Struct EndpointDescriptor 

#[repr(C)]
pub struct EndpointDescriptor {
    pub fn_ptr: fn(u64) -> u64,
    pub index: u8,
    pub arg_registers: u8,
    pub arg_meta: u8,
    pub _pad: [u8; 5],
}

Descriptor written into the .nub.endpoints ELF section by the endpoint attribute macro. nub-linker reads this section at link time and turns each record into a nub_program::ProgramBlob::endpoints entry.

Layout is #[repr(C)] so the linker can decode the section as a flat array of fixed-size records. On RISC-V64 the function pointer occupies 8 bytes, followed by 8 bytes of metadata, for a total stride of 16 bytes.

Fields

fn_ptr: fn(u64) -> u64

RISC-V address of the endpoint function. The linker maps this to a PVM PC via its instruction-mapping table.

index: u8

Endpoint index (key in the program’s endpoints map).

arg_registers: u8

Number of register args the caller supplies.

arg_meta: u8

Opaque metadata byte, passed through to nub_program::Endpoint::arg_meta. nub does not interpret it; a personality may (JAVM reads it as the arg-cnode size).

_pad: [u8; 5]

Reserved for alignment / future expansion.