fixed32 not working as expected => const. 4 bytes

[mp1609]

Hi there,

I am using a header, to identify what communication is done. So I thought, when I use "fixed32" for all of my header data, my header should have the same size (3x4byte = 12bytes) all the time, independet of the values.

Cargo.toml:

[...]

[dependencies]
prost = "0.13.4"

[build-dependencies]
prost-build = "0.13.4"

So my idea is, that every message has the "MsgHeader" so, I know on every message which type it is.

So here is my definition of the header:

backend.proto

syntax = "proto3";

package backend;

message MsgHeader {
    fixed32  pkgType = 1;   // package type
    fixed32  pkgNr   = 2;   // package number
    fixed32  pkgSize = 3;   // package size
}

message PkgHeader {    
    MsgHeader msgHeader             = 1;
}

So my generated rust file looks like:

// This file is @generated by prost-build.
#[derive(Clone, Copy, PartialEq, ::prost::Message)]
pub struct MsgHeader {
    /// package type
    #[prost(fixed32, tag = "1")]
    pub pkg_type: u32,
    /// package number
    #[prost(fixed32, tag = "2")]
    pub pkg_nr: u32,
    /// package size
    #[prost(fixed32, tag = "3")]
    pub pkg_size: u32,
}
#[derive(Clone, Copy, PartialEq, ::prost::Message)]
pub struct PkgHeader {
    #[prost(message, optional, tag = "1")]
    pub msg_header: ::core::option::Option<MsgHeader>,
}

In my understanding, the struct PkgHeader must have a minimum of 3x4bytes but also a constant size.

So when I use the code in my main:

use prost::Message;

mod backend {    
    include!("protobuf/backend.rs");
}

fn main() { 
    let mut pkg_header = backend::PkgHeader {
        msg_header: Some(backend::MsgHeader {
            pkg_type : 0,
            pkg_nr   : 0,
            pkg_size : 0,
        }),
    };

    let minPkgSize = pkg_header.encode_to_vec().len();
    eprintln!("minPkgSize {}", minPkgSize); // <<<<<<<<< the size is here 2


    let mut pkg_header2 = backend::PkgHeader {
        msg_header: Some(backend::MsgHeader {
            pkg_type : 123456789,
            pkg_nr   : 123456789,
            pkg_size : 123456789,
        }),
    };

    let minPkgSize2 = pkg_header2.encode_to_vec().len();

    eprintln!("minPkgSize2 {}", minPkgSize2); //  <<<<<<<<<  the size is here 17
}

Perhaps a noob question, sorry for that, but I have no idea whats wrong.

Also the definitions says alway 4bytes:
https://stackoverflow.com/questions/59195191/when-to-use-fixed-value-protobuf-type-or-under-what-scenarios