title: Why do the older C language specs require function-local variables to be declared up-front?
body: <p>In the C programming language, all of the language revisions I have worked with enforced up-front variable declarations before any non-declarative/assignative expressions would be evaluated.  C++ seems to have waived this requirement from all versions.  I also recognize more modern version of C have waived this requirement as well, but I have yet to use any of those standards. </p>
  
  <p>The question I have is this: <strong>What historic reason was there for preventing the C language from declaring freely on-demand instead of up front?</strong></p>
  
  <p>Obviously there are a number of reasons that come to mind from an engineering standpoint, but none of them seem especially plausible to me.</p>
  
  <ol>
  <li>Preventing an obscure compiler behavioral error from occurring (such as infinite parsing loops, a massive memory bloat for evaluation, or some weird corner cases with Macros.)</li>
  <li>Preventing undesirable compiler output.  This could be anything from symbol output muddling the debug process and the ease of development of debugging tools, to unexpected stack storage orders.</li>
  <li>Readability.  I find this hard to swallow as well, seeing as C, while designed for readability compared to other languages of the era, did not enforce this type of structure nearly anywhere else. (Unless you see prototyping as being a similar enforcement, but if I recall prototypes were added in the '89 spec.)</li>
  <li>Implementation complexity and practical reasons.  This is the one I'm most inclined to believe.  As engineers we have to make certain considerations in order to ship a viable product in a time-frame allotted. While I will grant that the professional landscape for Computer Science and Software Engineering have both changed dramatically, Business is still business. At the end of the day I'm sure Bell wanted a finished product that could be used in the Unix programming environment to showcase what they had achieved.</li>
  </ol>
  
  <p>Does anyone have any good sources backing up any of the above? Did I miss something entirely?  We can speculate from dawn till dusk, but I'm looking for good hard references.</p>