How do you pass a struct to a function efficiently?

Pass a pointer (struct big *) instead of the struct by value, so the function gets an 8-byte address instead of copying possibly thousands of bytes — add const if it won't modify it.

struct big_data {
    int values[1000];
    char name[256];
};

// BAD - copies entire struct (4000+ bytes!)
void process_copy(struct big_data data) {
    // Only modifies local copy
    data.values[0] = 100;
}

// GOOD - passes 8-byte pointer
void process_ptr(struct big_data *data) {
    // Modifies original
    data->values[0] = 100;
}

// GOOD - const pointer if you won't modify
void print_data(const struct big_data *data) {
    printf("%s\n", data->name);
    // data->values[0] = 5;  // ERROR - can't modify const
}

When to pass by value:

• Small structs (≤ 16 bytes typically)
• When you need a local copy anyway
• When you want to guarantee no side effects

When to pass by pointer:

• Large structs
• When you need to modify the original
• For consistency in an API