AR sound sync - document data offsets

The compiler has added gaps, to align our struct for hardware compatibility.

* made the gaps explicit
* added  `__attribute__ ((packed))`, to ensure that the data layout is the same on all platforms (extensa, risc-v, 8266)

Author: softhack007

usermods/audioreactive/audio_reactive.h

@@ -994,19 +994,21 @@ class AudioReactive : public Usermod {
     int8_t mclkPin = MCLK_PIN;
     #endif
 #endif
-    // new "V2" audiosync struct - 40 Bytes
-    struct audioSyncPacket {
-      char    header[6];      //  06 Bytes
-      float   sampleRaw;      //  04 Bytes  - either "sampleRaw" or "rawSampleAgc" depending on soundAgc setting
-      float   sampleSmth;     //  04 Bytes  - either "sampleAvg" or "sampleAgc" depending on soundAgc setting
-      uint8_t samplePeak;     //  01 Bytes  - 0 no peak; >=1 peak detected. In future, this will also provide peak Magnitude
-      uint8_t frameCounter;   //  01 Bytes  - track duplicate/out of order packets
-      uint8_t fftResult[16];  //  16 Bytes
-      float  FFT_Magnitude;   //  04 Bytes
-      float  FFT_MajorPeak;   //  04 Bytes
+    // new "V2" audiosync struct - 44 Bytes
+    struct __attribute__ ((packed)) audioSyncPacket {  // WLEDMM "packed" ensures that there are no additional gaps
+      char    header[6];      //  06 Bytes  offset 0
+      uint8_t gap1[2];        // gap added by compiler: 02 Bytes, offset 6
+      float   sampleRaw;      //  04 Bytes  offset 8  - either "sampleRaw" or "rawSampleAgc" depending on soundAgc setting
+      float   sampleSmth;     //  04 Bytes  offset 12 - either "sampleAvg" or "sampleAgc" depending on soundAgc setting
+      uint8_t samplePeak;     //  01 Bytes  offset 16 - 0 no peak; >=1 peak detected. In future, this will also provide peak Magnitude
+      uint8_t frameCounter;   //  01 Bytes  offset 17 - track duplicate/out of order packets
+      uint8_t fftResult[16];  //  16 Bytes  offset 18
+      uint8_t gap2[2];        // gap added by compiler: 02 Bytes, offset 34
+      float  FFT_Magnitude;   //  04 Bytes  offset 36
+      float  FFT_MajorPeak;   //  04 Bytes  offset 40
     };
 
-    // old "V1" audiosync struct - 83 Bytes - for backwards compatibility
+    // old "V1" audiosync struct - 83 Bytes payload, 88 bytes total - for backwards compatibility
     struct audioSyncPacket_v1 {
       char header[6];         //  06 Bytes
       uint8_t myVals[32];     //  32 Bytes
@@ -1897,6 +1899,26 @@ class AudioReactive : public Usermod {
       DEBUGSR_PRINTLN(enabled ? F("true.") : F("false."));
       USER_FLUSH();
 
+      // dump audiosync data layout
+      #if defined(SR_DEBUG)
+      {
+        audioSyncPacket data;
+        USER_PRINTF("\naudioSyncPacket_v1 size = %d\n", sizeof(audioSyncPacket_v1));                                                         // size 88
+        USER_PRINTF("audioSyncPacket size = %d\n", sizeof(audioSyncPacket));                                                               // size 44
+        USER_PRINTF("|  char    header[6]     offset = %2d   size = %2d\n", offsetof(audioSyncPacket, header[0]), sizeof(data.header));           // offset  0 size 6
+        USER_PRINTF("|  uint8_t gap1[2]       offset = %2d   size = %2d\n", offsetof(audioSyncPacket, gap1[0]), sizeof(data.gap1));               // offset  6 size 2
+        USER_PRINTF("|  float   sampleRaw     offset = %2d   size = %2d\n", offsetof(audioSyncPacket, sampleRaw), sizeof(data.sampleRaw));        // offset  8 size 4
+        USER_PRINTF("|  float   sampleSmth    offset = %2d   size = %2d\n", offsetof(audioSyncPacket, sampleSmth), sizeof(data.sampleSmth));      // offset 12 size 4
+        USER_PRINTF("|  uint8_t samplePeak    offset = %2d   size = %2d\n", offsetof(audioSyncPacket, samplePeak), sizeof(data.samplePeak));      // offset 16 size 1
+        USER_PRINTF("|  uint8_t frameCounter  offset = %2d   size = %2d\n", offsetof(audioSyncPacket, frameCounter), sizeof(data.frameCounter));  // offset 17 size 1
+        USER_PRINTF("|  uint8_t fftResult[16] offset = %2d   size = %2d\n", offsetof(audioSyncPacket, fftResult[0]), sizeof(data.fftResult));     // offset 18 size 16
+        USER_PRINTF("|  uint8_t gap2[2]       offset = %2d   size = %2d\n", offsetof(audioSyncPacket, gap2[0]), sizeof(data.gap2));               // offset 34 size 2
+        USER_PRINTF("|  float   FFT_Magnitude offset = %2d   size = %2d\n", offsetof(audioSyncPacket, FFT_Magnitude), sizeof(data.FFT_Magnitude));// offset 36 size 4
+        USER_PRINTF("|  float   FFT_MajorPeak offset = %2d   size = %2d\n", offsetof(audioSyncPacket, FFT_MajorPeak), sizeof(data.FFT_MajorPeak));// offset 40 size 4
+        USER_PRINTLN(); USER_FLUSH();
+      }
+      #endif
+
       #if defined(ARDUINO_ARCH_ESP32) && defined(SR_DEBUG)
       DEBUGSR_PRINTF("|| %-9s min free stack %d\n", pcTaskGetTaskName(NULL), uxTaskGetStackHighWaterMark(NULL)); //WLEDMM
       #endif