C - Creating an initialisation macro for an array of structs

Question

I would like to do this with a macro:

typedef struct _TIMER_llist {
    struct _TIMER_llist *next;
    uint32_t time;
    const int id;
} TIMER_llist;

TIMER_llist _timer_llists[] =
{
    { .id = 1, .next = &_timer_llists[1] },
    { .id = 2, .next = &_timer_llists[2] },
    { .id = 3, .next = &_timer_llists[3] },
    { .id = 4, .next = &_timer_llists[4] },
    { .id = 5, .next = &_timer_llists[5] },
    { .id = 6, .next = &_timer_llists[6] },
    { .id = 7, .next = &_timer_llists[7] },
    { .id = 8, .next = &_timer_llists[8] },
    { .id = 9, .next = &_timer_llists[9] },
    { .id = 10, .next = &_timer_llists[10] },
    { .id = 11, .next = &_timer_llists[11] },
    { .id = 12, .next = &_timer_llists[12] },
    { .id = 13, .next = 0 } };

//This won't work, because const .id
TIMER_llist _timer_llists[1];
void init() {
    _timer_llists[0].id = 1;
    _timer_llists[0].next = 0;
}

Instead of writing a line for each buffer entry I would like to use a

#define NUMBER_ENTRIES 13

as this gets really unhandy, if I want to do 64 entries or so...

Unfortunatly I don't have any experience with the C preprocessor, so I'm curious if:

there is a better non macro solution
there is an easy way with gcc to accomplish this macro operation

Besides, and that is more a theoretical aspect, is execution speed. AfaIk the example in the first post would be just a memcpy(.data[?], _timer_llists, sizeof(_timer_llists)), while a programmatical approach would require at least a counter, maybe an if statement for the last case, and a function overhead (ok, maybe inline). On the other hand this would save space in .data.

In this particular case I am using gcc-avr, but this question came to my mind repeatedly and I would like to have an universal approach.

No, there is no easy way. C preprocessor is not supporting looping or recursion. There are some X-macro tricks, but they are not universal. — Eugene Sh.
– Eugene Sh., Commented Nov 1, 2017 at 14:48
You could initialize the array programmatically with 3-4 lines of code. — Jabberwocky
– Jabberwocky, Commented Nov 1, 2017 at 14:53
You can do it with boost.preproceasor (yes you can use boost.preprocessor with C). — n. m. could be an AI
– n. m. could be an AI, Commented Nov 1, 2017 at 15:05
One complicating factor is the null pointer for the last entry; it is different from the rest. Any scheme will need to account for that. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 1, 2017 at 18:22

H Walters · Accepted Answer · 2017-11-02 02:30:24Z

3

If you use boost preprocessor (see n.m.'s comment), you can simply do this:

#include <boost/preprocessor/enum.hpp>
#include <boost/preprocessor/arithmetic/inc.hpp>
#include <boost/preprocessor/arithmetic/dec.hpp>

#define TIMER_ENTRY(_, n, data) \
   { .id = BOOST_PP_INC(n), .next = &data[BOOST_PP_INC(n)] }
#define DECL_TIMER_LLIST(NAME_, COUNT_) \
   TIMER_llist NAME_[] = \
   { \
      BOOST_PP_ENUM(BOOST_PP_DEC(COUNT_), TIMER_ENTRY, NAME_) \
      , { .id = COUNT_, .next = 0 } \
   }

BOOST_PP_ENUM is almost everything you need.

BOOST_PP_INC and BOOST_PP_DEC supplement the special start case (start count at 1 instead of default 0)/end case (count up to n-1 then add a different line).

Here's a "live" sample.

(FYI, unless you're programming on hardware you found in a time capsule, I wouldn't even take the overhead of programmatic alternatives into consideration when comparing approaches).

edited Nov 2, 2017 at 2:30

answered Nov 2, 2017 at 1:57

H Walters

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Comments

Jabberwocky · Accepted Answer · 2017-11-01 15:14:52Z

If you really need to do a initialisation rather than doing the initialisation programmatically the best you can do with macros is something like this:

#define TIMERINIT(a)      {.id = a, .next = &_timer_llists[a] },
#define END_TIMERINIT(a)  {.id = a, .next = 0 },

TIMER_llist _timer_llists[] =
{
  TIMERINIT(1)
  TIMERINIT(2)
  TIMERINIT(3)
  TIMERINIT(4)
  ...
  TIMERINIT(12)
  END_TIMERINIT(13)
};

It's not perfect but still better than writing everything by hand.

Now the programmatic solution (that's the easiest and most flexible solution IMO):

#define NUMBER_ENTRIES 13
TIMER_llist _timer_llists[NUMBER_ENTRIES];

int main()
{
  for (int i = 0; i < NUMBER_ENTRIES; i++)
  {
    _timer_llists[i].id = i + 1;
    _timer_llists[i].next = &_timer_llists[i + 1];
  }
  ...
}

For NUMBER_ENTRIES larger than approximately 8, the generated code will most likely be shorter with this solution.

Another approach would be to write a special program that generates the initialisation table by writing it into a .h file and include that .h file into your .c file. Your build process should then call that special program prior the comnpilation.

Lundin · Accepted Answer · 2017-11-01 15:16:54Z

You can use "X-macros". It is not necessarily an improvement over hard-coding everything, but it would look like this:

#define TIMER_INIT_LIST \
  X(1)   \
  X(2)   \
  X(3)   \
  X(4)   \
  X(5)   \
  X(6)   \
  X(7)   \
  X(8)   \
  X(9)   \
  X(10)  \
  X(11)  \
  X(12)  \
  X(13)  \

int main()
{
  TIMER_llist _timer_llists[] =
  {
    #define X(n) [n-1] = {.id = n, .next = &_timer_llists[n] },
      TIMER_INIT_LIST
    #undef X
  };

  return 0;
}

At the same time, I improved the code by using array index designated initializers, to ensure data integrity even if the X macro list is modified or unsorted.

The macros expand to something like this:

[0] = {.id = 1, .next = .next = &_timer_llists[1] },
[1] = {.id = 2, .next = .next = &_timer_llists[2] },
...

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