cannot use external const for array declaration

[mterber]

Describe the bug
The following code

@[CConst (binding = "MAX_FROM_C", header = "strb.h")]
extern const MAX: nat8

@[EntryPoint]
activity Main () ()
    var arr: [MAX]nat8
    await true
end

leads to this compile error:

~/git/str-test$ blechc bug.blc 
error: The expression MAX must be a compile-time constant.
 --> bug.blc:6:15 [typing]

Means the external C constant cannot be used for declaring the array size in Blech.

Expected behaviour
I would expect that above approach works since MAX actually is a compile-time constant.

Or is this a deliberate restriction since Blech cannot verify that MAX is actually constant at compile time?

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

[FriedrichGretz]

You already gave the answer. :-) Since by design we do not parse and interpret C code we cannot know for sure what value MAX has. Only local Blech constants can be used in this declaration.

[mterber]

Okay, thanks for the quick reply! :-)

Does that mean that there is no way in sharing the size of an array between Blech and C?

• If I define the array size in C I cannot use it for the array declaration in Blech.
• On the other hand, if I define the array size in Blech using const there is be no representation in memory which I could use in C.

[FriedrichGretz]

But you could define the size in Blech using param. Would that be a solution?

[mterber]

Unfortunately not – at least not as of my understanding. I tried the following:

1. Use param directly:

module

param SIZE: nat8 = 42

struct Test
    var arr: [SIZE]nat8
end

This leads to

~/tmp$ blechc test.blc 
error: The expression SIZE must be a compile-time constant.
 --> test.blc:6:15 [typing]

2. Use const and param in combination:

module

const SIZE_FOR_BLECH: nat8 = 42
param SIZE_FOR_C: nat8 = SIZE_FOR_BLECH

struct Test
    var arr: [SIZE_FOR_BLECH]nat8
end

This compiles, but still does not solve the problem. The param is generated as a static variable inside test.c:

/*
** parameters
*/

static blc_nat8 blc_01test01_SIZE_FOR_C = 42;

This means I cannot access blc_01test01_SIZE_FOR_C from outside, e.g. another C module, and – even if I could – I cannot use a variable for declaring the size of an array in C in a struct (which does not make any sense by the way), e.g.

int SIZE = 6;

struct test {
  char str[SIZE];
};

leads to

main.c:13:8: error: variably modified ‘str’ at file scope
   13 |   char str[SIZE];
      |        ^~~

[FriedrichGretz]

Oh yes, sorry, you are right.
We cannot use param because their intention was to be re-parameterized in a binary.

The C error makes sense in my opinion. SIZE is a global variable which can be changed after compile time and then the size of the str array would not match the value of SIZE anymore. It is then not clear what size the type "struct test" has.

So sharing constants between Blech and C is an issue at the moment. I'll reopen.

[FriedrichGretz]

One possible way to fix this is to generate a const declaration into the .h file even though we substitute their occurrences in the generated code.