What is alloca()? [Generated by bison from flex]

[2014_5001 at uwovax.uwo.ca]

I was trying to compile FLEX with TurboC 2.0.  Not having a yacc, I
ran bison on parse.y.

With a minor change in the source, I managed to get the whole thing
compiled.  Unfortunately, it requests the presence of an alloca()
routine, apparently with a prototype like:
  void *alloca(size_t s);
(As well as I can tell from the context and the type casting).
It does not appear to figure in parse.y, so it appears likely to me
that it is put in by my bison.  What does alloca() do?
It does not appear in the TurboC 2.0 library, nor in Unix SysV
man pages.

-- 
Alexander Pruss, at one of: Department of Applied Mathematics, Astronomy,
Mathematics, or Physics                     University of Western Ontario 
pruss at uwovax.uwo.ca         pruss at uwovax.BITNET          A5001 at nve.uwo.ca