Initialize an array of structs

Question

My code looks like this

struct {
    int valid;
    int pid;
    int heading;
    int speed;
} car;

struct car road[10];    //a road filled with cars

However, my compiler is complaining:

"incomplete struct/union/enum car: road"

What am I doing wrong? I looked at examples online and they show that this is how to initialize arrays of structs.

If it isn't obvious, I'm trying to make an array of car structs

Answer 1

Change :

struct {
    int valid;
    int pid;
    int heading;
    int speed;
} car;

which declares only one instance of an unnamed structure , with the name car , to :

struct car{
    int valid;
    int pid;
    int heading;
    int speed;
};

which declares a new type struct car .The struct 's name needs to be next to the keyword struct .

---

You can also do :

typedef struct {
    int valid;
    int pid;
    int heading;
    int speed;
} car;

and then you only refer to this struct as car , not struct car .

See this link for more information on structs and their syntax.

Answer 2

This:

struct {
  ...
} car;

declares a single instance of an anonymous structure, called car . You can view the struct {... } part as analogous with any other type name, except a new type is used. Since you don't give the structure a name, you can't create more than the car instance.

This might seem like a pointless thing to do, but it can be really useful when you want to group things together.

What you meant was this:

struct car {
 ...
};

which declares a new struct type, called struct car . You can use this name to refer to the declaration, and eg later create a bunch of instances like you did:

struct car road[10];

Answer 3

Option 1 (without typedef):

struct car {
    int valid;
    int pid;
    int heading;
    int speed;
};

struct car road[10];

Option 2 (with typedef with unnamed struct):

typedef struct {
    int valid;
    int pid;
    int heading;
    int speed;
} car_t;

car_t road[10];

Option 3 (with typedef with named struct):

typedef struct car {
    int valid;
    int pid;
    int heading;
    int speed;
} car_t;

car_t road[10];
//or
struct car road[10];

---

The Linux Coding Style states:

It's a mistake to use typedef for structures...