Thread: Flush Input Buffer

Originally Posted by mabuhay

what means "fflush() is illegal"? you shouldn't use it anymore?
I normally use fflush(stdin) after fgets()...

fflush() is ok, when it is used in the proper manner. Read the FAQ on it.

Originally Posted by kermit

fflush() is ok

My compiler, VC++ 6.0, doesn't like it. fflush() requires one parameter.

Originally Posted by mabuhay

I'm just too lazy to write the parameters...
but, does someone had ever problems using fflush(stdin)?

Even if fflush(stdin) were somehow "valid", it doesn't even make logical sense. When you flush an output buffer, all data that is buffered will be written out. The logical opposite of this when applied to input buffers is that all data that is buffered will be read in. Guess what does this? Functions that read from the stream (fgets/scanf/fread/getc/etc.).

You don't want to discard input, you want to process it and deal with it.

Originally Posted by Ancient Dragon

My compiler, VC++ 6.0, doesn't like it. fflush() requires one parameter.

ah - haha

I tend to write my functions that way (with the parentheses) just to indicate I am referring to a function. I never considered that it would be confusing that way though.

Originally Posted by cwr

...You don't want to discard input, you want to process it and deal with it.

Yes, but if I use fgets() I stored my input in a variable but something is still in the input. Or why does it jups over if I use fgets() in the same code again?

Originally Posted by Prelude

...You're probably thinking of using scanf,...

I don't want to use scanf(), because you can't define the input-length globally.

Ok, can someone give me a code example how to use fgets() correctly? If i want to take three lines of text one after other, each with maximal 50 characters.

Originally Posted by mabuhay

Yes, but if I use fgets() I stored my input in a variable but something is still in the input. Or why does it jups over if I use fgets() in the same code again?

Really?

I don't want to use scanf(), because you can't define the input-length globally.

You can, it's just not particularly convenient.

Ok, can someone give me a code example how to use fgets() correctly? If i want to take three lines of text one after other, each with maximal 50 characters.

Well, I can illustrate an issue with fgets:

Code:

#include <stdio.h>

#define NLINES 3

int main(void)
{
    char buf[NLINES][50];
    int i;
    for (i = 0; i < NLINES; i++)
    {
        printf("Enter line %d: ", i);
        fflush(stdout);
        fgets(buf[i], sizeof buf[i], stdin);
    }
    for (i = 0; i < NLINES; i++)
        printf("Line %d was \"%s\"\n", i, buf[i]);
    return 0;
}

Output:

Code:

Enter line 0: Line number zero
Enter line 1: A really long line one that exceeds fifty characters
Enter line 2: Line 0 was "Line number zero
"
Line 1 was "A really long line one that exceeds fifty characte"
Line 2 was "rs
"

The problem being that fgets will stop reading if it hits the size limit. One way to check to see if fgets has read a whole line is to check for the presence of a \n on the end, and discard further input with another fgets (which may overflow again), or a getchar loop, until you reach the actual end of line.

Originally Posted by cwr

One way to check to see if fgets has read a whole line is to check for the presence of a \n on the end

That may not always work either if the input stream does not contain a '\n'. For example the last line of a file may not terminate with '\n', in that case fgets() will not put one at the end of the line either.

True, so you should also check for a NULL return or an feof on the stream.