Fix recvmsg msghdr outputs

src/lib/libsyscall.js

@@ -530,7 +530,8 @@ var SyscallsLibrary = {
     // write the source address out
     var name = {{{ makeGetValue('message', C_STRUCTS.msghdr.msg_name, '*') }}};
     if (name) {
-      var errno = writeSockaddr(name, sock.family, DNS.lookup_name(msg.addr), msg.port);
+      var namelen = message + {{{ C_STRUCTS.msghdr.msg_namelen }}};
+      var errno = writeSockaddr(name, sock.family, DNS.lookup_name(msg.addr), msg.port, namelen);
 #if ASSERTIONS
       assert(!errno);
 #endif
@@ -546,12 +547,15 @@ var SyscallsLibrary = {
       }
       var length = Math.min(iovlen, bytesRemaining);
       var buf = msg.buffer.subarray(bytesRead, bytesRead + length);
-      HEAPU8.set(buf, iovbase + bytesRead);
+      HEAPU8.set(buf, iovbase);
       bytesRead += length;
       bytesRemaining -= length;
     }
 
-    // TODO set msghdr.msg_flags
+    {{{ makeSetValue('message', C_STRUCTS.msghdr.msg_controllen, '0', 'i32') }}};
+    {{{ makeSetValue('message', C_STRUCTS.msghdr.msg_flags, '0', 'i32') }}};
+
+    // TODO report truncation in msghdr.msg_flags
     // MSG_EOR
     // End of record was received (if supported by the protocol).
     // MSG_OOB

src/struct_info.json

@@ -232,7 +232,10 @@
                 "msg_name",
                 "msg_namelen",
                 "msg_iov",
-                "msg_iovlen"
+                "msg_iovlen",
+                "msg_control",
+                "msg_controllen",
+                "msg_flags"
             ]
         }
     },

src/struct_info_generated.json

@@ -1030,6 +1030,9 @@
         },
         "msghdr": {
             "__size__": 28,
+            "msg_control": 16,
+            "msg_controllen": 20,
+            "msg_flags": 24,
             "msg_iov": 8,
             "msg_iovlen": 12,
             "msg_name": 0,

src/struct_info_generated_wasm64.json

@@ -1030,6 +1030,9 @@
         },
         "msghdr": {
             "__size__": 56,
+            "msg_control": 32,
+            "msg_controllen": 40,
+            "msg_flags": 48,
             "msg_iov": 16,
             "msg_iovlen": 24,
             "msg_name": 0,

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
-  "a.out.js": 268195,
+  "a.out.js": 268229,
   "a.out.nodebug.wasm": 588191,
-  "total": 856386,
+  "total": 856420,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/sockets/test_udp_recvmsg.c

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2026 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <arpa/inet.h>
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif
+
+static int server_fd;
+static int client_fd;
+static struct sockaddr_in destination;
+static bool sent;
+
+static void succeed(void) {
+  close(server_fd);
+  close(client_fd);
+  puts("done");
+#ifdef __EMSCRIPTEN__
+  emscripten_cancel_main_loop();
+#else
+  _exit(0);
+#endif
+}
+
+static void main_loop(void) {
+  if (!sent) {
+    assert(sendto(client_fd, "onetwo", 6, 0, (struct sockaddr*)&destination,
+                  sizeof(destination)) == 6);
+    sent = true;
+  }
+
+  // Receive the single datagram scattered across two iovecs.
+  char first[3];
+  char second[3];
+  char control[16];
+  struct sockaddr_in source;
+  struct iovec iovecs[2] = {
+      {.iov_base = first, .iov_len = sizeof(first)},
+      {.iov_base = second, .iov_len = sizeof(second)},
+  };
+  struct msghdr message = {
+      .msg_name = &source,
+      .msg_namelen = sizeof(source),
+      .msg_iov = iovecs,
+      .msg_iovlen = 2,
+      .msg_control = control,
+      .msg_controllen = sizeof(control),
+  };
+
+  ssize_t len = recvmsg(server_fd, &message, 0);
+  if (len < 0) {
+    assert(errno == EAGAIN || errno == EWOULDBLOCK);
+    return;
+  }
+
+  assert(len == 6);
+  assert(memcmp(first, "one", 3) == 0);
+  assert(memcmp(second, "two", 3) == 0);
+  assert(message.msg_namelen == sizeof(struct sockaddr_in));
+  assert(source.sin_family == AF_INET);
+  assert(message.msg_controllen == 0);
+  assert(message.msg_flags == 0);
+  succeed();
+}
+
+int main(void) {
+  server_fd = socket(AF_INET, SOCK_DGRAM, 0);
+  client_fd = socket(AF_INET, SOCK_DGRAM, 0);
+  assert(server_fd >= 0 && client_fd >= 0);
+
+  struct sockaddr_in address = {
+      .sin_family = AF_INET,
+      .sin_port = htons(0),
+  };
+  inet_pton(AF_INET, "127.0.0.1", &address.sin_addr);
+  assert(bind(server_fd, (struct sockaddr*)&address, sizeof(address)) == 0);
+
+  socklen_t length = sizeof(destination);
+  assert(getsockname(server_fd, (struct sockaddr*)&destination, &length) == 0);
+  assert(fcntl(server_fd, F_SETFL, O_NONBLOCK) == 0);
+
+#ifdef __EMSCRIPTEN__
+  emscripten_set_main_loop(main_loop, 0, 0);
+#else
+  while (true) {
+    main_loop();
+    usleep(1000);
+  }
+#endif
+  return 0;
+}

test/test_sockets.py

@@ -500,6 +500,11 @@ def test_noderawsockets_udp(self):
     # ephemeral port, the client sends a datagram, the server echoes it back.
     self.do_runf('sockets/test_udp_echo.c', 'done\n', cflags=['-sNODERAWSOCKETS'])
 
+  def test_noderawsockets_udp_recvmsg(self):
+    # recvmsg scatters a datagram across multiple iovecs at the right offsets
+    # and updates msg_namelen/msg_controllen/msg_flags in the caller's msghdr.
+    self.do_runf('sockets/test_udp_recvmsg.c', 'done\n', cflags=['-sNODERAWSOCKETS'])
+
   @also_with_proxy_to_pthread
   def test_noderawsockets_udp_connect(self):
     # Connected UDP: sendto() with an address gives EISCONN, send() reaches the