Thread: fflush(stdin)

Someone says that this isn't defined. Is it possible that it works, even though it isn't defined?

> Someone says that this isn't defined
Everyone - especially those who wrote the ANSI-C standard

> Is it possible that it works
Undefined things never work. Part of 'could do absolutely anything' (which is what undefined means) includes doing what you expect/want, but that shouldn't be mistaken for working.

All manner of things could happen which could result in your program suddenly doing something different.

Prelude,
I'm in the middle of reading C in 21 Days (1997 edition), and it uses fflush(stdin) all over the place, as well as void main, and global variables. I'm shocked!

Prelude,
Oh well, it only cost me 20 bucks used. Although C in 21 Days has a lot more errors that a book like Deitel, for example, it's easier to understand for a fairly new guy like me. Deitels can't relate to dumb guys like me. By the way, did you say in another thread that you didn't like being called he...

Prelude,
This forum is almost 100% guys. It's hard to tell from your picture, but, you sound like a guy, that's why the comment caught my attention. By the way, at work do you keep several C books around to refer to, if necessary? Do you know C++ and Java?

You sound like a guy because you don't sound touchy-feely. I thought all the whiz-kids were into Java these days? Gotta go rollerblading...

> void main( void )
> fflush( stdin );
> gets( string );
Could this example be any worse?

> But I hate hacking code together like that
The simple way out of this mess is to always use fgets to read the input. It will read input up to the next newline or until the buffer is full.

And it has a simple interface to tell you whether you have reached the end of file, or there's been an i/o error (it returns NULL).

Once you have the input in an array, a wide variety of approaches are possible, including sscanf, strncmp, strtok, a for loop, etc

Yeah, that is what I thought - I assumed (incorrectly) that the following code would work. But anything that is greater than the 10 bytes that I want fgets() to read is stored in the stdin buffer until the next call to fgets() where it is then stored in the second array, buffer2. Have I incorrectly assumed that fgets() would discard everything that is not required, or do I have to do some cleaning up before the next call the fgets()?

Code:

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

int main(void)
{
	char buffer1[80];
	char buffer2[80];
	
	printf("Enter a string: ");
	fgets(buffer1, 10, stdin);
	printf("Enter another string: ");
	fgets(buffer2, 10, stdin);

	printf("\nstring1 : %s", buffer1);
	printf("\nstring2 : %s", buffer2);

	return 0;
}

fgets doesn't throw away any excess input, it's there for the next call to fgets.

printf("Enter a string: ");
fgets(buffer1, 10, stdin);

What you would then do (to see if you had any excess input), would be this
if ( strchr( buffer1, '\n' ) == NULL ) {
// there is no newline in this buffer, must have been a long line
// throw away the excess
}

But reading directly into the final result is perhaps not a good idea, normally you want to validate the input in some way, or perhaps remove the newline which fgets appends.

char buff[BUFSIZ];
fgets( buff, BUFSIZ, stdin );
// now move 10 chars from buff to buffer1, if your tests on buff show that this is a good (and safe) thing to do

BUFSIZ is a macro value in stdio.h

Ahhh, that is good advice - thanks