How to avoid pressing enter with getchar()

Question

In the next code:

#include <stdio.h>

int main(void) {   
  int c;   
  while ((c=getchar())!= EOF)      
    putchar(c); 
  return 0;
}

I have to press Enter to print all the letters I entered with getchar, but I don't want to do this, what I want to do is to press the letter and immediately see the the letter I introduced repeated without pressing Enter. For example, if I press the letter 'a' I want to see an other 'a' next to it, and so on:

aabbccddeeff.....

But when I press 'a' nothing happens, I can write other letters and the copy appears only when I press Enter:

abcdef
abcdef

How can I do this?

I am using the command cc -o example example.c under Ubuntu for compiling.

Answer 1

On a linux system, you can modify terminal behaviour using the stty command. By default, the terminal will buffer all information until Enter is pressed, before even sending it to the C program.

A quick, dirty, and not-particularly-portable example to change the behaviour from within the program itself:

#include<stdio.h>

int main(void){
  int c;
  /* use system call to make terminal send all keystrokes directly to stdin */
  system ("/bin/stty raw");
  while((c=getchar())!= '.') {
    /* type a period to break out of the loop, since CTRL-D won't work raw */
    putchar(c);
  }
  /* use system call to set terminal behaviour to more normal behaviour */
  system ("/bin/stty cooked");
  return 0;
}

Please note that this isn't really optimal, since it just sort of assumes that stty cooked is the behaviour you want when the program exits, rather than checking what the original terminal settings were. Also, since all special processing is skipped in raw mode, many key sequences (such as CTRL-C or CTRL-D) won't actually work as you expect them to without explicitly processing them in the program.

You can man stty for more control over the terminal behaviour, depending exactly on what you want to achieve.

Answer 2

This depends on your OS, if you are in a UNIX like environment the ICANON flag is enabled by default, so input is buffered until the next '\n' or EOF. By disabling the canonical mode you will get the characters immediately. This is also possible on other platforms, but there is no straight forward cross-platform solution.

EDIT: I see you specified that you use Ubuntu. I just posted something similar yesterday, but be aware that this will disable many default behaviors of your terminal.

#include<stdio.h>
#include <termios.h>            //termios, TCSANOW, ECHO, ICANON
#include <unistd.h>     //STDIN_FILENO


int main(void){   
    int c;   
    static struct termios oldt, newt;

    /*tcgetattr gets the parameters of the current terminal
    STDIN_FILENO will tell tcgetattr that it should write the settings
    of stdin to oldt*/
    tcgetattr( STDIN_FILENO, &oldt);
    /*now the settings will be copied*/
    newt = oldt;

    /*ICANON normally takes care that one line at a time will be processed
    that means it will return if it sees a "\n" or an EOF or an EOL*/
    newt.c_lflag &= ~(ICANON);          

    /*Those new settings will be set to STDIN
    TCSANOW tells tcsetattr to change attributes immediately. */
    tcsetattr( STDIN_FILENO, TCSANOW, &newt);

    /*This is your part:
    I choose 'e' to end input. Notice that EOF is also turned off
    in the non-canonical mode*/
    while((c=getchar())!= 'e')      
        putchar(c);                 

    /*restore the old settings*/
    tcsetattr( STDIN_FILENO, TCSANOW, &oldt);


    return 0;
}

You will notice, that every character appears twice. This is because the input is immediately echoed back to the terminal and then your program puts it back with putchar() too. If you want to disassociate the input from the output, you also have to turn of the ECHO flag. You can do this by simply changing the appropriate line to:

newt.c_lflag &= ~(ICANON | ECHO); 

Answer 3

getchar() is a standard function that on many platforms requires you to press ENTER to get the input, because the platform buffers input until that key is pressed. Many compilers/platforms support the non-standard getch() that does not care about ENTER (bypasses platform buffering, treats ENTER like just another key).

Answer 4

I/O is an operating system function. In many cases, the operating system won't pass typed character to a program until ENTER is pressed. This allows the user to modify the input (such as backspacing and retyping) before sending it to the program. For most purposes, this works well, presents a consistent interface to the user, and relieves the program from having to deal with this. In some cases, it's desirable for a program to get characters from keys as they are pressed.

The C library itself deals with files, and doesn't concern itself with how data gets into the input file. Therefore, there's no way in the language itself to get keys as they are pressed; instead, this is platform-specific. Since you haven't specified OS or compiler, we can't look it up for you.

Also, the standard output is normally buffered for efficiency. This is done by the C libraries, and so there is a C solution, which is to fflush(stdout); after each character written. After that, whether the characters are displayed immediately is up to the operating system, but all the OSes I'm familiar with will display the output immediately, so that's not normally a problem.

Answer 5

Since you are working on a Unix derivative (Ubuntu), here is one way to do it - not recommended, but it will work (as long as you can type commands accurately):

echo "stty -g $(stty -g)" > restore-sanity
stty cbreak
./your_program

Use interrupt to stop the program when you are bored with it.

sh restore-sanity

• The 'echo' line saves the current terminal settings as a shell script that will restore them.
• The 'stty' line turns off most of the special processing (so Control-D has no effect, for example) and sends characters to the program as soon as they are available. It means you cannot edit your typing any more.
• The 'sh' line reinstates your original terminal settings.

You can economize if 'stty sane' restores your settings sufficiently accurately for your purposes. The format of '-g' is not portable across versions of 'stty' (so what is generated on Solaris 10 won't work on Linux, or vice versa), but the concept works everywhere. The 'stty sane' option is not universally available, AFAIK (but is on Linux).

Answer 6

I like Lucas answer, but I would like to elaborate it a bit. There is a built-in function in termios.h named cfmakeraw() which man describes as:

cfmakeraw() sets the terminal to something like the "raw" mode of the
old Version 7 terminal driver: input is available character by
character, echoing is disabled, and all special processing of
terminal input and output characters is disabled. [...]

This basically does the same as what Lucas suggested and more, you can see the exact flags it sets in the man pages: termios(3).

Use case

int c = 0;
static struct termios oldTermios, newTermios;

tcgetattr(STDIN_FILENO, &oldTermios);
newTermios = oldTermios;

cfmakeraw(&newTermios);

tcsetattr(STDIN_FILENO, TCSANOW, &newTermios);
c = getchar();
tcsetattr(STDIN_FILENO, TCSANOW, &oldTermios);

switch (c) {
    case 113: // q
        printf("\n\n");
        exit(0);
        break;
    case 105: // i
        printf("insert\n");
        break;
    default:
        break;

Answer 7

You could include the 'ncurses' library, and use getch() instead of getchar().

Answer 8

yes you can do this on windows too, here's the code below, using the conio.h library

#include <iostream> //basic input/output
#include <conio.h>  //provides non standard getch() function
using namespace std;

int main()
{  
  cout << "Password: ";  
  string pass;
  while(true)
  {
             char ch = getch();    

             if(ch=='\r'){  //when a carriage return is found [enter] key
             cout << endl << "Your password is: " << pass <<endl; 
             break;
             }

             pass+=ch;             
             cout << "*";
             }
  getch();
  return 0;
}

Answer 9

I've had this problem/question come up in an assignment that I'm currently working on. It also depends on which input you are grabbing from. I am using

/dev/tty

to get input while the program is running, so that needs to be the filestream associated with the command.

On the ubuntu machine I have to test/target, it required more than just

system( "stty -raw" );

or

system( "stty -icanon" );

I had to add the --file flag, as well as path to the command, like so:

system( "/bin/stty --file=/dev/tty -icanon" );

Everything is copacetic now.

Answer 10

By default, the C library buffers the output until it sees a return. To print out the results immediately, use fflush:

while((c=getchar())!= EOF)      
{
    putchar(c);
    fflush(stdout);
}