Same C code different results

Veteq · 07-22-2004, 09:36 AM

Code:

# include <stdio.h>

int num;
main()
{
        system("clear");
        printf("enter any number: ");
        fflush ( stdin);
        scanf("%d", &num );

        printf("the number entered was %d\n", num);
        fflush(stdin);
        printf("press any key\n");
        getchar();
}

This is a very simple program to test a theory.

When I compile this code under AIX (CC compiler), the program waits at "Press any key" before exiting the program (getchar() command). The same code in linux (CC compiler) skips the pause and exits the program before any key is pressed.
what am I missing?

please help

Veteq

SheldonPlankton · 07-22-2004, 11:03 AM

what are the terminal settings?

Also portability is not on of C's strong points. If portability is important to you use a language that ports well (shell script, Perl, Python, Java ... )

infamous41md · 07-22-2004, 12:10 PM

fflush() is not suppposed to be called on input streams, when u do call it u it may or may not do anything on the stream. im guessing that on linux it actually flushes stdin, and on aix it doesn't.

The_Nerd · 07-22-2004, 02:09 PM

I've found that when using getchar, it will often pass imidiadtly from the keypress done of scanf.

Try using two getchars.
That is pretty cheesy though.

Oh well! I think that is the problem, its up to you to figure out how to fix it.

Sorry!

aluser · 07-22-2004, 02:46 PM

The problem is that the scanf() leaves a \n waiting on the input stream, and the getchar() grabs that immediately. I managed to make scanf take a single \n off the stream like this:

Code:

# include <stdio.h>

int num;
main()
{
        system("clear");
        printf("enter any number: ");
        scanf("%d%*1[\n]", &num);

        printf("the number entered was %d\n", num);
        printf("press any key\n");
        getchar();
}

clearly, still ugly as sin, but my man page seems to suggest that it is portable. I think AIX is simply doing the wrong thing.

When you actually want to make a screen-oriented app, this crap is probably the reason you're supposed to use ncurses.

aluser · 07-22-2004, 02:48 PM

oh, and the "any key" needs to be enter because your terminal is going to be line buffered. again there's some way to fix this with curses.

Hko · 07-22-2004, 04:45 PM

Quote:

Originally posted by aluser
I managed to make scanf take a single \n off the stream like this:

Code:

scanf("%d%*1[\n]", &num);

clearly, still ugly as sin, but my man page seems to suggest that it is portable.

Nice hack. Who would think of that...

arvind_sv · 07-22-2004, 08:18 PM

Hi,

Yes, a nice hack, but it fails for inputs like, "12 \n", ie, an extra space before the "\n". The actual portable way to do this (flush the input stream) is to use a while loop:

Code:

scanf ("%d", &num);
while (getchar () != '\n')
    ;

Arvind

Veteq · 07-22-2004, 09:10 PM

Thank you for all the help,
I am going to try a few of the sugestions.....

Once again thank you, I always find the answer here ....in this forum

aluser · 07-22-2004, 09:18 PM

arvind_sw is probably right. the only question I have about it is that it seems from the OP that scanf was magically eating the \n on AIX and not linux, so if that's the case you could end up waiting in the wrong place. Then again if AIX is broken maybe mine would too. Mine definitely sucked about not eating random characters after the number, in any event.

Kumar · 07-23-2004, 01:24 AM

Well.....I too tried many different things. But the best way I guess, is to use two getch() calls instead of one. The first one will collect the \n while the second will read the data for you.

jlliagre · 07-23-2004, 07:00 AM

As said before, fflushing stdin is outside the standard C library specifications, so there is no point blaming O/S reliability or language portability there.

scanf is also unreliable when the input come from a user, as it may crash if, for example, non numeric values are entered.

Here is a version I guess is more portable

Code:

#include <stdio.h>

int num;
main()
{
        char line[32];
        system("clear");
        printf("enter any number: ");
        fgets(line, 31, stdin);
        sscanf(line, "%d", &num);
        printf("the number entered was %d\n", num);
        printf("press any key\n");
        getchar();
}

itsme86 · 07-23-2004, 08:22 AM

Instead of a loop you can also just use fgets() on stdin after the scanf(). Or just use fgets() in the first place and then use strtod() or atoi(). I really can't stand scanf() and I never use it on non-trusted input.

jlliagre · 07-23-2004, 03:49 PM

Quote:

Instead of a loop you can also just use fgets() on stdin after the scanf()

There is no loop in what I suggest ??

Quote:

I really can't stand scanf() and I never use it on non-trusted input.

sscanf hasn't the issues scanf has, and can reliably be used, I agree atoi or strtod are enough for this simple case though.

itsme86 · 07-23-2004, 03:59 PM

I was was referring to this loop that arvind suggested:

Code:

scanf ("%d", &num);
while (getchar () != '\n')
    ;