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; }

Does this answer your question? [How to initialize a struct in accordance with C programming language standards](https://stackoverflow.com/questions/330793/how-to-initialize-a-struct-in-accordance-with-c-programming-language-standards) — जलजनक, Mar 30 '22 at 12:49
Remember struct is not a "class", because **C isn't OOP**. So you are creating a type. It would be the same if in the the definition of `int` you try to initialize always with the value 5. — Oscar Gonzalez, Mar 30 '22 at 12:58

score 5 · Accepted Answer · answered Mar 30 '22 at 12:28

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
    // ...
}

score 1 · Answer 2 · answered Mar 30 '22 at 12:28

1

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.

answered Mar 30 '22 at 12:28

Eric Postpischil

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Default initialization for a struct in C

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