard input is buffered, in order to catch a key that was hit on the fly you have to configure TERM IO.

Near the top of main() add a call to (see code below)

term_nonblocking();

to read a key that was pressed on the fly, without waiting for a carriage return (CR).

Code:

struct termios stdin_orig;  // Structure to save parameters

void term_reset() {
        tcsetattr(STDIN_FILENO,TCSANOW,&stdin_orig);
        tcsetattr(STDIN_FILENO,TCSAFLUSH,&stdin_orig);
}

void term_nonblocking() {
        struct termios newt;
        tcgetattr(STDIN_FILENO, &stdin_orig);
        fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK); // non-blocking
        newt = stdin_orig;
        newt.c_lflag &= ~(ICANON | ECHO);
        tcsetattr(STDIN_FILENO, TCSANOW, &newt);

        atexit(term_reset);
}

Note: term_reset() will be called automatically when your program exits, (to reset the terminal parameters).

You can call the now non-blocking getchar() anywhere in your program to detect a key press

int i = getchar();

and check if a key was pressed:

if (i > 0) {
    // key was pressed, code in `i`
}

In your program for instance:

int key = 0;

while (... && key <= 0) {
   // ... 
   key = getchar();
}

Note: if you want the output to be unbuffered, call setbuf(stdout, NULL);

(comment from @stacey : getchar() may return 0 or -1 when no key is available)

You might want to `ungetc(i, stdin)` if you read a character so that it can be processed again by the outer loop. — Jonathan Leffler, Jun 05 '13 at 14:11
@JonathanLeffler Thanks. This is valuable information indeed. — Déjà vu, Jun 05 '13 at 15:07
Note that getchar() returns -1 or 0 if a key hasn't been pressed. I used `(key <= 0)` in my while condition to get around this. As it stands, the while loop ends immediately if nothing is pressed. — stanri, Jul 05 '16 at 07:17
@stacey thanks, edited the answer as it may help other people. — Déjà vu, Jul 05 '16 at 12:15

score 0 · Accepted Answer · answered Jun 06 '13 at 07:48

Thank you people.... even though my answer is a little out of box, I think ring0's approach is much better. Here is what I did.

Added code inside the outer while loop which allows the compiler to take argument for switch case from a file, say, data.dat
Added similar code in each of the nested while loops to continue if the variable retrieved from the data.dat is what is required to run the loop, if not, break.
Since in linux I can run processes in the back ground, I ran the main program in the background, and edited data.dat accordingly.

This method is working for me but will definitely try ring0's method too.

score 0 · Answer 3 · edited May 23 '17 at 11:54

I had a similar desire for an old system that I have to maintain. I wanted to add a "quit" as well as provide some timing. I stripped out all proprietary code and left the shell of what I learned from this site. Thank you.

I ended up using the above example by ring0. I wanted to comment but I'm not at that level yet.

I also took some information from here by Ulf and Walter

I could not use fcntl because of a global variable that was used in our code that conflicted with something within fcntl and I did not want to figure out how to avoid that conflict.

Anyway, here is an example. It worked for me on Debian (older version). Your mileage may vary.

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

#define SLEEP  500000. /*usec*/

#include <termios.h>
struct termios stdin_orig;

void term_reset() {
  tcsetattr(STDIN_FILENO, TCSANOW, &stdin_orig);
  tcsetattr(STDIN_FILENO, TCSAFLUSH, &stdin_orig);
}

void term_nonblocking() { 
  struct termios term;
  tcgetattr(STDIN_FILENO, &stdin_orig);
  term = stdin_orig;
  term.c_iflag |= IGNBRK;
  //term.c_iflag &= ~(INLCR | ICRNL | IXON | IXOFF);
  term.c_lflag &= ~(ICANON);// | ECHO | ECHOK | ECHOE | ECHONL | ISIG | IEXTEN);*/
  term.c_cc[VMIN] = 0;
  term.c_cc[VTIME] = 0;
  /*   fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK); // non-blocking */
  tcsetattr(STDIN_FILENO, TCSANOW, &term);
} 
// end mikes timing

int main (void)
{
  int key=0;
  // mikes test variables
  long mnloops = 0;
  double mttot = 0.0;//total time elapsed in difference between start-end.
  term_nonblocking();
  // end mikes test variables

  /*main loop*/
  do{
    //mikes timing
    puts("Start Do");
    time_t mnow;
    struct timeval mtv;
    struct timezone mtz;
    time(&mnow);
    gettimeofday(&mtv, &mtz);
    double startTime = (double)mnow + mtv.tv_usec/1e6;
    // end mikes timing
    usleep(SLEEP);
    puts("got hey");
    //mikes timing
    time(&mnow);
    gettimeofday(&mtv, &mtz);
    mttot += (((double)mnow + mtv.tv_usec/1e6) - startTime);
    mnloops++;
    key = getchar();
    // end mikes timing
  }while (1 && key != 'q');/* end of main loop*/
  term_reset();
  printf("DONE: average time per loop = %-10.4e\n",mttot/mnloops);
  return 0;
}

How to break a while loop in C on linux by taking any value from keyboard?

3 Answers3