line buffered stdin

line buffered stdin

[Bastian Ruppert]

Hello,

under Linux stdin is line buffered, that means you can read one byte with
following statement:

read(0,&myvar,1);

You get the byte after typing one or more chars AND return.

But i need this function to return after every char entered in stdin.

Is there a function that reads stdin without line buffering?

Is it possible to disable the linebuffer or to reduce the buffersize to one?
Does this kind of manupilation have consequences for the hole system or 
just for
the app
doing this?

Any Ideas are very welcome,
thank you.

Regards
Bastian.

Re: line buffered stdin

[Manish Katiyar]

On Mon, Jan 11, 2010 at 9:57 PM, Bastian Ruppert <ng@max01.eu> wrote:
> Hello,
>
> under Linux stdin is line buffered, that means you can read one byte with
> following statement:
>
> read(0,&myvar,1);
>
> You get the byte after typing one or more chars AND return.
>
> But i need this function to return after every char entered in stdin.
>
> Is there a function that reads stdin without line buffering?
>
> Is it possible to disable the linebuffer or to reduce the buffersize to one?
First two links in google search.

http://www.linux.org/docs/ldp/howto/NCURSES-Programming-HOWTO/init.html
http://www.flipcode.com/archives/_kbhit_for_Linux.shtml
> Does this kind of manupilation have consequences for the hole system or just
> for
> the app
> doing this?
>
> Any Ideas are very welcome,
> thank you.
>
> Regards
> Bastian.
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Re: line buffered stdin

[ern0]

> Is there a function that reads stdin without line buffering?

You should set some terminal attr:

   termios oldt;
   [...]
   tcgetattr(STDIN_FILENO,&oldt);
   termios newt = oldt;
   newt.c_lflag &= (~ICANON & ~ECHO);
   tcsetattr(STDIN_FILENO,TCSANOW,&newt);

and restore the old state when you're done:

   tcsetattr(STDIN_FILENO,TCSANOW,&oldt);

If you interrupt your program, you will see no echo, which is annoying. 
Type - blind mode - at the shell prompt: "reset" (don't worry, it only 
resets the terminal).

Gnome-terminal also clears the scrollback buffer upon "reset".
-- 
ern0.scene.plus4.amiga.code.muzak
Haben Sie Fragen?

Re: line buffered stdin

[Glynn Clements]

Bastian Ruppert wrote:

> under Linux stdin is line buffered,

More precisely, it's line buffered if stdin is a tty, fully-buffered
otherwise.

> that means you can read one byte with
> following statement:
> 
> read(0,&myvar,1);
> 
> You get the byte after typing one or more chars AND return.
> 
> But i need this function to return after every char entered in stdin.
> 
> Is there a function that reads stdin without line buffering?

User-space buffering of a stream (i.e. a "FILE *") can be controlled
with setvbuf() (or the other functions described in the setvbuf(3)
manpage).

However, the terminal driver also performs line-buffering by default
(so that you can edit the current line with Backspace, Ctrl-W,
Ctrl-U). This can be disabled with tcsetattr() (see ern0's reply for
details), although this also disables the use of Ctrl-D to generate
EOF.

> Is it possible to disable the linebuffer or to reduce the buffersize to one?
> Does this kind of manupilation have consequences for the hole system or 
> just for the app doing this?

setvbuf() only applies to a specific process. tcsetattr() affects the
terminal device, so you must restore the state upon termination (both
normal termination via exit() or return from main(), and abnormal
termination via a signal), upon suspension (i.e. SIGTSTP; you can't
catch SIGSTOP), or if you invoke a child process (via e.g. system())
which might need to use the terminal.

-- 
Glynn Clements <glynn@gclements.plus.com>