C Structs and Typedef

Structs

Declaring a struct

struct car {
    char *name;
    float price;
    int speed;
};

int main(void){
    struct car saturn;
    saturn.name = "Saturn SL/2";
    saturn.price = 15999.99;
    saturn.speed = 175;
}

• Commonly used with typedef

#include <stdio.h>

typedef struct {
    char *name;
    int leg_count, speed;
} animal;

int main(void){
    animal z;
}

Passing structs to functions

• It’s far more common to pass a pointer to a struct to a function

void set_price(struct car *c, float new_price){
    // This won't work because dot operator only works on `struct`, not pointer to struct
    // c.price = new_price;
    c->price = new_price;
}

int main(void){
    struct car saturn = {.speed=175, .name="Saturn SL/2"};
    set_price(&saturn, 799.99);
    printf("Price: %f\n", saturn.price);
}

Arrow Operator

void set_price(struct car *c, float new_price){
    // This won't work because dot operator only works on `struct`, not pointer to struct
    // c.price = new_price;

    // This works but it's non-idiomatic
    // (*c).price = new_price;
    c->price = new_price;
}

Padding bytes

• C is allowed to add padding bytes within of after a struct as it sees fit. They’re not committed to be directly adjacent in memory.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct foo {
    int a;
    char b;
};

int main(){
    printf("%zu\n", sizeof(int)+sizeof(char));
    printf("%zu\n",sizeof(struct foo));
}

• To findout where those structure are

#include <string.h>
#include <stddef.h>

struct foo {
    int a;
    char b;
    int c;
    char d;
};

int main(){
    printf("%zu\n", offsetof(struct foo, a));
    printf("%zu\n", offsetof(struct foo, b));
    printf("%zu\n", offsetof(struct foo, c));
    printf("%zu\n", offsetof(struct foo, d));
    // 0 4 8 12
}

Unions