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?

if 18, 36 or 54 are only options, then I would probably just write it with some #ifdefs. — j_kubik
– j_kubik, Commented Jun 18, 2014 at 4:32
@xmojmr can you give example how we initializing above structure with memset? — Jayesh Bhoi
– Jayesh Bhoi, Commented Jun 18, 2014 at 4:42

Sanket Parmar · Accepted Answer · 2014-06-18 04:48:47Z

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

Community · Accepted Answer · 2017-05-23 12:14:31Z

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

I knew there had to be a preprocessor solution. The issue started with #define MAX *something*; a problem born in the preprocessor. The solution had to come from there too. ;) And as I suspected, it's loathsome (yet I can't look away)! +1 for not letting me down. :)

AnT stands with Russia · Accepted Answer · 2014-06-18 07:29:16Z

Well, there's is no direct and immediate syntax in standard C to specify an initializer that would do what you want. If you wanted to initialize the whole thing with zeros, then = { 0 } would work regardless of size, but that 0xff makes it a completely different story. GCC compiler supports a non-standard extension that works in such cases (see Sanket Parmar's answers for details), but alas it is not standard.

There's also a non-standard memcpy hack that is sometimes used to fill memory regions with repetitive patterns. In your case it would look as follows

my_struct strct_arr[MAX] = { { 0xff, 0, 0 } }; memcpy(strct_arr + 1, strct_arr, sizeof strct_arr - sizeof *strct_arr);

But this is a hack, since it relies on memcpy doing its copying in byte-by-byte fashion and in strictly left-to-right direction (i.e. from smaller memory addresses to larger ones). However, that's not guaranteed by the language specification. If you want to "legalize" this trick, you have to write your own version of my_memcpy that works in that way specifically (byte-by-byte, left-to-right) and use it instead. Of course, this is formally a cyclic solution that is not based entirely on initializer syntax.

Even if this memcpy "works" it would be because the implementation of memcpy has a loop in it, so you lose nothing performance-wise by explicitly putting the loop in your own code.

Community · Accepted Answer · 2017-05-23 12:23:35Z

Paraphrasing Jonathan Leffler's solution:

struct my_struct { char c, int a; int b; } #define MAX 135 #define INIT_X_1 { 0xff, 0, 0 } #define INIT_X_2 INIT_X_1, INIT_X_1 #define INIT_X_4 INIT_X_2, INIT_X_2 #define INIT_X_8 INIT_X_4, INIT_X_4 #define INIT_X_16 INIT_X_8, INIT_X_8 #define INIT_X_32 INIT_X_16, INIT_X_16 #define INIT_X_64 INIT_X_32, INIT_X_32 #define INIT_X_128 INIT_X_64, INIT_X_64 struct my_struct strct_arr[MAX] = { #if (MAX & 1) INIT_X_1, #endif #if (MAX & 2) INIT_X_2, #endif #if (MAX & 4) INIT_X_4, #endif #if (MAX & 8) INIT_X_8, #endif #if (MAX & 16) INIT_X_16, #endif #if (MAX & 32) INIT_X_32, #endif #if (MAX & 64) INIT_X_64, #endif #if (MAX & 128) INIT_X_128, #endif };

Mr Wildplasser, This is the way I have tried it my self after some struggle. But the thing is, I cannot restrict my initialization up to 54 or 62 or even 128. It's really hard to get an idea on MAX number value which will be varied by the user configuration. So, I have created a perl script which will generate one macro based on the configured MAX value. In this way, I don't make C code burden in run time.
Well, it is not that hard to extend this solution to 512, 1024, ... and beyond. Everery doubling will only cost you a few (four) extra lines. But codegeneration is always an option.

Jonathan Leffler · Accepted Answer · 2014-06-18 14:36:58Z

Just for sake of variety, since you know the array will be a multiple of 18, you could use something like this:

#define INIT_X_1 { 0xff, 0, 0 } #define INIT_X_3 INIT_X_1, INIT_X_1, INIT_X_1 #define INIT_X_9 INIT_X_3, INIT_X_3, INIT_X_3 #define INIT_X_18 INIT_X_9, INIT_X_9 my_struct strct_arr[MAX] = { INIT_X_18, #if MAX > 18 INIT_X_18, #if MAX > 36 INIT_X_18, #endif #endif };

This will work without needing C99 support (it would even work with pre-standard C), GCC extensions, or Boost Preprocessor library. In every other respect, the other solutions are better.

I see that you have considered it up to 54 times. I am afraid that if my array size is 64, will it initialize 64 times with structure values. Can you give any possibilities if I need to continue up to the max value, which is a configurable value?
Use wildplasser's generalization of my solution, as long as you have an upper bound to work with. Otherwise, you'll need to use an extra-linguistic mechanism such as you mention in your comments below his answer. I tailored my answer to the multiples of 18 you mentioned in your question. Failing that, consider using a single initializer and copying that to your array at runtime. It won't take long compared to everything else that you're going to do.

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