title: C standard prohibition on warnings by default
body: <p>Seeing yet another <a href="https://stackoverflow.com/questions/36664754/c-program-crashes-while-trying-to-open-a-file-with-variable-filename">question</a> the answer to which would have been obvious had the questioner compiled with <code>-Wall</code> got me thinking.</p>
  
  <p>Is there a 'C standard' based reason why <code>-Wall</code> cannot be enabled by default by the compiler? </p>
  
  <p>As far as I know none of the major compilers do this (and certainly historically none of them did it), and I'd like to know whether this is adherence to the standard, or some other reason (inertia, back compatibility or whatever). Speculating as to the other reason is probably off-topic (opinion based), but I think asking whether a standard requires this behaviour is on-topic (factual).</p>