setvbuf not able to make stdin unbuffered

Question

My main intention was to make getchar return as soon as it gets a character instead of waiting for the ENTER key. I tried this

int main()
{
    setvbuf(stdin,NULL,_IONBF,0);
    getchar();

    return 0;
}

Comparing this with the prototype of setvbuf

setvbuf ( FILE * stream, char * buffer, int mode, size_t size );

it should set stdin to unbuffered mode.

But still getchar() keeps waiting for ENTER

I've seen related posts like this

Printing while reading characters in C

which are suggesting alternate methods to make stdin unbuffered. But I am curious to know as to why setvbuf method does not work

Neologism

You must call setvbuf() before any "movement" on the stream ... so first thing in main(). – Dandy

@Dandy I updated my post to reflect what I tried. But still it doesn't work – Neologism

I think the problem is not with setvbuf(). I tried your program with and without setvbuf() and the behaviour is different. Without setvbuf() all characters up to and including the ENTER are consumed (even though it is consumed only after typing ENTER); with setvbuf() only the first character is consumed, the remaining characters are used as a following bash command. – Dandy

Answer 1

The terminal driver doesn't return anything until you hit return, even if the read() operation would accept what's already there.

To get character-by-character input from a terminal, you have to get it out of canonical mode into raw or cbreak mode, and that requires different operations altogether. Take a look at the POSIX manual on 'General Terminal Interface' for how to control the terminal. Or consider using the curses library.

See also: Canonical vs non-canonical terminal input

Hols

setvbuf(stdin,NULL,_IONBF,0); is supposed to do that right? I mean setvbuf works fine for all file pointers but not for stdin? – Neologism

No. setvbuf() just tells the standard I/O library in your program to use no buffer; it doesn't tell the terminal driver anything. – Hols

Answer 2

In case you are trying this under Linux or another Unix-like system, it is the terminal that buffers the input and only passes an entire line. You can use ncurses to circumvent this:

#include <ncurses.h>

int main()
{
    initscr();
    getch();
    endwin();

    return 0;
}

Compile with:

gcc -o main main.c -lncurses

Allhallowtide

Thanks for the reply. I saw these alternate methods, but I was curious as to why setvbuf doesn't work – Neologism