It's ugly, but it passes

.clang-format

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+---
+BasedOnStyle: Google

.gitignore

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+cmake-build-*

0_smoketest/CMakeLists.txt

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+add_executable(0_smoketest main.cpp)

0_smoketest/main.cpp

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+//
+// Created by bspeice on 10/5/24.
+//
+#include <arpa/inet.h>
+#include <sys/epoll.h>
+#include <ifaddrs.h>
+#include <netdb.h>
+#include <unistd.h>
+
+#include <array>
+#include <iostream>
+#include <stdexcept>
+#include <vector>
+
+#define CHECK_SYSCALL(expr) [&](){ const auto ret = expr; if (ret < 0) { throw std::runtime_error(#expr); } else { return ret; } }()
+
+int main(int argc, char** argv) {
+  if (argc != 2) {
+    throw std::runtime_error("Usage: ./0_smoketest <interface>");
+  }
+
+  // Get the address tied to our intended interface
+  const auto interface = std::string_view(argv[1]);
+
+  ifaddrs* interface_addrs{};
+  CHECK_SYSCALL(getifaddrs(&interface_addrs));
+
+  while (interface_addrs) {
+    if (interface == interface_addrs->ifa_name && interface_addrs->ifa_addr->sa_family == AF_INET) {
+      break;
+    }
+    interface_addrs = interface_addrs->ifa_next;
+  }
+
+  if (!interface_addrs) {
+    throw std::runtime_error("Expected interface not found");
+  }
+
+  std::array<char, 1024> name_buffer{};
+  getnameinfo(interface_addrs->ifa_addr, sizeof(sockaddr_in), name_buffer.data(), name_buffer.size(), nullptr, 0, NI_NUMERICHOST);
+
+  auto* address = reinterpret_cast<sockaddr_in*>(interface_addrs->ifa_addr);
+  address->sin_port = htons(3490); // Port must be in network byte order
+
+  std::cout << "Address: " << std::string(name_buffer.data()) << ' ' << ntohs(address->sin_port) << '\n';
+
+  // Set up a socket
+  const auto socket_fd = CHECK_SYSCALL(socket(AF_INET, SOCK_STREAM, 0));
+  int enable = 1;
+  setsockopt(socket_fd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(enable));
+
+  CHECK_SYSCALL(bind(socket_fd, reinterpret_cast<sockaddr*>(address), sizeof(sockaddr_in)));
+  CHECK_SYSCALL(listen(socket_fd, 10));
+
+  // Set up epoll
+  const auto epoll_fd = CHECK_SYSCALL(epoll_create(10));
+
+  epoll_event event{EPOLLIN};
+  event.data.fd = socket_fd;
+  CHECK_SYSCALL(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, socket_fd, &event));
+
+  std::vector<epoll_event> events{10};
+  std::vector<std::byte> buffer(1024);
+
+  while (true) {
+    std::cout << "Waiting for events..." << '\n';
+    const auto num_fds = CHECK_SYSCALL(epoll_wait(epoll_fd, events.data(), events.size(), -1));
+    std::cout << "Received events=" << num_fds << '\n';
+
+    for (auto i = 0; i < num_fds; i++) {
+      if (events[i].data.fd == socket_fd) {
+        // Connection attempt; add to the interest list and continue
+        const auto connection_fd = CHECK_SYSCALL(accept(socket_fd, nullptr, nullptr));
+        event.data.fd = connection_fd;
+        CHECK_SYSCALL(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, connection_fd, &event));
+        std::cout << "Adding connection=" << connection_fd << '\n';
+        continue;
+      }
+
+      // Received data on a connection socket, read it and send it back
+      const auto bytes_read = CHECK_SYSCALL(read(events[i].data.fd, buffer.data(), buffer.size()));
+      std::cout << "Received bytes=" << bytes_read << '\n';
+
+      if (bytes_read == 0) {
+        // Connection closed
+        std::cout << "Closing fd=" << events[i].data.fd << '\n';
+        CHECK_SYSCALL(epoll_ctl(epoll_fd, EPOLL_CTL_DEL, events[i].data.fd, &event));
+        CHECK_SYSCALL(close(events[i].data.fd));
+        continue;
+      }
+
+      // Blocking I/O, I know, it's bad
+      std::cout << "Sending bytes=" << bytes_read << '\n';
+      CHECK_SYSCALL(write(events[i].data.fd, buffer.data(), bytes_read));
+    }
+  }
+}

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.29)
+project(solar_hackers)
+
+add_subdirectory(0_smoketest)