// The name of the (VALUE, PREVIOUS) record of this type and name.
#define dynamic_record(type,name) __ ## type ## _ ## name ## _record

// The name of the stack of dynamic bindings for this type and name.  Each
// stack is a linked list of records.  The final record is the global one,
// all new records are allocated on the stack by uses of dynamic_bind.
#define dynamic_stack(type,name) __ ## type ## _ ## name ## _stack

// The name of the cleanup function that is invoked by GCC whenever control
// is transferred outside of a dynamic_bind block.
#define dynamic_pop(type,name) __ ## type ## _ ## name ## _pop

#define defdynamic_aux(type, initform, record, stack, pop) \
    struct record {                                        \
        type value;                                        \
        int validp;                                        \
        struct record* previous;                           \
    } record = {initform, 1, 0};                           \
    struct record* stack = &record;                        \
    void pop(struct record* x) {stack = stack->previous;}  \

#define dynamic_bind_aux(initform, record, stack, pop)        \
    for(struct record __record __attribute__((cleanup (pop))) \
            = {initform, 1, stack};                           \
        stack->validp && (stack = &__record);                 \
        stack->validp = 0)                                    \

#define defdynamic(type, name, initform)       \
    defdynamic_aux(type,                       \
                   initform,                   \
                   dynamic_record(type,name),  \
                   dynamic_stack(type,name),   \
                   dynamic_pop(type,name))     \

#define dynamic_bind(type, name, initform)       \
    dynamic_bind_aux(initform,                   \
                     dynamic_record(type, name), \
                     dynamic_stack(type, name),  \
                     dynamic_pop(type,name))     \

#define dynamic_ref(type, name)         \
        dynamic_stack(type,name)->value \

/////////////////////////////////

defdynamic(int, x, 5);

int bar() {
    return dynamic_ref(int, x);
}

int foo() {
    dynamic_bind(int, x, 42) {
        return bar();
    }
}

int main(int argc, char** argv) {
    return foo();
}