Flush Input Buffer

Since fflush() is illegal, and rewind() is not guaranteed to work, is this legal, and more importantly, function the way I'm intending it to?

Code:

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

// Flush input buffer (actually, any buffer)
//
// Parameters:
//    FILE* stream    Pointer to file to be 'debuffered'.
// Returns:
//    bool - false if error, true if successful.
// Notes:
//    For output streams, does not mimic fflush() in that remaining
//    data is lost.
inline bool Flush_Stream(FILE* stream)
{
    void* p = malloc(BUFSIZ);
    if (p == NULL)
        return false;

    setbuf(stream, p);
    return true;
}

I don't believe so. At least, not if I'm understanding the man page correctly. Actually, there is no standard way you can have unbuffered input, so you don't even need to try.

Wait a second. Flushing the input stream and unbuffering it are two different things. Unbuffering refers to making it non-buffered. Meaning, as soon as you press a key, it is sent immediately. (IE: Not buffered.)

Flushing the stream simply means to clear out any pending input. That's a snap to do:

Code:

void fflushstdin( void )
{
    int c;
    while( (c = fgetc( stdin )) != EOF && c != '\n' );
}

Quzah.

Originally Posted by quzah

Wait a second. Flushing the input stream and unbuffering it are two different things. Unbuffering refers to making it non-buffered. Meaning, as soon as you press a key, it is sent immediately. (IE: Not buffered.)

Maybe I didn't use the best terminology, but I did have "debuffered" in quotation marks, so a winnar is me. :-)

Originally Posted by quzah

Flushing the stream simply means to clear out any pending input. That's a snap to do:

Code:

void fflushstdin( void )
{
    int c;
    while( (c = fgetc( stdin )) != EOF && c != '\n' );
}

That is true, but the relevant FAQ entry states that that will not work if there is no input pending. What I'm trying to accomplish is create a panacea that works whenever wherever.

Well, if there were one that worked in all occasions, it'd already be in the relevant FAQ entry.


Quzah.

Can you use feof() on stdin to see if there's any input pending?

I suppose it would work if there was EOF left in the buffer but I wouldn't rely on it.

> Can you use feof() on stdin to see if there's any input pending?
No, that tells you there will never be any more input.

> void* p = malloc(BUFSIZ);
Way to go with the huge memory leak!

Originally Posted by Salem

> void* p = malloc(BUFSIZ);
Way to go with the huge memory leak!

What memory leak?

Originally Posted by setbuf() man page

When the first I/O operation occurs on a file, malloc(3) is called, and a buffer is obtained.

Well how many free() calls are there for every Flush_Stream() call - bzzzzzt - NONE!!!!

Code:

inline bool Flush_Stream(FILE* stream) {
    void* p = malloc(BUFSIZ);
    if (p == NULL)
        return false;

    setbuf(stream, p);

    free(p);
    return true;
}

Great - now setbuf() is using free'd memory - nice bun-fight when someone else gets that same block in a following malloc call.

Huh? But the free() call is after the setbuf() call.

I take it you haven't read the manual page on setbuf() to know that it holds onto the pointer you give it, and then uses it as the buffer for future I/O operations.

No, I haven't, sorry. So where would you put the free()?

You can't polish a turd.


Quzah.