Add a udp program and a ping test script

ping.py

@@ -0,0 +1,18 @@
+import socket
+import sys
+import time
+
+sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+addr = ('localhost', 26099)
+sock.bind(addr)
+
+
+print("pinging...", file=sys.stderr)
+while True:
+        buf, raddr = sock.recvfrom(4096)
+        print("receive: " + buf.decode("utf-8"))
+        buf = "this is a ping to port 6200!".encode('utf-8')
+        sock.sendto(buf, ("127.0.0.1", 6200))
+        buf = "this is a ping to reply!".encode('utf-8')
+        sock.sendto(buf, raddr)
+        time.sleep(1)

user/src/bin/udp.rs

@@ -0,0 +1,45 @@
+#![no_std]
+#![no_main]
+
+use alloc::string::String;
+use user_lib::{connect, write, read};
+
+#[macro_use]
+extern crate user_lib;
+#[macro_use]
+extern crate alloc;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("udp test open!");
+    
+    let udp_fd = connect(10 << 24 | 0 << 16 | 2 << 8 | 2, 2001, 26099);
+
+    if udp_fd < 0 {
+        println!("failed to create udp connection.");
+        return -1;
+    }
+
+    let buf = "Hello rCoreOS user program!";
+
+    println!("send <{}>", buf);
+
+    write(udp_fd as usize, buf.as_bytes());
+
+    println!("udp send done, waiting for reply.");
+
+    let mut buf = vec![0u8; 1024];
+
+    let len = read(udp_fd as usize, &mut buf);
+
+    if len < 0 {
+        println!("can't receive udp packet");
+        return -1;
+    }
+
+    let recv_str = String::from_utf8_lossy(&buf[..len as usize]);
+
+    println!("receive reply <{}>", recv_str);
+
+    0
+}