Is function return an array possible in Vivado HLS?

Question

A function like that：

int * getRandom( ) {

   static int  r[10];
   int i;

   /* set the seed */
   srand( (unsigned)time( NULL ) );

   for ( i = 0; i < 10; ++i) {
      r[i] = rand();
      printf( "r[%d] = %d\n", i, r[i]);
   }

   return r;
}

Is this one possible to be used in Vivado HLS? If possible, how can I initialize an array of unknown size because I cannot use static and malloc anymore?

You cannot, in standard C, return an array from a function — you can return a pointer OK (so the code shown is permissible, though it clearly has re-entrancy and threading isssues). If you can't use `static` or `malloc()` et al, then you need to pass the array to the function for it to fill in instead of returning the array. Then it is the caller's responsibilty to allocate the space. — Jonathan Leffler, Aug 09 '16 at 03:08
See also [`srand()` — why call it only once](http://stackoverflow.com/questions/7343833/srand-why-call-it-only-once/). — Jonathan Leffler, Aug 09 '16 at 03:09
@JonathanLeffler So you mean i can set an global array as function arguments and give value to each element so i can get the array without using static and malloc? — Varrian, Aug 09 '16 at 03:26
Yes, or a local array, or an any-other-type of array you care to think of. I think the appropriate implementation might be: `void getRandom(int n_vals, int *i_vals) { for (int i = 0; i < n_vals; i++) i_vals[i] = rand(); }` but the possible variations are legion. You can reinstate the printing if you really want it; you can even call `srand()` if you really want to, but you should only call that once. You can then have `void somefunc(void) { int data[20]; getRandom(15, data); …use data…; }` or `static int data[20]; void somefunc(void) { getRandom(18, data); …use data…; }` or other variants. — Jonathan Leffler, Aug 09 '16 at 03:29

score 3 · Accepted Answer · edited May 23 '17 at 10:27

^{Converting comments into an answer.}

You cannot, in standard C, return an array from a function — you can return a pointer OK (so the code shown is permissible, though it clearly has re-entrancy and threading issues). If you can't use static or malloc() et al, then you need to pass the array to the function for it to fill in instead of returning the array. Then it is the caller's responsibility to allocate the space.

See also srand() — why call it only once.

So you mean I can set a global array as function arguments and give value to each element so I can get the array without using static and malloc?

Yes, or a local array, or an any-other-type of array you care to think of. I think the appropriate implementation might be:

void getRandom(int n_vals, int *i_vals)
{
    for (int i = 0; i < n_vals; i++)
        i_vals[i] = rand();
}

but the possible variations are legion. You can reinstate the printing if you really want it; you can even call srand() if you really want to (but you should only call that once). You can then use it like:

void somefunc(void)
{
    int data[20];
    getRandom(15, data);
    …use data…;
}

or

static int data[20];

void somefunc(void)
{
    getRandom(18, data);
    …use data…;
}

or other variants (such as not using static in front of the file-scope definition of data — converting it into a global variable). (Yes, you'd probably use 10 as in the question, or 20 as the amount of space in the array — but 15 and 18 are also OK values in their context.)

Well, I don't know if I totally agree: in **Vivado HLS** you can use the option for the array "partition -> complete". In this way you can create an hardware accelerator that passes all the data in parallel (to another hw accelerator for example). I want to remark that we are speaking about FPGA: this is C language used to describe and to create HW — Leos313, Oct 20 '16 at 09:48

Is function return an array possible in Vivado HLS?

1 Answers1