Question

I have a array of somestruct_t I’m adding to but don’t know in advance how many elements it will need. So I malloc 5 slots of space to start with but I need to encapsulate it in another struct that saves the array boundary and whats used so far so I don’t overflow. I’ll realloc if it looks like it will overflow.

typedef struct {
  somestruct_t[] *data;
  uint32_t max_size;
  uint32_t used;
} something_box_t;

Is this the idiomatic way to do this?

Answer 1

One thing that’s done in C sometimes is to use a zero length array at the end of a struct and to allocate extra space for a variable number of elements. I’m not sure if zero length arrays are legal in C (they’re not in C++) but it’s commonly supported.

struct something_box_t {
  uint32_t max_size;
  uint32_t used;
  somestruct_t data[];
};

something_box_t *box = malloc(sizeof(something_box_t) + N*sizeof(somestruct_t));
box.max_size = N;
box.used = 0;

box.data[0] = ...;
box.data[1] = ...;

Of course, if you can switch to C++ you won’t have to hack together a solution on your own. You could just do std::vector<somestruct_t> box; and have a solution far easier to use, as performant if not more performant, and more reliable than what you’re likely to write in C.