Q: What is alloca and why is its use discouraged?
A: alloca allocates memory which is automatically freed when the function which called alloca returns. That is, memory allocated with alloca is local to a particular function's ``stack frame'' or context.
alloca cannot be written portably, [footnote] and is difficult to implement on machines without a conventional stack. Its use is problematical (and the obvious implementation on a stack-based machine fails) when its return value is passed directly to another function, as in fgets(alloca(100), 100, stdin). [footnote]
For these reasons, alloca is not Standard and cannot be used in programs which must be widely portable, no matter how useful it might be. Now that C99 supports variable-length arrays (VLA's), they can be used to more cleanly accomplish most of the tasks which alloca used to be put to.
See also question 7.22.
Additional links: an article by Gordon L. Burditt describing difficulties in implementing and using alloca
Rationale Sec. 4.10.3