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copied!<p>You're doing IO incorrectly, the POSIX manual and all other related documentation explicitly says never to mix IO done on <code>FILE *</code>s and file descriptors. You have very blatantly broken this rule. This rule is in place because <code>FILE *</code>s use <strong>buffering</strong> an this means that after a call to <a href="http://linux.die.net/man/3/fgets" rel="nofollow noreferrer"><code>fgets</code></a> there will be nothing left for <code>read</code> to get because <code>fgets</code> already read all pending data into a buffer that is kept in the <code>FILE *</code> structure.</p> <p>So since there's no way to check if an ISO C IO method will block, we have to use file descriptors only.</p> <p>Since we know that <code>STDIN_FILENO</code> is just the number 0, we can use </p> <pre><code>fcntl (0, F_SETFL, O_NONBLOCK); </code></pre> <p>this will turn all <a href="http://linux.die.net/man/2/read" rel="nofollow noreferrer"><code>read</code></a>s on file descriptor 0 to non-blocking mode, if you want to use a different file descriptor so that you can leave 0 alone then just use <a href="http://linux.die.net/man/2/dup" rel="nofollow noreferrer"><code>dup</code></a> to duplicate it.</p> <p>This way, you can stay away from <a href="http://linux.die.net/man/2/poll" rel="nofollow noreferrer"><code>poll</code></a> completely and implement <code>ngetc</code> as</p> <pre><code>ssize_t ngetc (char *c) { return read (0, c, 1); } </code></pre> <p>or better yet, a macro</p> <pre><code>#define ngetc(c) (read (0, (c), 1)) </code></pre> <p>Thus you get a simple implementation for what you're looking for.</p> <p><strong>Edit:</strong> If you are still worried about the terminal buffering the input, you can always change the terminal's settings, see <a href="https://stackoverflow.com/questions/4506704/how-to-disable-line-buffering-of-input-in-xterm-from-program">How to disable line buffering of input in xterm from program?</a> for more information on how to do this.</p> <p><strong>Edit:</strong> The reason that one could not use <code>fgetc</code> instead of <code>read</code> is for the same reason that using <code>fgets</code> won't work. When one of the <code>FILE *</code> IO functions is run, it reads all the data from the associated file descriptor. But once that happens, <code>poll</code> will never return because it's waiting on a file descriptor that's always empty, and the same thing will happen with <code>read</code>. Thus, I suggest that you follow the advice of the documentation and <strong>never</strong> mix <em>streams</em> (IO using <code>fgets</code>, <code>fgetc</code>, etc.) and <em>file descriptors</em> (IO using <code>read</code>, <code>write</code>, etc.)</p>

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