Default initialization for a struct in C

Question

I want to do something like this in plain C:

struct data_msg {
    uint8_t id = 25;
    uint8_t       data1;
    uint32_t      data2;
}

I need the id to be set to 25 by default so that when I create an instance of the struct, the id is already set to 25, like this:

struct data_msg      tmp_msg;
printf("ID: %d", tmp_msg.id); // outputs ID: 25

Is there a way to do this in C? I know it can be done in C++, but have not figured a way in C.

Doing this in C will throw errors:

struct data_msg { uint8_t id = 25; }

Answer 1

Unfortunately, you can't, but if you do this a lot, you could create a constant that you use for initialization:

struct data_msg {
    uint8_t       id;
    uint8_t       data1;
    uint32_t      data2;
};

const struct data_msg dm_init = {.id = 25};

int main(void) {
    struct data_msg var = dm_init;  // var.id is now 25, data1 = 0, data2 = 0
    // ...
}

Answer 2

The C standard does not provide any feature for this. You may set initial values when defining an object, but there is no provision for setting default or initial values when defining a type.