Correct. Note that alloca() can be an alias for malloc() in stackless Cs. If the stack assumption is invalid, then a checker will not trip over a guard page,

Correct. Note that alloca() can be an alias for malloc() in stackless Cs. If the stack assumption is invalid, then a checker will not trip over a guard page, and it will at best trip over memory over-commit (which may still be desirable). If the guard page assumption is invalid, then a checker will cause failure just as surely as not checking (provided you intended to use the whole allocation anyways), and either way the failure will be undefined, therefore this is still portable C!