I worry you are mixing up alloca and malloc. Alloca allocates off the stack. All modern platforms have irritatingly small stack sizes, and if you overflow them

I worry you are mixing up alloca and malloc. Alloca allocates off the stack. All modern platforms have irritatingly small stack sizes, and if you overflow them your code just defaults with an annoyingly imprecise error. There is no overcommitting with stacks in practice.