fun

    Once you bind it, the function is available inside the C type as if it was a class method:

    1. C.cos(1.5) # => 0.0707372

    You can omit the parentheses if the function doesn’t have parameters (and omit them in the call as well):

    If the return type is void you can omit it:

    1. lib C
    2.   fun srand(seed : UInt32)
    3. end
    4. C.srand(1_u32)

    You can bind to variadic functions:

    Function names in a lib definition can start with an upper case letter. That’s different from methods and function definitions outside a lib, which must start with a lower case letter.

    Function names in Crystal can be different from the C name. The following example shows how to bind the C function name SDL_Init as LibSDL.init in Crystal.

    1. lib LibSDL
    2.   fun init = SDL_Init(flags : UInt32) : Int32
    3. end

    The C name can be put in quotes to be able to write a name that is not a valid identifier:

    This can also be used to give shorter, nicer names to C functions, as these tend to be long and are usually prefixed with the library name.

    Types in C Bindings

    • Primitive types (, …, Int64, UInt8, …, UInt64, Float32, Float64)
    • Pointer types (Pointer(Int32), which can also be written as Int32*)
    • Function types (Function(Int32, Int32), which can also be written as Int32 -> Int32)
    • Other struct, union, enum, type or alias declared previously.
    • Void: the absence of a return value.
    • : similar to Void, but the compiler understands that no code can be executed after that invocation.
    • Crystal structs marked with the @[Extern] annotation

    Refer to the for the notation used in fun types.

    The standard library defines the LibC lib with aliases for common C types, like int, short, size_t. Use them in bindings like this:

    1. lib MyLib
    2. end

    Note

    The C char type is UInt8 in Crystal, so a char* or a const char* is UInt8*. The Char type in Crystal is a unicode codepoint so it is represented by four bytes, making it similar to an Int32, not to an UInt8. There’s also the alias LibC::Char if in doubt.