Why is the input displayed after the enter key has been pressed?

Question

#include <stdio.h>

/* copy input to output */

main()
{
    int c;

    c = getchar();

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

    return 0;
}

In the given code, the program displays the input character. It reads every character one by one (into the variable 'c') and outputs the same read characters simultaneously. The program terminates when the EOF character is given as input.

When I ran the code in my IDE (Code::Blocks 16.01) and input a string, eg: Hi! My name is C.\n

The output is displayed after '\n' and not simultaneously. Isn't the output supposed to be as - "HHii!! MMyy nnaammee iiss CC.."?

Bold letters indicate the output.

After putchar() returns, the character are held in an internal buffer until the program exits/completes. — ManmeetP, Nov 16 '17 at 06:27
`#include int main (void) { int c; while ( (c = getchar ()) != EOF ) putchar (c); return 0; }` — ManmeetP, Nov 16 '17 at 06:34
Can you once try above program structure? and share output please. — ManmeetP, Nov 16 '17 at 06:34
@ManmeetP I did. Isn't it just a compressed version of the same code? And the output is still the same. It takes the whole input into an internal buffer and then prints it after the newline('\n') character. — Mohammed Arshaan, Nov 16 '17 at 06:39
@AA Where exactly should I write the code `fflush(stdout);` ? — Mohammed Arshaan, Nov 16 '17 at 06:39
@ManmeetP `stdout` not `stdin`. Here more https://stackoverflow.com/a/12450125/8918119 — Mihayl, Nov 16 '17 at 06:44
Right, as I suggested, _After putchar() returns, the character are held in an internal buffer until the program exits/completes_, fflush() is the method for it. USE: putchar(c); fflush(stdout); — ManmeetP, Nov 16 '17 at 06:48
See [Canonical vs non-canonical terminal input](https://stackoverflow.com/questions/358342). — Jonathan Leffler, Nov 16 '17 at 08:13

score 0 · Answer 1 · answered Nov 16 '17 at 10:35

include:

#include <unistd.h>
#include <termios.h>

before your code in main insert:

struct termios term, term_orig;
if(tcgetattr(STDIN_FILENO, &term_orig)) {
        printf("tcgetattr failed\n");
        return -1;
    }
term = term_orig;
term.c_lflag &= ~ICANON;
if (tcsetattr(STDIN_FILENO, TCSANOW, &term)) {
    printf("tcsetattr failed\n");
    return -1;

}

after your code insert:

if (tcsetattr(STDIN_FILENO, TCSANOW, &term_orig)) {
    printf("tcsetattr failed\n");
   return -1;
}

score 0 · Answer 2 · answered Nov 16 '17 at 11:39

Because by default, input from terminal is line-buffered. So your program get a whole line after you press Enter.

To disable buffering in Unix & Linux systems, try this:

#include <unistd.h>
#include <termios.h>

int disableLineBuffer(void){
    struct termios term, oldterm;
    if (tcgetattr(STDIN_FILENO, &term)) return -1;
    oldterm = term;
    term.c_lflag &= ~(ECHO | ICANON);
    if (tcsetattr(STDIN_FILENO, TCSANOW, &term)) return -1;
    return 0;
}

In Windows, go and do this instead:

#include <windows.h>

BOOL WINAPI DisableLineBuffer(void){
    HANDLE hInput = GetStdHandle(STD_INPUT_HANDLE);
    DWORD mode;
    if (!GetConsoleMode(hInput, &mode)) return FALSE;
    mode &= ~ENABLE_LINE_INPUT;
    if (!SetConsoleMode(hInput, mode)) return FALSE;
    return TRUE;
}

Be sure to revert changes to the console before your program exits, or you may have trouble doing anything else in your terminal.

Why is the input displayed after the enter key has been pressed?

2 Answers2