setvbuf not able to make stdin unbuffered

c buffer stdin getchar unbuffered
Pavan Manjunath picture Pavan Manjunath · Apr 20, 2012 · Viewed 7.3k times · Source

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

Answer

Jonathan Leffler picture Jonathan Leffler · Apr 20, 2012

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

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