xous 0.9.70

System call interface for Xous
Documentation 
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use core::sync::atomic::{AtomicU32, Ordering};

// By making this repr(C), the layout of this struct becomes well-defined
// and no longer shifts around.
// By marking it as `align(4096) we define that it will be page-aligned,
// meaning it can be sent between processes.
#[repr(C, align(4096))]
struct ConnectRequest {
    name: [u8; 64],
    len: u32,
    _padding: [u8; 4096 - 4 - 64],
}

impl Default for ConnectRequest {
    fn default() -> Self { ConnectRequest { name: [0u8; 64], len: 0, _padding: [0u8; 4096 - 4 - 64] } }
}

impl ConnectRequest {
    pub fn new(name: &str) -> Option<Self> {
        let mut cr: ConnectRequest = Default::default();
        let name_bytes = name.as_bytes();

        // Ensure the bytes won't blow out the buffer
        if name_bytes.len() > 64 {
            return None;
        }

        // Copy the string into our backing store.
        for (&src_byte, dest_byte) in name_bytes.iter().zip(&mut cr.name) {
            *dest_byte = src_byte;
        }

        // Set the string length to the length of the passed-in String,
        // or the maximum possible length. Which ever is smaller.
        cr.len = 64usize.min(name.as_bytes().len()) as u32;

        // If the string is not valid, set its length to 0.
        if core::str::from_utf8(&cr.name[0..cr.len as usize]).is_err() {
            return None;
        }

        if cr.len == 0 { None } else { Some(cr) }
    }
}

/// Request a connection to the nameserver-managed `name`.
pub fn connect(name: &str) -> Option<crate::CID> {
    let mut request = ConnectRequest::new(name)?;
    let ns_cid = nameserver();
    let memory_range = unsafe {
        crate::MemoryRange::new(
            &mut request as *mut ConnectRequest as usize,
            core::mem::size_of::<ConnectRequest>(),
        )
        .unwrap()
    };
    let response = crate::send_message(
        ns_cid,
        crate::Message::new_lend_mut(
            6, /* BlockingConnect */
            memory_range,
            None,
            crate::MemoryAddress::new(request.len as usize),
        ),
    )
    .expect("unable to perform lookup");
    if let crate::Result::MemoryReturned(_, _) = response {
        let response_ptr = &request as *const ConnectRequest as *const u32;
        let result = unsafe { response_ptr.read() };

        if result == 0 {
            let cid = unsafe { response_ptr.add(1).read() };
            // let mut token = [0u32; 4];
            // token[0] = unsafe { response_ptr.add(2).read() };
            // token[1] = unsafe { response_ptr.add(3).read() };
            // token[2] = unsafe { response_ptr.add(4).read() };
            // token[3] = unsafe { response_ptr.add(5).read() };
            // println!("Successfully connected to {}. CID: {}, token: {:?}", name, cid, token);
            Some(cid)
        } else {
            let _error = unsafe { response_ptr.add(1).read() };
            // println!("Error connecting to {}. Type: {}  Code: {}", name, result, _error);
            None
        }
    } else {
        None
    }
}

/// Return the connection ID of the nameserver
pub(crate) fn nameserver() -> crate::CID {
    static NAMESERVER_CID: AtomicU32 = AtomicU32::new(0);

    let cid = NAMESERVER_CID.load(Ordering::Relaxed);
    if cid != 0 {
        return cid;
    }

    let cid = crate::connect(crate::SID::from_bytes(b"xous-name-server").unwrap()).unwrap();
    NAMESERVER_CID.store(cid, Ordering::Relaxed);
    cid
}