peek at stdin, flush stdin


J

Johnathan Doe

I'm trying to find a way to reliably peek at stdin, and if anything's
waiting, flush stdin so that it clears the stream ready to wait for a
character.

The problem I have is that in an application, if I call say scanf() to
get some input, occasionally the newline is still left hanging around in
the buffer. If there's any junk left over in stdin after getting input
I'd like to clear it out. And then when going to read input again
later, check for any junk and clear it out before prompting and
attempting to read input.

I've found a way to do this quite well by using:

scanf("%c%*c", &ch);

which gets a menu choice of a single character, and sucks up the newline
still in the stream. But, if any extra stuff is entered, this is not
very robust. It's all still left in the input stream. Because most of
my programs run in blocking mode, I can sit there in a loop and read
everything until stdin drains out. I've tried going into non-blocking
mode to "peek" at anything that might be there, get it, and then return,
but I couldn't make it work. It also seems like a bit of a nasty hack.

Any ideas?

Many thanks for your help.

Johnathan
 
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T

Thomas stegen

Johnathan said:
I'm trying to find a way to reliably peek at stdin, and if anything's
waiting, flush stdin so that it clears the stream ready to wait for a
character.

char c;
scanf("%c%*[^\n]%*c", &c);

Should do the trick. It does not clear stdin if more than
one line was entered in some way.
 
C

Chris Torek

char c;
scanf("%c%*[^\n]%*c", &c);

Should do the trick. It does not clear stdin if more than
one line was entered in some way.

Unfortunately, this technique does not work *unless* there is
at least one character other than newline following the one
desired input character. The reason is that the %[ directive
("%*[^\n]" => scan an unlimited length sequence of characters
other than newline, discarding each character scanned) fails
if the number of characters that match the scanset is zero.

In other words, scanning an input stream containing "y\n" puts the
'y' into c, then finds a newline and fails the %[ directive, and
then stops (and scanf() returns 1 -- one successful convert and
assign occurred). The newline remains in the stream. If the stream
contains "yes\n", on the other hand, the %c converts the 'y' as
desired, the %[ skips the 'e' and 's' as desired, than the %*c
discards the newline as desired. (Here scanf() again returns 1.)

The trick is to separate this into *two* scanf() calls:

r1 = scanf("%c%*[^\n]", &c);
r2 = scanf("%*c");

If it ever gets anything at all, the second scanf() just gets one
newline character, so it can be replaced with getchar(). The return
value from the first scanf() should be inspected first, to make
sure that the stream has not already hit end-of-file.

These all have a flaw: if the character stuck into "c" is a newline,
they read one more line, so you actually need *three* scanf() calls:

r1 = scanf("%c", &c);
... check for EOF and newline; if not ...
r2 = scanf("%*[^\n]"); /* read and discard all non-newlines */
if (r2 != EOF)
r3 = scanf("%*c"); /* then read and discard the newline */

A tiny function that calls getchar() in a loop may be more readable
to future programmers:

int get_one_letter_response_on_a_line(void) {
int c, first;

/* get and save the first character */
first = c = getchar();

/* if it was not newline, keep getting more (but stop on EOF) */
while (c != '\n' && c != EOF)
c = getchar();

/* in any case, return the first one, which may be '\n' or EOF */
return first;
}

Now you can just have:

int c;
...
c = get_one_letter_response_on_a_line();
switch (c) {
case 'y': case 'Y': /* handle "yes" */ break;
case 'n': case 'N': /* handle "no" */ break;
case EOF: /* handle EOF */ break;
default: puts("y(es) or n(o), please"); ...
}

for instance.
 
J

Johnathan Doe

Thomas stegen wrote:

Thankyou for teaching me about [^\n] : it's not in my textbook!

Kind Regards.
 
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