What is alloca and why is its use discouraged?

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Why is alloca not considered good practice?

What is alloca and why is its use discouraged?

Answer 1

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.

It cannot be written portably, 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).

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.