Using fflush(stdin)

Simple: this is undefined behavior, since fflush is meant to be called on an output stream. This is an excerpt from the C standard:

int fflush(FILE *ostream);

ostream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

So it's not a question of "how bad" this is. fflush(stdin) is plainly wrong, and you mustn't use it, ever.


Converting comments into an answer — and extending them since the issue reappears periodically.

Standard C and POSIX leave fflush(stdin) as undefined behaviour

The POSIX, C and C++ standards for fflush() explicitly state that the behaviour is undefined, but none of them prevent a system from defining it.

ISO/IEC 9899:2011 — the C11 Standard — says:

§7.21.5.2 The fflush function

¶2 If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

POSIX mostly defers to the C standard but it does mark this text as a C extension.

[CX] For a stream open for reading, if the file is not already at EOF, and the file is one capable of seeking, the file offset of the underlying open file description shall be set to the file position of the stream, and any characters pushed back onto the stream by ungetc() or ungetwc() that have not subsequently been read from the stream shall be discarded (without further changing the file offset).

Note that terminals are not capable of seeking; neither are pipes or sockets.

Microsoft defines the behaviour of fflush(stdin)

Microsoft and the Visual Studio runtime defines the define the behaviour of fflush() on an input stream.

If the stream is open for input, fflush clears the contents of the buffer.

M.M notes:

Cygwin is an example of a fairly common platform on which fflush(stdin) does not clear the input.

This is why this answer version of my comment notes 'Microsoft and the Visual Studio runtime' — if you use a non-Microsoft C runtime library, the behaviour you see depends on that library.

Linux documentation and practice seem to contradict each other

Surprisingly, Linux nominally documents the behaviour of fflush(stdin) too, and even defines it the same way (miracle of miracles).

For input streams, fflush() discards any buffered data that has been fetched from the underlying file, but has not been consumed by the application.

I remain a bit puzzled and surprised at the Linux documentation saying that fflush(stdin) will work. Despite that suggestion, it most usually does not work on Linux. I just checked the documentation on Ubuntu 14.04 LTS; it says what is quoted above, but empirically, it does not work — at least when the input stream is a non-seekable device such as a terminal.

demo-fflush.c

#include <stdio.h>

int main(void)
{
    int c;
    if ((c = getchar()) != EOF)
    {
        printf("Got %c; enter some new data\n", c);
        fflush(stdin);
    }
    if ((c = getchar()) != EOF)
        printf("Got %c\n", c);

    return 0;
}

Example output

$ ./demo-fflush
Alliteration
Got A; enter some new data
Got l
$

This output was obtained on both Ubuntu 14.04 LTS and Mac OS X 10.11.2. To my understanding, it contradicts what the Linux manual says. If the fflush(stdin) operation worked, I would have to type a new line of text to get information for the second getchar() to read.

Given what the POSIX standard says, maybe a better demonstration is needed, and the Linux documentation should be clarified.

demo-fflush2.c

#include <stdio.h>

int main(void)
{
    int c;
    if ((c = getchar()) != EOF)
    {
        printf("Got %c\n", c);
        ungetc('B', stdin);
        ungetc('Z', stdin);
        if ((c = getchar()) == EOF)
        {
            fprintf(stderr, "Huh?!\n");
            return 1;
        }
        printf("Got %c after ungetc()\n", c);
        fflush(stdin);
    }
    if ((c = getchar()) != EOF)
        printf("Got %c\n", c);

    return 0;
}

Example output

Note that /etc/passwd is a seekable file. On Ubuntu, the first line looks like:

root:x:0:0:root:/root:/bin/bash

On Mac OS X, the first 4 lines look like:

##
# User Database
# 
# Note that this file is consulted directly only when the system is running

In other words, there is commentary at the top of the Mac OS X /etc/passwd file. The non-comment lines conform to the normal layout, so the root entry is:

root:*:0:0:System Administrator:/var/root:/bin/sh

Ubuntu 14.04 LTS:

$ ./demo-fflush2 < /etc/passwd
Got r
Got Z after ungetc()
Got o
$ ./demo-fflush2
Allotrope
Got A
Got Z after ungetc()
Got B
$

Mac OS X 10.11.2:

$ ./demo-fflush2 < /etc/passwd
Got #
Got Z after ungetc()
Got B
$

The Mac OS X behaviour ignores (or at least seems to ignore) the fflush(stdin) (thus not following POSIX on this issue). The Linux behaviour corresponds to the documented POSIX behaviour, but the POSIX specification is far more careful in what it says — it specifies a file capable of seeking, but terminals, of course, do not support seeking. It is also much less useful than the Microsoft specification.

Summary

Microsoft documents the behaviour of fflush(stdin). Apparently, it works as documented on the Windows platform, using the native Windows compiler and C runtime support libraries.

Despite documentation to the contrary, it does not work on Linux when the standard input is a terminal, but it seems to follow the POSIX specification which is far more carefully worded. According to the C standard, the behaviour of fflush(stdin) is undefined. POSIX adds the qualifier 'unless the input file is seekable', which a terminal is not. The behaviour is not the same as Microsoft's.

Consequently, portable code does not use fflush(stdin). Code that is tied to Microsoft's platform may use it and it will work, but beware the portability issues.

POSIX way to discard unread terminal input from a file descriptor

The POSIX standard way to discard unread information from a terminal file descriptor (as opposed to a file stream like stdin) is illustrated at How can I flush unread data from a tty input queue on a Unix system. However, that is operating below the standard I/O library level.


According to the standard, fflush can only be used with output buffers, and obviously stdin isn't one. However, some standard C libraries provide the use of fflush(stdin) as an extension. In that case you can use it, but it will affect portability, so you will no longer be able to use any standards-compliant standard C library on earth and expect the same results.

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