Good way around initializer element is not constant error in C?

Question

Getting an initializer element is not constant compile error when using avr-gcc. Is there any good way to do what I'm trying to do here?

file.c

#include "file.h"
#include "../notes/octave_two_notes.h"

//F2 is defined in octave_two_notes.h
//This is the file giving me the compilation error
struct Song_Note F2_500ms = { .note = F2, .duration_ms = 500 };

song_note.h

#include "note.h"

struct Song_Note {
    struct Note note;
    uint16_t duration_ms;
} Song_Note;

octave_two_notes.h

extern struct Note F2;

octave_two_notes.c

#define CALC_CTC_FREQ(clock_freq, prescaler, note_freq_hz) ( (uint32_t) clock_freq / ( (uint16_t) note_freq_hz * (uint16_t) prescaler * 2) - 1)

struct Note F2 = {.freq_hz = 87, .ocr_val = CALC_CTC_FREQ(16000000, 8, 87)};

note.h

#include <stdint.h>

struct Note {
    uint16_t freq_hz;
    uint16_t ocr_val;
} Note;

Unfortunately C doesn't allow using the value of a variable as the initializer for another variable at file scope; so you either have to redo all of this to use macros; or move some of it to block scope — M.M, Oct 23 '17 at 05:41
@AjayBrahmakshatriya file scope variables are already static; and adding `const` will make no difference — M.M, Oct 23 '17 at 05:50
@AjayBrahmakshatriya m.m means "static storage duration". Internal linkage with `static` doesn't change anything here. Neither does `const`. — Antti Haapala -- Слава Україні, Oct 23 '17 at 06:40
@AjayBrahmakshatriya no, instead there should be an external declaration, since the `const` etc approach wouldn't work anyway. — Antti Haapala -- Слава Україні, Oct 23 '17 at 06:44
The only option I can see right now is change the struct to contain a pointer to struct Note and use `.note = &F2` — Ajay Brahmakshatriya, Oct 23 '17 at 06:47
@AnttiHaapala I understand your point now on why `const` wouldn't work. I was misreading the comments. Thanks. — Ajay Brahmakshatriya, Oct 23 '17 at 06:53

score 0 · Accepted Answer · answered Oct 23 '17 at 09:26

0

First of all, these variables should all be declared as const. Mainly because you want them in flash, not in RAM.

const will unfortunately not solve the main problem though, because the C language doesn't even regard const-qualified variables as constant expressions. And variables declared at file scope have "static storage duration" and must therefore be initialized with a constant expression.

The only solution C provides is pretty much to declare the initializer list as an inelegant macro, in this case it has to be inside a header file:

// header file

#define CALC_CTC_FREQ ...

#define F2_INIT                              \
{                                            \
  .freq_hz = 87,                             \
  .ocr_val = CALC_CTC_FREQ(16000000, 8, 87)  \
}

and in the .c file, use that macro:

// c file
const struct Note F2 = F2_INIT;

and then in a .c file elsewhere:

const struct Song_Note F2_500ms = { .note = F2_INIT, .duration_ms = 500 };

Note that the CALC_CTC_FREQ part should be fine, everything inside it is a constant expression and it is evaluated at compile-time.

answered Oct 23 '17 at 09:26

Lundin

195,001
40
254
396

Alternatively, one could use convert `Song_Note.note` into a pointer... though this would use more memory per "_complete `Song_Note`_" and add a level of indirection, it may end up using less memory (depending on pointer size), as presumably there are many "_notes_" per "_song_". – Attie Oct 23 '17 at 09:32
@Attie Worse, it would add run-time overhead whenever the struct is accessed. I find this hard to justify even though it isn't much overhead. Please note that AVR is one of the least code-effective CPUs still manufactured. Every CPU tick helps. – Lundin Oct 23 '17 at 09:42
The run-time cost is what I was referring to with "_add a level of indirection_"... I had rather presumed that a simple 8-bit micro would have had less of an issue with such things due to the lack of any pipelining, cache, etc... Happy to be put straight though, and appreciate the comment, thanks :-) – Attie Oct 23 '17 at 09:46
Your solution solves the OP's problem, but: Unfortunately, the only thing you achieve that way is have the values *both* in Flash and in RAM (on an AVR, at least). If you really want to put these values into Flash *only*, you need to add a PROGMEM attribute. – tofro Oct 23 '17 at 16:59

Good way around initializer element is not constant error in C?

1 Answers1