How to initialize an array of structures with variable index

Question

I have structure like below

typedef struct
{
    int a;
    int b;
    int c;
} my_struct;

and in another file I have declared a variable of this my_struct type, like below.

my_struct strct_arr[MAX];

Where MAX is a macro which is a configurable value that is a multiple of 18 (18 or 36 or 54 and so on.. it may go up to 18*n times).

I have to initialize the structure with {0xff,0,0}. So, how to initialize whole array of structure my_struct strct_arr[MAX]; with my initial values without using any kind of loops.

I am expecting the output as below:

my_struct strct_arr[MAX]={
    {0xff,0,0},
    {0xff,0,0},
    {0xff,0,0},
    {0xff,0,0},
    …
};

But without knowing MAX value, how to initialize it?

Answer 1

There is GCC extension for this. Try this

#define MAX 18
my_struct strct_arr[MAX]={ [0 ... (MAX - 1)] = {0xff,0,0}};

Check https://gcc.gnu.org/onlinedocs/gcc-4.2.1/gcc/Designated-Inits.html

Answer 2

Yes, this is possible using the C preprocessor!

#include <stdio.h>
#include <boost/preprocessor/repetition/repeat.hpp>

#define INITS(z, n, t) { 0xFF, 0, 0 },
#define REP(item, n) BOOST_PP_REPEAT(n, INITS, item)

#define MAX 123

typedef struct { int a,b,c; } my_struct;

my_struct ms[] = { REP(, MAX) };

int main()
{
    // Check it worked
    printf("%d\n", (int)(sizeof ms / sizeof *ms));
}

Note: boost is a package of C++ stuff, however the boost/preprocessor just uses the preprocessor features which are common to both languages. If your implementation doesn't allow this #include by default, you can find a copy of repeat.hpp from the boost source code.

Also, BOOST_PP_REPEAT defaults to a max of 256. If your MAX is bigger than this, you can edit repeat.hpp to allow bigger values, it should be obvious what to do from there.

Note: this post describes a system for recursive macro that would not require the same sort of implementation as repeat.hpp uses, but I haven't been able to get it to work.

Credit: this post