WrongWay reversible iterator, C (gcc)

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//WrongWay reversible iterator

#include <stdio.h>
typedef float some;
typedef struct {
    size_t pos, start, end;
    some *ptr;
    some *(*next)(object);
} obj, *object;
//iterator next
some *next (object obj) {
    return obj->pos <= obj->end? &obj->ptr[obj->pos++] : NULL;
}
//iterator previous
some *prev (object obj) {
    return obj->pos >= obj->start? &obj->ptr[obj->pos--] : NULL;
}
//shortcuts
#define ITER(name, start, end) {start, start, end, name, &next}
#define FOREACH(i, j) for (some *j;(j = i.next(&i));)
#define NEXT(i) *i.next(&i)
#define REVERSE(i) { if(i.pos>i.start){i.next = prev, i.pos=i.end;}    \
                   else{ i.next = next, i.pos = i.start;}}while(0)
int main (void) {
    some i[]={0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0};
    obj k = ITER(i, 2, 6);
    printf ("%1.2f\n", NEXT(k));
    printf ("%1.2f\n", NEXT(k));
    FOREACH (k, j) printf ("%1.2f ", *j);
    REVERSE(k); putchar ('\n');
    FOREACH (k, j) printf ("%1.2f ", *j);
    REVERSE(k); putchar ('\n');
    FOREACH (k, j) printf ("%1.2f ", *j);
    REVERSE(k); putchar ('\n');
    printf ("%1.2f\n", NEXT(k));
    printf ("%1.2f\n", NEXT(k));
    FOREACH (k, j) printf ("%1.2f ", *j);
    return 0;
}

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Compilation time: 0.12 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,34 sec