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

One of the most memorable bugs I had was to do with an inline function that used alloca. It manifested itself as a stack overflow (because it allocates on the stack) at random points of the program's execution.

In the header file:

void DoSomething() {
   wchar_t* pStr = alloca(100);
   //......
}

In the implementation file:

void Process() {
   for (i = 0; i < 1000000; i++) {
     DoSomething();
   }
}

So what happened was the compiler inlined DoSomething function and all the stack allocations were happening inside Process() function and thus blowing the stack up. In my defence (and I wasn't the one who found the issue; I had to go and cry to one of the senior developers when I couldn't fix it), it wasn't straight alloca, it was one of ATL string conversion macros.

So the lesson is - do not use alloca in functions that you think might be inlined.