Flush Input Buffer

**exvor** · 07-23-2005

Code:

inline bool Flush_Stream(FILE* stream)
 {
    void* p = malloc(BUFSIZ);
    if (p == NULL)
        {
            return false;
        }
    else
        setbuf(stream, p);
      
    free(p);
    return true;
}

there we go that looks less insane :P
sorry im anal about {} in if statements also you needed an else to be more clear what you wanted to happen in the if statement.

**quzah** · 07-23-2005

Which still doesn't fix the issue Salem last commented on. In any case, this function still won't do what the origional poster wanted it to, no matter what you do to it.

Quzah.

**exvor** · 07-23-2005

I just read the man page for setbuf and yea your right that does not make alot of sense.

lol well at least you got me to look at the man page

now i know what setbuf does YAY !

**TKMan** · 10-27-2005

Code:

void flush_stdin(void)
{
char ch = 0;

while ((ch = getchar()) != '\n' && ch != EOF);
}

This code would always work if you could insert a char into stdin before the loop. Is this possible with any standard c function?

**Prelude** · 10-27-2005

What an entertaining thread.

I wonder when people will realize that if you do things right to begin with, there's no need to "flush" the input stream.

>This code would always work if you could insert a char into stdin before the loop.
Really? What if char is unsigned?

**pianorain** · 10-27-2005

Ha...I'm kind of glad this thread got resurrected. It was fun to read the first time, and it's still a pretty good read now.

Code:

void flush_stdin(void)
{
char ch = 0;

while ((ch = getchar()) != '\n' && ch != EOF);
}

I could have sworn that getchar() returns an int value and EOF is an int value.

**TKMan** · 10-27-2005

Code:

while ((ch = getchar()) != '\n' && ch != EOF);

I took this loop from the faq on this forum, so I guess it´s correct... So do you know a function that inserts chars into stdin?

**Prelude** · 10-27-2005

>I took this loop from the faq on this forum, so I guess it´s correct...
That part is correct, but your little function isn't. Maybe you should read the FAQ more closely. Or better yet, declare ch as unsigned char and see what happens when you assume that getchar returns char.

>So do you know a function that inserts chars into stdin?
Not the way you're thinking of it.

**TKMan** · 10-27-2005

how would you flush stdin?

**Ancient Dragon** · 10-27-2005

Yes I agree this is an entertaining thread

What everyone failed to mention was that setbuf() must be called immediately after the stream is opened and before any i/o takes place. That pretty much makes the function unusable for stdin unless called at the start of the program in main(). passing NULL to setbuf() makes the stream unbuffered.

**cwr** · 10-27-2005

There are platform specific methods to rid the input stream of any pending data. Certain unix-like systems provide fpurge (and __fpurge on linux) to do this. As you've discovered, there's no completely portable.

Also, as was said above, there's never any point to doing this anyway. If a user has put stuff on the input stream, that user will expect it to be dealt with, right? In a text based interactive program, you are prompting a user for input. The valid way to deal with "unwanted" cruft on the stream is to parse it and produce an error if appropriate.

If you start trying to "flush" the input stream, you get into deep doodoo when the ambitious user tries to automate your program by piping pre-cooked data into your application. In short, your program becomes far less useful.

**TKMan** · 10-28-2005

But if I use scanf it can always happen that there will be chars left in stdin. Can could I prevent this with scanf or do I have to use another function?

**Prelude** · 10-28-2005

>How could I prevent this with scanf?
Don't mix scanf with other input methods. Use scanf intelligently and you'll have no problems. But an accepted solution is not to use scanf at all in favor of a combination of fgets to read a line and sscanf to parse it.