Why is the use of alloca() not considered good practice?

Question

alloca() allocates memory on the stack rather than on the heap, as in the case of malloc(). So, when I return from the routine the memory is freed.

Answer 1

Processes only have a limited amount of stack space available - far less than the amount of memory available to malloc().

By using alloca() you dramatically increase your chances of getting a Stack Overflow error (if you're lucky, or an inexplicable crash if you're not).