Using the system("clear") command in Vista

**maverick95** · 08-13-2008

i am a C newbie attempting to clear the screen in a program. When I wrote a quick test program,

Code:

 
#include <stdio.h>
#include <stdlib.h>

main()

{

system("clear");

}

it compiled perfectly, but when I ran it gave the error message "sh: clear: command not found". I also tried the same but replacing "clear" with "cls", but received a similar error message. Can anyone help with this?

**matsp** · 08-13-2008

What compiler are you using? "sh: " seems like you are using Cygwin, which simulates a unix environment.

--
Mats

**maverick95** · 08-13-2008

yes i'm using cygwin

**whiteflags** · 08-13-2008

You really ought to look around for help with the problem you are having with cygwin, maybe someone in the cygwin community has had this problem before you. The web would know. Perhaps do it in a portable way in the meantime?

Code:

void doclear (void)
{
  int i;
  for (i = 0; i < 1200; i++) {
    putchar('\n');
  }
  fflush(stdout);
}

**maverick95** · 08-13-2008

thanks citizen, 'til i get it sorted online that works fine. looks a bit scrappy when it runs but it'll do for now

**dwks** · 08-13-2008

Why 1200? The default number of lines of history in Vista is 500, I'm pretty sure. (Though I set mine to 1000.)

You could use the ASCII escape sequence to clear the screen. I think this is it. http://www.cygwin.com/ml/cygwin/1999-08/msg00285.html
It should work on Vista, I would think.

**whiteflags** · 08-13-2008

>> Why 1200?

I didn't think about it that hard, I just wanted to reach the bottom of most monitors, and 1200 is the tallest height in pixels that came to mind.

**dwks** · 08-13-2008

. . . but we're talking lines here. A line is usually at least 8 pixels high.

If you want to handle standard terminals, print 25 newlines. That should work on most DOS and Linux terminals. Unless the user has made their terminal larger, of course, but then it's their problem.

Personally, I dislike programs that clear the screen. The history is there for a reason -- it serves to show what you've typed in the past! If you have sensitive information, like a password, try to figure out an alternative way to display it, such as not echoing the characters -- which also happens to be non-standard.

**whiteflags** · 08-13-2008

By the way op, I did find something else. Looks like you need to install a library called ncurses.

**King Mir** · 08-13-2008

Originally Posted by dwks

If you want to handle standard terminals, print 25 newlines.

Or 50 for full screen mode.

**dwks** · 08-13-2008

But that depends on the user's screen resolution, and we've come in a circle.

Why not just install ncurses? . . . .

BTW, you might want to use something like this for semi-better portability:

Code:

    system(
#ifdef WIN32
        "cls"
#else
        "clear"
#endif
        );

At least that way the program will still work if you compile it with Dev-C++ . . . .

**King Mir** · 08-13-2008

It doesn't depend on the resolution, as the width is still 80 characters. It does however depend on the font.

**dwks** · 08-13-2008

I think you're confused. The code is printing a number of newlines. The width of the screen does not matter a bit -- a newline scrolls down a line, no matter how wide that line is or what was previously printed on the line.

What does matter is how many lines are visible on the screen at once -- that is, how tall the console is vertically. Usually this is 25 for DOS or 24 for Linux, but it varies, of course.

Making a console fullscreen, in XP, at least, increases the height of the console window to the height of the screen. The number of lines the console is at this point does indeed depend on your screen resolution.

Therefore, the number of newlines that you have to print to clear the screen varies depending on the screen resolution, if you assume you users will be messing with the size of the console.

Personally, I'd go with 25, or better yet, install clear, which does a much better job. (It resets the cursor position to the upper-left corner of the screen as well as clearing it.)

[edit] One thing I didn't think of -- were you talking about ALT-ENTER fullscreen mode? In that case, you might be right -- although on my old Windows 98 computer, a fullscreen terminal is still in the same mode (80x25, I think) by default. Switching to fullscreen shouldn't affect the number of lines displayed.

Which might be what you were referring to.

Except, of course, that the font has no affect on this either. Unless larger numbers of lines cause the font to look smaller. (Do you have a CRT?)

I think I'm overanalyzing something here.

[/edit]

**King Mir** · 08-13-2008

Yeah I was referring to Alt+Enter. It's 80 by 50 on XP at least. Vista console does not support it, unfortunately. Cygwin does though. I imagine it would have to use the same dimensions to look right, although it may use a taller font.

I just remember this from when I was doing a full screen ascii program. It was 80x50-1 characters to refresh the screen.

**dwks** · 08-13-2008

Yeah, I was trying to run a bunch of old full-screen DOS programs on Vista the other day. It didn't work. I guess Microsoft decided that they opened up too many security holes.

It sounds like you're talking about clearing the screen with spaces. Note that you don't have to: ordinary newlines work just fine. And then you only have to worry about the screen's height, not its width as well.

Another note: I don't think printing newlines or spaces properly clears the colour attributes of a Windows console. Another thing that clear and cls do that printing chars does not.

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