Implementing UDP Networking in Wave: Direct recvfrom Syscall

[LunaStev]

#Wave Blog Link

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On September 5, 2025, Wave successfully achieved its first UDP network communication.
This post documents that milestone.

Although Wave is still in a pre-beta stage and currently runs only on Linux x86-64,
this experiment proves that Wave can directly interact with the operating system to perform network I/O.

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udp.mp4

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recvfrom in C

Normally, in C, receiving UDP packets looks like this:

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                 struct sockaddr *src_addr, socklen_t *addrlen);

This is a familiar libc function.
But internally, it’s just a thin wrapper that calls the Linux kernel’s syscall.

Inside glibc, the implementation is essentially:

return syscall(SYS_recvfrom, sockfd, buf, len, flags, src_addr, addrlen);

So recvfrom() is not magic — it’s just calling system call number 45.

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C vs Wave: Call Flow

C Call Flow

sequenceDiagram
    participant C as C Code
    participant L as glibc Library
    participant K as Linux Kernel
    participant N as Network Stack

    C->>L: recvfrom call
    L->>K: "syscall SYS_recvfrom"
    K->>N: forward packet
    N-->>K: data
    K-->>L: result (n bytes)
    L-->>C: result (n bytes)

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Wave Call Flow

sequenceDiagram
    participant W as Wave Code + inline asm
    participant K as Linux Kernel
    participant N as Network Stack

    W->>K: "mov rax=45"
    K->>N: forward packet
    N-->>K: data
    K-->>W: result (n bytes)

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👉 The only difference:

• C goes through glibc wrapper

• Wave talks to the kernel directly

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Final Wave Code

const AF_INET: i32 = 2;
const SOCK_DGRAM: i32 = 2;
const SYS_SOCKET: i64 = 41;
const SYS_BIND: i64 = 49;
const SYS_RECVFROM: i64 = 45;
const SYS_WRITE: i64 = 1;

fun main() {
    // 1. Create socket
    var sockfd: i64;
    asm {
        "mov rax, 41"      // SYS_SOCKET
        "syscall"
        out("rax") sockfd
        in("rdi") AF_INET
        in("rsi") SOCK_DGRAM
        in("rdx") 0
    }
    println("socket fd = {}", sockfd);

    // 2. Bind to port 8080
    var addr: array<i8, 16> = [2, 0, 0x1F, 0x90, 0,0,0,0, 0,0,0,0, 0,0,0,0];

    var ret: i64;
    asm {
        "mov rax, 49"      // SYS_BIND
        "syscall"
        out("rax") ret
        in("rdi") sockfd
        in("rsi") &addr
        in("rdx") 16
    }
    println("bind ret = {}", ret);

    // 3. Prepare buffer
    var buf: array<i8, 128>;
    var src: array<i8, 16>;
    var srclen: i32 = 16;

    // 4. Receive data
    var n: i64;
    asm {
        "mov rax, 45"      // SYS_RECVFROM
        "syscall"
        out("rax") n
        in("rdi") sockfd
        in("rsi") &buf
        in("rdx") 128
        in("r10") 0        // flags = 0 (blocking)
        in("r8")  &src
        in("r9")  &srclen
    }

    println("recvfrom got {} bytes", n);

    // 5. Write to stdout
    var ret2: i64;
    asm {
        "mov rax, 1"       // SYS_WRITE
        "syscall"
        out("rax") ret2
        in("rdi") 1
        in("rsi") &buf
        in("rdx") n
    }
}

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How It Works

1. Socket Creation
   Calls SYS_SOCKET (41) with parameters (AF_INET, SOCK_DGRAM) to create a UDP socket.

2. Binding
   Calls SYS_BIND (49) to bind the socket to port 8080.
   The addr array represents a raw sockaddr_in structure.

3. Receiving Data
   Calls SYS_RECVFROM (45) with correct register mapping:

   • rdi = sockfd

   • rsi = buffer pointer

   • rdx = buffer length

   • r10 = flags (0, blocking mode)

   • r8 = src_addr pointer

   • r9 = addrlen pointer

4. Writing Output
   Calls SYS_WRITE (1) to print the received data directly to stdout.

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Running the Program

1. Run Wave program:

   ./wavec run test/test61.wave

2. In another terminal, send a UDP packet:

   echo "hi" | nc -u 127.0.0.1 8080

3. Output:

   hi
   socket fd = 3
   bind ret = 0
   recvfrom got 3 bytes
   

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Significance

This experiment shows that Wave can now:

• Directly call Linux system calls

• Implement UDP networking without libc

• Receive real packets and interact with the OS network stack

Even though Wave is still pre-beta with no standard library,
we have demonstrated that it can already act as a system programming language.

Future extensions will include:

• sendto() for UDP sending

• connect() / accept() for TCP

• HTTP library built in Wave

• Asynchronous I/O for high-performance servers