Question

When I have an array in a struct, the meaning is totally clear to me: When the struct is defined, memory for the whole array is reserved and when I copy the struct, all the array contents is copied.

typedef struct {
    uint8_t type;
    struct {
        uint8_t length;
        uint8_t data[5];
    } m;
} s;

But when I use uint8_t data[], what does that mean? I guessed it might be the same as uint8_t *data but it isn’t. When I try to assign to it like this:

s the_s;
uint8_t something[] = {1, 2, 3};
the_s.m.data = something;

the compiler gives me

cannot assign array type objects

Answer 1

An array of an incomplete type as the last member of a struct is a C99 feature called the flexible array member feature.

In this statement

the_s.m.data = something;

you are trying to assign an array but in C, arrays cannot be assigned.