CODEWITHSUNDEEP
cstruct
Kampi
Kampi

Reputation: 1891

Initialize struct with macro

I have this structure

typedef struct {
    int Length;
    int Data[];
} MyStruct;

And this macro to initialize the struct

#define FillStruct(...)     { .Length = sizeof((int[]){__VA_ARGS__}), .Data = {__VA_ARGS__} }

So I can initialize the struct with

MyStruct Obj = FillStruct(1, 2, 3, 4, 5);

But this doesn´t work. I got this error:

non-static initialization of a flexible array member

I can compile the code when I change Obj into a static initialization

static MyStruct Obj = FillStruct(1, 2, 3, 4, 5);

but now the debugger gives me some weird informations about this object.

enter image description here

So how can I solve this problem? I´m looking for a solution to intialize this struct during compile time on an embedded device (AVR MCU). In this case I can´t use solutions with malloc etc. And the data in this struct is constant and got stored in the program memory. So the application only reads this data.

Upvotes: 0

Views: 310

Answers (1)

Kampi
Kampi

Reputation: 1891

I found one solution. The compiler throw this error, because I declared and initialize the structure inside of main. Moving this segment out of main will solve this problem.

The final solution looks like this:

#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) 2 * N
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)

#define FillStruct(...)     { .Length = VA_NARGS(__VA_ARGS__), .Data = {__VA_ARGS__} }

const MyStruct Obj = FillStruct(1, 2, 3, 4, 5);

int main(void)
{
   int B = Obj.Data[2];
   // B = 3
}

So everything is fine now. It seems that the wrong debugger output also appears with const keyword. But now I can remove const and static to get the corrent Length (but not Data) in the structure.

Upvotes: 1

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