Berry FFI questions

[Conaclos]

Hi! And thanks for this awesome project! I am considering to use Berry for a low-tech project with a very limited memory (64Ko of RAM and 2M of flash), but good computation resources.

The documentation is really helpful, however there is some gray area.
Here some of my questions:

1. What is the second parameter (argc) of be_pcall? Is it an index in the virtual stack?
2. Is it possible to push some values on the virtual stack and call a berry function with the previously pushed values?
   As an example, I would like to push a string and then call print with this string.
3. More generally how can I interact with the virtual stack in a berry script?
4. How can I create native closure?
5. What is the difference between an error code BE_SYNTAX_ERROR/BE_IO_ERROR and an exception named syntax_error/io_error? If I get an error code BE_SYNTAX_ERROR/BE_IO_ERROR, have I the guarantee that the string syntax_error/io_error is on the virtual stack?

Thanks!

[s-hadinger]

• What is the second parameter (argc) of be_pcall? Is it an index in the virtual stack?

The second argument in be_pcall is the number of arguments passed through the virtual stack to the function. On return, all arguments are still on the stack but the slot that contained the function is replaced with the return value.

Example, let's say the function takes 2 arguments:

• push the function
• push arg1
• push arg2
• call be_pcall(vm, 2)
• result is in slot -3
• be_pop(vm, 3) to clear the stack from all 3 values

• Is it possible to push some values on the virtual stack and call a berry function with the previously pushed values?
  As an example, I would like to push a string and then call print with this string.

I'm not sure to understand. Just consider what I posted above, it should make it clearer about what you can and can't do

• More generally how can I interact with the virtual stack in a berry script?
  No you can't directly. The arguments are passed to your function's argument list. There is a special construct if you have an non-predictable arbitrary length number of arguments. If you define more arguments than sent by the caller, the extraneous are set to nil

• How can I create native closure?

I have never done it. You have some examples in be_maplib.c in function m_iter where it creates a native closure with upvals.

5. What is the difference between an error code BE_SYNTAX_ERROR/BE_IO_ERROR and an exception named syntax_error/io_error? If I get an error code BE_SYNTAX_ERROR/BE_IO_ERROR, have I the guarantee that the string syntax_error/io_error is on the virtual stack?

I don't quite remember the details. be_pcall (protected call) will return the error code like BE_IO_ERROR but not throw an exception. On the contrary be_call is not protected and will throw the exception.

[Conaclos]

The second argument in be_pcall is the number of arguments passed through the virtual stack to the function. On return, all arguments are still on the stack but the slot that contained the function is replaced with the return value.

Thanks! This is more clear now.
I managed to call a native function with some parameter using be_pushntvfunction.

However, I still don't know how to call a function defined in a berry script.
I don't see any be_push_berry_function_name.
Am I missing something?

[s-hadinger]

There is no direct way. You need to find and push the berry function on top of the stack.

One way would be to use be_getglobal(vm, "func_name"); if the name is global.

[Conaclos]

Thanks! This was really helpful!