The 'counted_by' attribute is used on flexible array members. The
argument for the attribute is the name of the field member in the same
structure holding the count of elements in the flexible array. This
information can be used to improve the results of the array bound sanitizer
and the '__builtin_dynamic_object_size' builtin.
This example specifies the that the flexible array member 'array' has the
number of elements allocated for it in 'count':
struct bar;
struct foo {
size_t count;
/* ... */
struct bar *array[] __attribute__((counted_by(count)));
};This establishes a relationship between 'array' and 'count', specifically
that 'p->array' must have *at least* 'p->count' number of elements available.
It's the user's responsibility to ensure that this relationship is maintained
through changes to the structure.
In the following, the allocated array erroneously has fewer elements than
what's specified by 'p->count'. This would result in an out-of-bounds access not
not being detected:
struct foo *p;
void foo_alloc(size_t count) {
p = malloc(MAX(sizeof(struct foo),
offsetof(struct foo, array[0]) + count *
sizeof(struct bar *)));
p->count = count + 42;
}The next example updates 'p->count', breaking the relationship requirement that
'p->array' must have at least 'p->count' number of elements available:
struct foo *p;
void foo_alloc(size_t count) {
p = malloc(MAX(sizeof(struct foo),
offsetof(struct foo, array[0]) + count *
sizeof(struct bar *)));
p->count = count + 42;
}
void use_foo(int index) {
p->count += 42;
p->array[index] = 0; /* The sanitizer cannot properly check this access */
}