Taking Input in c program without
pressing enter
IIT Bombay
Problem Statement
While giving input to c or c++ programs, enter/return key has to be
pressed. Some programs require keyboard recognition and ability to give
input without pressing enter/return key.
Proposed Solutions
The operating system used is Ubuntu 12.041 and IDE used is Code::Blocks2 .
The programs written and tested are console application projects. Link for
Code::Block manual is given in reference [1]. Below we have discussed some
of the method to take input in c/c++ programs without pressing enter/return key.
1. Modifying terminal behaviour using stty command
In this method, stty is used to modify terminal behaviour. Terminal
buffers all information until enter/return key is pressed. When the
enter/return key is pressed the buffered information is sent to the c
program. This buffering can be suppressed using ssty and input will
be sent to c program as soon as key is pressed. The code used is [2]
1 #include<stdio.h>
2
3 int main(void){
4
int c;
5
/* use system call to make terminal send all keystrokes
directly to stdin */
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system ("/bin/stty raw");
1
TM
R
Ubuntu 12.04 with intel Core
i3-2120 CPU @ 3.30GHzx4 processor 4 GB RAM
and 32-bit architecture and 64 bit architecture.
2
Version:10.05
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14 }
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;
Listing 1: Using stty command
This program does the check for users default terminal behaviour
and assumes after program exits the behaviour expected is ‘cooked’ [3].
In raw mode many key sequence like CTRL-C or CTRL-D won’t work
with explicitely processing them in the program. Further information
on stty can be obtained using ‘man stty’
2. Disabling ICANON flag
In unix like environment ICANON flag is enabled by default. ICANON
flag enables input to be buffered until ‘\n’ or EOF os encountered.
Thus enter/return has to be pressed to give input to programs. By
disabling the canonical mode [4] characters are taken as soon as some
key is pressed.
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#include<stdio.h>
#include <termios.h>
#include <unistd.h>
//termios, TCSANOW, ECHO, ICANON
//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);
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29 }
/* 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;
Listing 2: Disabling ICANON flag
When above code is executed the output is echoed as soon as some
key is pressed. Echo can be suppressed by disabling echo flag. Echo
flag can be disabled by changing line 15 in listing 2 as given in listing 3
1
newt.c_lflag &= ~(ICANON | ECHO);
Listing 3: Disabling echo
References
[1] IIT Bombay. Code::block manual
http://www.it.iitb.ac.in/frg/wiki/images/e/e8/
CodeBlockManual.pdf.
[2] Stack Overflow. c - how to avoid press enter with any getchar() - stack
overflow
http://stackoverflow.com/questions/1798511/
how-to-avoid-press-enter-with-any-getchar.
[3] Wikipedia. Posix terminal interface - wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/POSIX_terminal_interface.
[4] W. Richard Stevens and Stephen A. Rago. Advanced Programming in
the UNIXr Environment
http: // infohost. nmt. edu/ ~ eweiss/ 222_ book/ 222_ book/
0201433079/ ch18lev1sec10. html .
Addition Wesley Professional,
second edition, June 17 2005.
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