Definition of Undefined behaviour : <p>I have read on <a href="http://en.cppreference.com/w/cpp/language/ub" rel="nofollow noreferrer" title="Undefined Behavior">cppreference</a> about Undefined behavior and understand the differences. What I didn't find though is what is implicit.</p>

<p>I would expect any behavior that is not explicitly defined to be unspecified behavior. But when I tried to find what <code>t</code> means in <code>fopen</code>'s format string, there was nothing in the <a href="http://en.cppreference.com/w/cpp/io/c/fopen" rel="nofollow noreferrer" title="fopen">cppreference</a> on it and nowhere does it state that it is implementation defined. What bugs me even more is that in notes it is explicitly stated about the <code>filename</code> parameter.</p>

<p>On the other hand <a href="https://msdn.microsoft.com/en-us/library/yeby3zcb.aspx" rel="nofollow noreferrer" title="fopen">msdn</a> defines among others the <code>t</code> parameter so that would mean that any behavior not explicitly stated as undefined can be implementation defined? Or is the cppreference just missing a sentence and I'm only chasing ghosts?</p>

<p>I guess what the question boils down for me is, if there is nothing written about something, what is it, undefined or implementation defined behavior?</p>

<p>PS. The question is about general case so please don't tell me how to use <code>fopen</code> "the right way" in the answers. The function was just an example.</p>
