Add ohm, an OpenHome Songcast sender audio backend

Streams AirPlay audio to OpenHome/Songcast receivers as OHU unicast PCM
(16/24-bit). Sends are paced on the PTP master clock so receivers slave
to the AirPlay group clock through their buffer-fill servo; MediaLatency
carries the phase, matching the wire behaviour of Linn's own sender
(zero flags and timestamps). delay() reports the newest frame's playtime
lead so shairport feeds a receiver latency ahead. Handles Join, Listen,
Leave and Resend with a frame history, and forwards track metadata and
metatext from the metadata hub.

With -r <name>, ohm_upnp controls the receiver over UPnP: discovered by
friendly name via SSDP at time of use, woken, switched to its Songcast
source and pointed at this sender when a session starts; volume follows
the AirPlay slider capped at the device's VolumeLimit; standby follows
some minutes after the session ends.

.dockerignore

@@ -2,3 +2,6 @@
 documents
 docker/Dockerfile
 docker/classic/Dockerfile
+reference
+ohm.txt
+out-ohm

Makefile.am

@@ -112,6 +112,10 @@ if USE_DUMMY
 shairport_sync_SOURCES += audio_dummy.c
 endif
 
+if USE_OHM
+shairport_sync_SOURCES += audio_ohm.c ohm_upnp.c
+endif
+
 if USE_AO
 shairport_sync_SOURCES += audio_ao.c
 endif

audio.c

@@ -62,6 +62,9 @@ extern audio_output audio_pipe;
 #ifdef CONFIG_STDOUT
 extern audio_output audio_stdout;
 #endif
+#ifdef CONFIG_OHM
+extern audio_output audio_ohm;
+#endif
 
 static audio_output *outputs[] = {
 #ifdef CONFIG_ALSA
@@ -93,6 +96,9 @@ static audio_output *outputs[] = {
 #endif
 #ifdef CONFIG_DUMMY
     &audio_dummy,
+#endif
+#ifdef CONFIG_OHM
+    &audio_ohm,
 #endif
     NULL};
 

configure.ac

@@ -98,6 +98,12 @@ if test "x$with_pipe" = "xyes" ; then
 fi
 AM_CONDITIONAL([USE_PIPE], [test "x$with_pipe" = "xyes" ])
 
+AC_ARG_WITH([ohm],[AS_HELP_STRING([--with-ohm],[build the OpenHome Songcast OHM/OHU unicast sender audio backend into Shairport Sync])])
+if test "x$with_ohm" = "xyes" ; then
+  AC_DEFINE([CONFIG_OHM], 1, [Include the OpenHome Songcast OHM/OHU unicast sender audio backend.])
+fi
+AM_CONDITIONAL([USE_OHM], [test "x$with_ohm" = "xyes"])
+
 # Check to see if we should include the System V initscript
 
 AC_ARG_WITH([systemv-startup],[AS_HELP_STRING([--with-systemv-startup],[install a System V startup script during a make install])])

docker/Dockerfile.ohm

@@ -0,0 +1,66 @@
+# Minimal cross-build of shairport-sync with the OHM Songcast sender backend,
+# for linux/arm64 (Raspberry Pi OS aarch64). Produces a glibc binary compatible
+# with Debian bullseye (glibc 2.31) and later (e.g. bookworm).
+#
+# Usage from the repo root:
+#   docker buildx build --platform linux/arm64 --target binary \
+#     -f docker/Dockerfile.ohm --output type=local,dest=./out-ohm .
+#
+# Then copy ./out-ohm/shairport-sync to the Pi.
+#
+# FFmpeg is taken from Debian packages (libavcodec-dev etc.) rather than built
+# from source, to keep the cross-build fast under emulation.
+
+ARG DEBIAN_VERSION=bullseye
+
+FROM --platform=linux/arm64 debian:${DEBIAN_VERSION} AS builder
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        build-essential \
+        autoconf \
+        automake \
+        libtool \
+        pkg-config \
+        git \
+        ca-certificates \
+        xxd \
+        libasound2-dev \
+        libplist-dev \
+        libplist-utils \
+        libssl-dev \
+        libsodium-dev \
+        libgcrypt-dev \
+        uuid-dev \
+        libavcodec-dev \
+        libavformat-dev \
+        libswresample-dev \
+        libavutil-dev \
+        libavahi-client-dev \
+        avahi-daemon \
+        libdbus-1-dev \
+        libglib2.0-dev \
+        libconfig-dev \
+        libpopt-dev \
+        libsndfile-dev \
+        libsoxr-dev \
+        xmltoman \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /shairport-sync
+COPY . .
+
+WORKDIR /shairport-sync/build
+RUN autoreconf -i ../
+RUN ../configure --sysconfdir=/etc --with-alsa \
+        --with-avahi --with-ssl=openssl \
+        --with-airplay-2 --with-ffmpeg --with-metadata \
+        --with-ohm --with-dummy --with-pipe --with-stdout \
+        --with-dbus-interface --with-mpris-interface \
+        --with-convolution --with-soxr
+RUN make -j"$(nproc)"
+
+# Target that yields just the binary on the host filesystem via --output.
+FROM --platform=linux/arm64 scratch AS binary
+COPY --from=builder /shairport-sync/build/shairport-sync /shairport-sync

metadata/hub.h

@@ -4,7 +4,7 @@
 #include "rtsp.h"
 #include <pthread.h>
 
-#define number_of_watchers 2
+#define number_of_watchers 3
 
 typedef enum {
   PS_NOT_AVAILABLE = 0,

ohm_upnp.c

@@ -0,0 +1,579 @@
+/*
+ * OpenHome receiver control for the ohm audio backend.
+ *
+ * This file is part of Shairport Sync.
+ * Copyright (c) Nils Schneider 2026
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "ohm_upnp.h"
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdatomic.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <strings.h>
+#include <sys/socket.h>
+#include <time.h>
+#include <unistd.h>
+
+#ifdef TEST_OHM
+#include <stdarg.h>
+static void debug(int level, const char *fmt, ...) {
+  if (level > 1)
+    return;
+  va_list ap;
+  va_start(ap, fmt);
+  fprintf(stderr, "upnp: ");
+  vfprintf(stderr, fmt, ap);
+  fprintf(stderr, "\n");
+  va_end(ap);
+}
+#define inform(...) debug(1, __VA_ARGS__)
+#else
+#include "config.h"
+#include "common.h"
+#endif
+
+#define NS_PER_SEC 1000000000LL
+#define NS_PER_MIN (60LL * NS_PER_SEC)
+
+#define OHM_CTL_HTTP_MAX (128 * 1024)
+#define OHM_CTL_TIMEOUT_S 3
+#define OHM_CTL_RETRY_NS (10LL * NS_PER_SEC)
+
+#define SSDP_ADDR "239.255.255.250"
+#define SSDP_PORT 1900
+#define SSDP_LOCATIONS_MAX 8
+#define SSDP_LOCATION_LEN 192
+
+#define CDB_PER_DB 100   /* shairport volume unit: dB * 100 */
+#define BMDB_PER_DB 1024 /* OpenHome volume unit: binary milli-dB */
+
+typedef struct {
+  char type[80]; /* full urn, e.g. "urn:av-openhome-org:service:Volume:4" */
+  char url[160]; /* controlURL path */
+} ohm_svc_t;
+
+/* Settings, copied in ohm_upnp_configure(). */
+static struct {
+  char *receiver;
+  char *iface;
+  char *name;
+  int standby_min;
+  int ohu_port;
+} set;
+
+/* The discovered device, valid while .found. Tick thread only. */
+static struct {
+  int found;
+  char host[64];
+  int port;
+  ohm_svc_t receiver, volume, product;
+  int vol_limit;
+  int vol_step; /* binary milli-dB per volume step */
+} dev;
+
+/* Requests from the backend callbacks to the tick thread. */
+static struct {
+  volatile int session;
+  volatile int activate;
+  volatile int vol_dirty;
+  volatile long vol_cdb;      /* dB * 100, <= 0 */
+  _Atomic uint64_t standby_at; /* 64-bit accesses must not tear on 32-bit targets */
+} req;
+
+static char *scratch; /* HTTP response buffer, tick thread only */
+static uint64_t retry_at;
+
+size_t ohm_xml_escape(char *dst, size_t dst_cap, const char *src) {
+  size_t w = 0;
+  dst[0] = '\0';
+  if (!src)
+    return 0;
+  for (const char *s = src; *s && w + 8 < dst_cap; s++) {
+    switch (*s) {
+    case '&':
+      memcpy(dst + w, "&amp;", 5);
+      w += 5;
+      break;
+    case '<':
+      memcpy(dst + w, "&lt;", 4);
+      w += 4;
+      break;
+    case '>':
+      memcpy(dst + w, "&gt;", 4);
+      w += 4;
+      break;
+    case '"':
+      memcpy(dst + w, "&quot;", 6);
+      w += 6;
+      break;
+    default:
+      dst[w++] = *s;
+    }
+  }
+  dst[w] = '\0';
+  return w;
+}
+
+static uint64_t ohm_ctl_mono_ns(void) {
+  struct timespec t;
+  clock_gettime(CLOCK_MONOTONIC, &t);
+  return (uint64_t)t.tv_sec * NS_PER_SEC + (uint64_t)t.tv_nsec;
+}
+
+static long ohm_ctl_lround(double d) { return (long)(d < 0 ? d - 0.5 : d + 0.5); }
+
+static const char *ohm_ctl_find_ci(const char *hay, const char *needle) {
+  size_t n = strlen(needle);
+  for (; *hay; hay++)
+    if (strncasecmp(hay, needle, n) == 0)
+      return hay;
+  return NULL;
+}
+
+/* ---- XML scraping ---- */
+
+/* Copy text from p up to the next '<' into out. */
+static int ohm_ctl_span(const char *p, char *out, size_t cap) {
+  const char *e = strchr(p, '<');
+  if (!e || (size_t)(e - p) >= cap)
+    return -1;
+  memcpy(out, p, e - p);
+  out[e - p] = '\0';
+  return 0;
+}
+
+/* Copy the text content of the first <tag> into out. */
+static int ohm_ctl_tag(const char *xml, const char *tag, char *out, size_t cap) {
+  char key[96];
+  snprintf(key, sizeof(key), "<%s>", tag);
+  const char *p = strstr(xml, key);
+  return p ? ohm_ctl_span(p + strlen(key), out, cap) : -1;
+}
+
+/* Fill svc from the <service> element whose serviceType starts with prefix. */
+static int ohm_ctl_service(const char *xml, const char *prefix, ohm_svc_t *svc) {
+  const char *p = strstr(xml, prefix);
+  if (!p || ohm_ctl_span(p, svc->type, sizeof(svc->type)) != 0)
+    return -1;
+  const char *end = strstr(p, "</service>");
+  const char *cu = strstr(p, "<controlURL>");
+  if (!cu || (end && cu > end))
+    return -1;
+  return ohm_ctl_span(cu + strlen("<controlURL>"), svc->url, sizeof(svc->url));
+}
+
+/* ---- HTTP ---- */
+
+/* Collapse chunked transfer-encoding in place; a chunk boundary must never
+ * split a tag. Returns the new body length. */
+static size_t ohm_ctl_dechunk(char *body, size_t body_len) {
+  char *end = body + body_len;
+  char *src = body;
+  char *dst = body;
+  while (src < end) {
+    char *num_end;
+    long sz = strtol(src, &num_end, 16);
+    if (num_end == src || sz <= 0)
+      break;
+    src = strstr(num_end, "\r\n");
+    if (src == NULL)
+      break;
+    src += 2;
+    if (src + sz > end)
+      sz = (long)(end - src);
+    memmove(dst, src, (size_t)sz);
+    dst += sz;
+    src += sz;
+    if (src + 2 <= end && src[0] == '\r')
+      src += 2;
+  }
+  *dst = '\0';
+  return (size_t)(dst - body);
+}
+
+/* One request/response exchange into scratch. Returns 0 on HTTP 200. */
+static int ohm_ctl_http(const char *host, int port, const char *req_buf, size_t req_len) {
+  int fd = socket(AF_INET, SOCK_STREAM, 0);
+  if (fd < 0)
+    return -1;
+  struct timeval tv = {.tv_sec = OHM_CTL_TIMEOUT_S};
+  setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
+  setsockopt(fd, SOL_SOCKET, SO_SNDTIMEO, &tv, sizeof(tv));
+  struct sockaddr_in sin = {.sin_family = AF_INET, .sin_port = htons(port)};
+  if (inet_pton(AF_INET, host, &sin.sin_addr) != 1 ||
+      connect(fd, (struct sockaddr *)&sin, sizeof(sin)) != 0 ||
+      send(fd, req_buf, req_len, 0) != (ssize_t)req_len) {
+    close(fd);
+    return -1;
+  }
+  size_t len = 0;
+  for (;;) {
+    ssize_t n = recv(fd, scratch + len, OHM_CTL_HTTP_MAX - 1 - len, 0);
+    if (n <= 0)
+      break;
+    len += (size_t)n;
+    if (len >= OHM_CTL_HTTP_MAX - 1)
+      break;
+  }
+  close(fd);
+  scratch[len] = '\0';
+
+  const char *sp = strchr(scratch, ' ');
+  if (sp == NULL || atoi(sp + 1) != 200)
+    return -1;
+  char *body = strstr(scratch, "\r\n\r\n");
+  if (body == NULL)
+    return -1;
+  body += 4;
+
+  body[-4] = '\0'; /* terminate the headers for the search */
+  int chunked = ohm_ctl_find_ci(scratch, "transfer-encoding: chunked") != NULL;
+  body[-4] = '\r';
+  if (chunked)
+    ohm_ctl_dechunk(body, len - (size_t)(body - scratch));
+  return 0;
+}
+
+static int ohm_ctl_get(const char *host, int port, const char *path) {
+  char req_buf[256];
+  int n = snprintf(req_buf, sizeof(req_buf),
+                   "GET %s HTTP/1.1\r\nHost: %s:%d\r\nConnection: close\r\n\r\n", path, host, port);
+  return ohm_ctl_http(host, port, req_buf, (size_t)n);
+}
+
+static int ohm_ctl_soap(const ohm_svc_t *svc, const char *action, const char *args) {
+  char body[6144];
+  int blen = snprintf(body, sizeof(body),
+                      "<?xml version=\"1.0\"?>"
+                      "<s:Envelope xmlns:s=\"http://schemas.xmlsoap.org/soap/envelope/\" "
+                      "s:encodingStyle=\"http://schemas.xmlsoap.org/soap/encoding/\">"
+                      "<s:Body><u:%s xmlns:u=\"%s\">%s</u:%s></s:Body></s:Envelope>",
+                      action, svc->type, args, action);
+  char req_buf[8192];
+  int rlen = snprintf(req_buf, sizeof(req_buf),
+                      "POST %s HTTP/1.1\r\nHost: %s:%d\r\n"
+                      "Content-Type: text/xml; charset=\"utf-8\"\r\n"
+                      "SOAPACTION: \"%s#%s\"\r\nContent-Length: %d\r\nConnection: close\r\n\r\n%s",
+                      svc->url, dev.host, dev.port, svc->type, action, blen, body);
+  return ohm_ctl_http(dev.host, dev.port, req_buf, (size_t)rlen);
+}
+
+/* ---- Discovery ---- */
+
+/* M-SEARCH for OpenHome receivers; collect response LOCATION urls. */
+static int ohm_ctl_ssdp(char locations[][SSDP_LOCATION_LEN], int max) {
+  int fd = socket(AF_INET, SOCK_DGRAM, 0);
+  if (fd < 0)
+    return 0;
+  struct timeval tv = {.tv_usec = 500000};
+  setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
+  if (set.iface) {
+    struct in_addr mcast_if;
+    if (inet_pton(AF_INET, set.iface, &mcast_if) == 1)
+      setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &mcast_if, sizeof(mcast_if));
+  }
+  const char *msearch = "M-SEARCH * HTTP/1.1\r\nHOST: " SSDP_ADDR ":1900\r\n"
+                        "MAN: \"ssdp:discover\"\r\nMX: 1\r\n"
+                        "ST: urn:av-openhome-org:service:Receiver:1\r\n\r\n";
+  struct sockaddr_in dst = {.sin_family = AF_INET, .sin_port = htons(SSDP_PORT)};
+  inet_pton(AF_INET, SSDP_ADDR, &dst.sin_addr);
+  sendto(fd, msearch, strlen(msearch), 0, (struct sockaddr *)&dst, sizeof(dst));
+
+  int n = 0;
+  char pkt[2048];
+  for (int i = 0; i < max && n < max; i++) {
+    ssize_t r = recv(fd, pkt, sizeof(pkt) - 1, 0);
+    if (r <= 0)
+      break;
+    pkt[r] = '\0';
+    const char *l = ohm_ctl_find_ci(pkt, "location:");
+    if (!l)
+      continue;
+    l += strlen("location:");
+    while (*l == ' ')
+      l++;
+    size_t len = strcspn(l, "\r\n");
+    if (len == 0 || len >= SSDP_LOCATION_LEN)
+      continue;
+    memcpy(locations[n], l, len);
+    locations[n][len] = '\0';
+    n++;
+  }
+  close(fd);
+  return n;
+}
+
+/* Fetch one device description; on a friendly-name match fill dev. */
+static int ohm_ctl_probe(const char *location) {
+  char host[64], path[128], fname[128];
+  int port = 80;
+  if (sscanf(location, "http://%63[^:/]:%d%127s", host, &port, path) != 3 &&
+      sscanf(location, "http://%63[^:/]%127s", host, path) != 2)
+    return -1;
+  if (ohm_ctl_get(host, port, path) != 0 ||
+      ohm_ctl_tag(scratch, "friendlyName", fname, sizeof(fname)) != 0 ||
+      !ohm_ctl_find_ci(fname, set.receiver))
+    return -1;
+  if (ohm_ctl_service(scratch, "urn:av-openhome-org:service:Receiver:", &dev.receiver) != 0 ||
+      ohm_ctl_service(scratch, "urn:av-openhome-org:service:Volume:", &dev.volume) != 0 ||
+      ohm_ctl_service(scratch, "urn:av-openhome-org:service:Product:", &dev.product) != 0)
+    return -1;
+  snprintf(dev.host, sizeof(dev.host), "%s", host);
+  dev.port = port;
+  inform("ohm: controlling receiver \"%s\" at %s:%d", fname, dev.host, dev.port);
+  return 0;
+}
+
+static void ohm_ctl_read_volume_caps(void) {
+  dev.vol_limit = 100;
+  dev.vol_step = BMDB_PER_DB;
+  char v[32];
+  if (ohm_ctl_soap(&dev.volume, "Characteristics", "") == 0) {
+    if (ohm_ctl_tag(scratch, "VolumeMax", v, sizeof(v)) == 0)
+      dev.vol_limit = atoi(v);
+    if (ohm_ctl_tag(scratch, "VolumeMilliDbPerStep", v, sizeof(v)) == 0 && atoi(v) > 0)
+      dev.vol_step = atoi(v);
+  }
+  if (ohm_ctl_soap(&dev.volume, "VolumeLimit", "") == 0 &&
+      ohm_ctl_tag(scratch, "Value", v, sizeof(v)) == 0 && atoi(v) > 0)
+    dev.vol_limit = atoi(v);
+  debug(1, "ohm: receiver volume limit %d, %d mdB/step", dev.vol_limit, dev.vol_step);
+}
+
+static int ohm_ctl_discover(void) {
+  char locations[SSDP_LOCATIONS_MAX][SSDP_LOCATION_LEN];
+  int n = ohm_ctl_ssdp(locations, SSDP_LOCATIONS_MAX);
+  for (int i = 0; i < n; i++) {
+    if (ohm_ctl_probe(locations[i]) == 0) {
+      ohm_ctl_read_volume_caps();
+      dev.found = 1;
+      return 0;
+    }
+  }
+  return -1;
+}
+
+/* shairport delivers attenuation in cdB over the declared range; the device
+ * runs 0..vol_limit steps. */
+static long ohm_ctl_steps_from_cdb(long cdb) {
+  double step_db = (double)dev.vol_step / BMDB_PER_DB;
+  long steps = dev.vol_limit + ohm_ctl_lround((double)cdb / CDB_PER_DB / step_db);
+  if (steps < 0)
+    steps = 0;
+  if (steps > dev.vol_limit)
+    steps = dev.vol_limit;
+  return steps;
+}
+
+/* ---- Actions ---- */
+
+/* ohu:// URI of this sender as reachable from the receiver. */
+static int ohm_ctl_our_uri(char *out, size_t cap) {
+  struct sockaddr_in sin = {.sin_family = AF_INET, .sin_port = htons(SSDP_PORT)};
+  inet_pton(AF_INET, dev.host, &sin.sin_addr);
+  int fd = socket(AF_INET, SOCK_DGRAM, 0);
+  if (fd < 0)
+    return -1;
+  struct sockaddr_in local;
+  socklen_t slen = sizeof(local);
+  int ok = connect(fd, (struct sockaddr *)&sin, sizeof(sin)) == 0 &&
+           getsockname(fd, (struct sockaddr *)&local, &slen) == 0;
+  close(fd);
+  if (!ok)
+    return -1;
+  char ip[INET_ADDRSTRLEN];
+  inet_ntop(AF_INET, &local.sin_addr, ip, sizeof(ip));
+  snprintf(out, cap, "ohu://%s:%d", ip, set.ohu_port);
+  return 0;
+}
+
+/* Switch the receiver's product to its Songcast source. The SourceXml body is
+ * XML-escaped inside the SOAP response, so count escaped <Source> elements up
+ * to the one of type Receiver. */
+static void ohm_ctl_select_source(void) {
+  if (ohm_ctl_soap(&dev.product, "SourceXml", "") != 0)
+    return;
+  const char *rcv = strstr(scratch, "&lt;Type&gt;Receiver&lt;/Type&gt;");
+  if (!rcv)
+    return;
+  int idx = -1;
+  for (const char *p = scratch; (p = strstr(p, "&lt;Source&gt;")) != NULL && p < rcv; p++)
+    idx++;
+  if (idx < 0)
+    return;
+  char arg[48];
+  snprintf(arg, sizeof(arg), "<Value>%d</Value>", idx);
+  ohm_ctl_soap(&dev.product, "SetSourceIndex", arg);
+}
+
+static int ohm_ctl_set_sender(const char *uri) {
+  char name[256], didl[768], meta[2304], args[2560];
+  ohm_xml_escape(name, sizeof(name), set.name ? set.name : "Shairport Sync");
+  snprintf(didl, sizeof(didl),
+           "<DIDL-Lite xmlns:dc=\"http://purl.org/dc/elements/1.1/\" "
+           "xmlns:upnp=\"urn:schemas-upnp-org:metadata-1-0/upnp/\" "
+           "xmlns=\"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/\">"
+           "<item id=\"0\" restricted=\"True\"><dc:title>%s</dc:title>"
+           "<res protocolInfo=\"ohu:*:*:*\">%s</res>"
+           "<upnp:class>object.item.audioItem</upnp:class></item></DIDL-Lite>",
+           name, uri);
+  ohm_xml_escape(meta, sizeof(meta), didl);
+  snprintf(args, sizeof(args), "<Uri>%s</Uri><Metadata>%s</Metadata>", uri, meta);
+  return ohm_ctl_soap(&dev.receiver, "SetSender", args);
+}
+
+static int ohm_ctl_activate(void) {
+  char uri[128];
+  if (ohm_ctl_soap(&dev.product, "SetStandby", "<Value>0</Value>") != 0)
+    return -1;
+  ohm_ctl_select_source();
+  if (ohm_ctl_our_uri(uri, sizeof(uri)) != 0 || ohm_ctl_set_sender(uri) != 0 ||
+      ohm_ctl_soap(&dev.receiver, "Play", "") != 0)
+    return -1;
+  debug(1, "ohm: receiver pointed at %s and playing", uri);
+  return 0;
+}
+
+static int ohm_ctl_push_volume(void) {
+  long steps = ohm_ctl_steps_from_cdb(req.vol_cdb);
+  char arg[48];
+  snprintf(arg, sizeof(arg), "<Value>%ld</Value>", steps);
+  if (ohm_ctl_soap(&dev.volume, "SetVolume", arg) != 0)
+    return -1;
+  debug(2, "ohm: receiver volume %ld/%d", steps, dev.vol_limit);
+  return 0;
+}
+
+/* Standby only if the receiver is still pointed at us. */
+static int ohm_ctl_standby(void) {
+  char uri[128];
+  if (ohm_ctl_soap(&dev.receiver, "Sender", "") != 0)
+    return -1;
+  if (ohm_ctl_our_uri(uri, sizeof(uri)) == 0 && !strstr(scratch, uri)) {
+    debug(1, "ohm: receiver plays another sender, skipping standby");
+    return 0;
+  }
+  if (ohm_ctl_soap(&dev.product, "SetStandby", "<Value>1</Value>") != 0)
+    return -1;
+  debug(1, "ohm: receiver put into standby");
+  return 0;
+}
+
+/* Run every due request in order; -1 invalidates the device. */
+static int ohm_ctl_run(uint64_t now) {
+  if (req.activate) {
+    if (ohm_ctl_activate() != 0)
+      return -1;
+    req.activate = 0;
+  }
+  if (req.vol_dirty) {
+    req.vol_dirty = 0; /* a newer volume arriving during the push re-sets it */
+    if (ohm_ctl_push_volume() != 0) {
+      req.vol_dirty = 1;
+      return -1;
+    }
+  }
+  if (req.standby_at && now >= req.standby_at) {
+    if (ohm_ctl_standby() != 0)
+      return -1;
+    req.standby_at = 0;
+  }
+  return 0;
+}
+
+/* ---- Backend interface ---- */
+
+void ohm_upnp_configure(const ohm_upnp_config_t *config) {
+  free(set.receiver);
+  free(set.iface);
+  free(set.name);
+  set.receiver = config->receiver_name ? strdup(config->receiver_name) : NULL;
+  set.iface = config->interface_ip ? strdup(config->interface_ip) : NULL;
+  set.name = config->display_name ? strdup(config->display_name) : NULL;
+  set.standby_min = config->standby_minutes;
+  set.ohu_port = config->ohu_port;
+}
+
+int ohm_upnp_enabled(void) { return set.receiver != NULL; }
+
+void ohm_upnp_session_begin(void) {
+  if (set.receiver == NULL || req.session)
+    return;
+  req.session = 1;
+  req.standby_at = 0;
+  req.activate = 1;
+}
+
+void ohm_upnp_session_end(void) {
+  if (set.receiver == NULL)
+    return;
+  req.session = 0;
+  req.standby_at =
+      set.standby_min > 0 ? ohm_ctl_mono_ns() + (uint64_t)set.standby_min * NS_PER_MIN : 0;
+}
+
+void ohm_upnp_set_volume(long vol_cdb) {
+  req.vol_cdb = vol_cdb;
+  req.vol_dirty = 1;
+}
+
+void ohm_upnp_tick(void) {
+  if (set.receiver == NULL)
+    return;
+  uint64_t now = ohm_ctl_mono_ns();
+  int standby_due = req.standby_at && now >= req.standby_at;
+  if (!req.activate && !req.vol_dirty && !standby_due)
+    return;
+  if (now < retry_at)
+    return;
+  if (scratch == NULL && (scratch = malloc(OHM_CTL_HTTP_MAX)) == NULL)
+    return;
+  if (!dev.found && ohm_ctl_discover() != 0) {
+    retry_at = now + OHM_CTL_RETRY_NS;
+    debug(1, "ohm: receiver \"%s\" not found, retrying", set.receiver);
+    return;
+  }
+  if (ohm_ctl_run(now) != 0) {
+    dev.found = 0;
+    retry_at = now + OHM_CTL_RETRY_NS;
+  }
+}
+
+void ohm_upnp_deinit(void) {
+  free(set.receiver);
+  set.receiver = NULL;
+  free(set.iface);
+  set.iface = NULL;
+  free(set.name);
+  set.name = NULL;
+  free(scratch);
+  scratch = NULL;
+  dev.found = 0;
+  req.session = 0;
+  req.activate = 0;
+  req.vol_dirty = 0;
+  req.standby_at = 0;
+}

ohm_upnp.h

@@ -0,0 +1,58 @@
+/*
+ * OpenHome receiver control for the ohm audio backend.
+ *
+ * This file is part of Shairport Sync.
+ * Copyright (c) Nils Schneider 2026
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#pragma once
+
+#include <stddef.h>
+
+/* XML-escape src into dst (NUL-terminated). Returns chars written (excluding NUL). */
+size_t ohm_xml_escape(char *dst, size_t dst_cap, const char *src);
+
+/* The backend declares volume as attenuation -OHM_UPNP_VOLUME_RANGE_CDB..0
+ * (dB * 100); shairport stretches the AirPlay slider over exactly this range. */
+#define OHM_UPNP_VOLUME_RANGE_CDB 6000
+
+typedef struct {
+  const char *receiver_name; /* friendly name to match; NULL disables the module */
+  const char *interface_ip;
+  const char *display_name;
+  int standby_minutes; /* 0 = never */
+  int ohu_port;
+} ohm_upnp_config_t;
+
+/* Strings are copied. */
+void ohm_upnp_configure(const ohm_upnp_config_t *config);
+int ohm_upnp_enabled(void);
+
+/* Callback side: cheap, thread-safe, never block. */
+void ohm_upnp_session_begin(void);
+void ohm_upnp_session_end(void);
+void ohm_upnp_set_volume(long vol_cdb); /* dB * 100, <= 0 */
+
+/* Does the network work (SSDP discovery, SOAP); call periodically from one
+ * thread. */
+void ohm_upnp_tick(void);
+
+void ohm_upnp_deinit(void);