Module libffi::low::type_tag

Type tags used in constructing and inspecting ffi_types.

For atomic types this tag doesn’t matter because libffi predeclares an instance of each one. However, for composite types (structs and complex numbers), we need to create a new instance of the ffi_type struct. In particular, the type_ field contains a value that indicates what kind of type is represented, and we use these values to indicate that that we are describing a struct or complex type.

Examples

Suppose we have the following C struct:

struct my_struct {
    uint16_t f1;
    uint64_t f2;
};

To pass it by value to a C function we can construct an ffi_type as follows using type_tag::STRUCT:

use std::ptr;
use libffi::low::{ffi_type, types, type_tag};

let mut elements = unsafe {
    [ &mut types::uint16,
      &mut types::uint64,
      ptr::null_mut::<ffi_type>() ]
};

let mut my_struct: ffi_type = Default::default();
my_struct.type_ = type_tag::STRUCT;
my_struct.elements = elements.as_mut_ptr();