This library provides a simplified and semi-automated way to map C functions to Berry functions with minimal effort and minimal code size.
This library was originally designed for LVGL mapping to Berry, which implies mapping of hundreds of C function. It was later extended as a generalized `C` mapping mechanism.
Warning: for this library to work, the `C` stack needs to have the same size for `int` and `void*` pointers (all 32 bits or all 64 bits). Otherwise the layout of the calling stack is too complex to handle. On ESP32 and most embedded 32 bits systems, this should not be an issue.
## Quick example
Let's create a simple module skeleton with 3 functions:
-`addint`: simple function that adds two ints
-`ftoc`: converts Fahrenheit real to Celsius real
-`yesno` that transforms an int into a constant string
Below we have the three pure `C` functions that we want to map:
``` C
/* sum two ints */
int addint(int a, int b)
{
return a + b;
}
/* returns 'yes' or 'no' depending on v being true */
const char* yesno(int v)
{
return v ? "yes" : "no";
}
/* fahrenheit to celsius, forcing to float to avoid using double libs */
`bool`|`intptr_t` with value `1` if `true` or `0` if `false`
`string`|`const char*` C string `NULL` terminated (cannot be modified)
`nil`|`void*` with value `NULL`
`comptr`|`void*` native pointer
`instance` of `bytes`|In case of an instance of type `bytes` or any subclass, the argument is converted to the pointer to the internal buffer `_buffer`. This is equivalent to a `C``struct`
`instance` of any other class|In case of an instance, the engine search for an instance variable `_p` or `.p`, and applies the conversion recursively.<br>This is handy when an instance contains a pointer to a native C structure as `comptr`.
### Argument type checking
This phase is optional and checks that there is the right number of arguments and the right types, according to the type definition described as a string.
Note: callbacks need an explicit type definition to be handled correctly
Argument type|Berry type expected
:---|:---
`i`|`int`
`f`|`real` (if arg is `int` it is silently converted to `real`)
`b`|`bool` (no implicit conversion, use `bool()` to force bool type)
`s`|`string`
`c`|`comptr` (native pointer)
`.`|**any** - no type checking for this argument
`-`|**skip** - skip this argument (handy to discard the `self` implicit argument for methods)
`(<class>)`|`instance` deriving from `<class>` (i.e. of this class or any subclass
`^<callback_type>^`|`function` which is converted to a `C` callback by calling `cb.make_cb()`. The optional `callback_type` string is passed as second argument to `cb.make_cb()` and Berrt `arg #1` (typically `self`) is passed as 3rd argument<br>See below for callbacks
`[<...>]`|arguments in brackets are optional (note: the closing bracket is not strictly necessary but makes it more readable)
Example:
`-ib(lv_obj)` means: 1/ skip arg1, 2/ arg2 must be `int`, 3/ arg3 must be `bool`, 4/ arg4 must be an instance of `lv_obj` or subclass and its attribute `_p` or `.p` is passed. The final `C` function is passed 3 arguments.
`ii[.]` means: the first two arguments must be `int` and there can be an optional third argument of any type.
### Return type
The return type defines how the `C` result (intptr_t) is converted to any other Berry type.
Return type definition|Berry return type
:---|:---
`''` (empty string)|`nil` - no return value, equivalent to `C``void`
`s`|`string` - `const char*` is converted to `C` string, a copy of the string is made so the original string can be modified
`b`|`bool` - any non-zero value is converted to `true`
`c`|`comptr` - native pointer
`<class>` or `<module.class>`|Instanciate a new instance for this class, pass the return value as `comptr` (native pointer), and pass a second argument as `comptr(-1)` as a marker for an instance cretion (to distinguish from an simple encapsulation of a pointer)
`+<varable>` of `=<variable>`|Used in class constructor `init()` functions, the return value is passed as `comptr` and stored in the instance variable with name `<variable>`. The variables are typically `_p` or `.p`. `=` prefix indicates that a `NULL` value is accepted, while the `+` prefix raises an exception if the function returns `NULL`. This is typically used when encapsulating a pointer to a `C++` object that is not supposed to be `NULL`.
The library introduces a new module `cb` used to create `C` callbacks that are mapped to Berry functions (native functions, native closures, Berry functions or closures).
Due to the nature of `C` callbacks, each callback must point to a different `C` address. For this reason, the library pre-defines `20` callback addresses of stubs. This should be enough for most use-case; increasing this limit requires to define additional stubs and increases slightly the code size.
The low-level `cb.gen_cb()` takes a Berry callable and returns a `C` callback address. The callback supports up to 5 `C` parameters. For each call, there are 5 Berry arguments passed as `int` converted from `intptr_t`. Each argument can be converted to a `comptr` with `introspect.toptr()` or converted to a `bytes()` structure.
```
> def inc(x) return x+1 end
> import cb
> print(cb.gen_cb(f))
<ptr:0x40148c18>
```
It is easy to convert an argument to a `bytes()` or `cbytes()` object. In such case, you need to create a `bytes()` object with 2 arguments: first a `comptr` pointer, second the buffer size (note: the buffer will have a fixed size). The `bytes()` buffer is mapped to the `C` structure in memory and can be read or written as long as the address is valid.
```
> # let's assume the callback receives as first argument a pointer to a buffer of 8 bytes
> def get_buf(a)
import introspect
var b = bytes(introspect.toptr(a), 8)
print(b)
end
> var c = cb.gen_cb(get_buf)
> # let's try manually the conversion with a dummy address
> import introspect
> get_buf(introspect.toptr(0x3ffb2340))
bytes('BD9613807023FB3F')
```
However some callbacks need more information to reuse the same callback in different locations. The `C` mapper will actually call `cb.make_cb(closure, name, self)` and let modules the opportunity to register specific callback handlers.
You can register a hanlder with `cb.add_handler(handler)` where `handler` receives the 3 following arguments `handler(cb:function, name:string, obj:instance)`. The handler must return a `comptr` if it has sucessfully allocated a callback, or return `nil` if it ignores this callback (based on its name for example). `gen_cb()` is called eventually if no handler handled it.
