Question

Think of a pointer-datatype, for instance to a floating-pointer number.

typedef float* flPtrt;

How would I allocate an array of 3 elements in the local scope? I guess using malloc and free withing the same scope produces overhead, but what’s the alternative?

void foo() {
    flPtrt ptr = malloc(sizeof(float)*3);
    // ...
    free(ptr);
}

Answer 1

I think what your’e looking for is the alloca() function.
I’m not sure it’s standard C, but it exists in GNU, and it worked on my visual studio.
So this is how you use it:

int n = 5;
int* a = (int*) alloca(sizeof(int) * n);

It creates an array of elements on the stack (rather than on the heap with malloc).
Advantages: less overhead, no need to free manually (when you return from your method, the stack folds back and the memory is lost)
Disadvantage: If you want to return a pointer from a method NEVER use alloca, since you will be pointing at something that no longer exists after exiting the function. One can also argue that the stack is usually smaller than the heap, so if you want larger space use malloc.
See more here