Struct C/C++ - Reading WAV Header 32 Bit Float - byte offset wrong

Question

I use a struct for reading Wav-Files in audio format 3 (=IEEE FLOAT 32 Bit) with a 58 byte wav-header. The problem: Byte 38 starts with the "fact" chunk. This seems to be ok. DwFactSize should have an offset of 42. Here the problem begins! The offset is 44 !!! First I thought, the problem is in padding bits. So I tried uints, unsigned chars and attribute ((packed)). This doesn't change anything.

This is the definition of the wav header:

0 - 3 'RIFF'/'RIFX' Little/Big-endian

4 - 7 wRiffLength length of file minus the 8 byte riff header

8 - 11 'WAVE'

12 - 15 'fmt '

16 - 19 wFmtSize length of format chunk minus 8 byte header

20 - 21 wFormatTag identifies PCM, ULAW etc

22 - 23 wChannels

24 - 27 dwSamplesPerSecond samples per second per channel

28 - 31 dwAvgBytesPerSec non-trivial for compressed formats

32 - 33 wBlockAlign basic block size

34 - 35 wBitsPerSample non-trivial for compressed formats

(until byte 35 like the usual 44 byte header)

36 - 37 wExtSize = 0 the length of the format extension

38 - 41 'fact'

42 - 45 dwFactSize = 4 length of the fact chunk minus 8 byte header

46 - 49 dwSamplesWritten actual number of samples written out

50 - 53 'data'

54 - 57 dwDataLength length of data chunk minus 8 byte header

---

output:

...

until here ... correct...

Fact[4] 38

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

DwFactSize 44

dwSamplesWritten 48

Data[4] 52

dwDataLength 56

Quick hack to show the offset problem:

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <cstring>
#include <iostream>
#include <cstddef>

typedef struct  WAV_HEADER
{
unsigned char       RIFF[4];        /* RIFF Header      */ //Magic header
unsigned long       ChunkSize;      /* RIFF Chunk Size  */
unsigned char       WAVE[4];        /* WAVE Header      */
unsigned char       fmt[4];         /* FMT header       */
unsigned long       Subchunk1Size;  /* Size of the fmt chunk: 16=PCM, 18=IEEE Float, 40=Extensible                     */
unsigned short      AudioFormat;    /* Audio format 1=PCM, 3=IEEE Float, 6=mulaw,7=alaw, 257=IBM Mu-Law, 258=IBM A-Law, 259=ADPCM, 65534=Extensible */
unsigned short      NumOfChan;      /* Number of channels 1=Mono 2=Stereo                   */
unsigned long       SamplesPerSec;  /* Sampling Frequency in Hz                             */
unsigned long       bytesPerSec;    /* bytes per second */
unsigned short      blockAlign;     /* 2=16-bit mono, 4=16-bit stereo , 6=24-bit stereo,8=32-bit stereo*/
unsigned short      bitsPerSample;  /* Number of bits per sample, inkl. hier=36 bytes  */
unsigned short      wExtSize;       /*  2 byte, wExtSize = 0  the length of the format extension   */
unsigned char       Fact[4];        /* "fact"  string   */
/*Problem here! -> +2bytes*/
unsigned long       DwFactSize;     /* Sampled data length,  inkl. hier bei 44 bytes */
unsigned long       dwSamplesWritten;
unsigned char       Data[4];        /* leerer string falls extensible -> Beginn nicht nach 44 sondern 68!  */
unsigned long       dwDataLength;   /* raw DataLength */
}__attribute__((packed)) wav_hdr;


int getFileSize(FILE *inFile);

int main(int argc,char *argv[])
{
char *file;
file = argv[1];
wav_hdr wavHeader;
FILE *wavFile;
int headerSize = sizeof(wav_hdr),filelength = 0;
wavFile = fopen(file,"r");
if(wavFile == NULL)
{
    printf("\nCan not open wave file. Usage: program [file] \n");
    exit(EXIT_FAILURE);
}

fread(&wavHeader,headerSize,1,wavFile);
filelength = getFileSize(wavFile);
fclose(wavFile);


std::cout << "\nRIFF " << offsetof(WAV_HEADER, RIFF) <<  std::endl;
std::cout << "\nChunkSize " << offsetof(WAV_HEADER, ChunkSize) <<  std::endl;
std::cout << "\nWAVE[4] " << offsetof(WAV_HEADER, WAVE) <<  std::endl;
std::cout << "\nfmt[4] " << offsetof(WAV_HEADER, fmt) <<  std::endl;
std::cout << "\nSubchunk1Size " << offsetof(WAV_HEADER, Subchunk1Size) <<  std::endl;
std::cout << "\nAudioFormat " << offsetof(WAV_HEADER, AudioFormat) <<  std::endl;
std::cout << "\nNumOfChan " << offsetof(WAV_HEADER, NumOfChan) <<  std::endl;
std::cout << "\nSamplesPerSec " << offsetof(WAV_HEADER, SamplesPerSec) <<  std::endl;
std::cout << "\nbytesPerSec " << offsetof(WAV_HEADER, bytesPerSec) <<  std::endl;
std::cout << "\nblockAlign " << offsetof(WAV_HEADER, blockAlign) <<  std::endl;
std::cout << "\nbitsPerSample " << offsetof(WAV_HEADER, bitsPerSample) <<  std::endl;
std::cout << "\nwExtSize (2) " << offsetof(WAV_HEADER, wExtSize) <<  std::endl;
std::cout << "\nFact[4] " << offsetof(WAV_HEADER, Fact) <<  std::endl;

std::cout << "\nDwFactSize " << offsetof(WAV_HEADER, DwFactSize) <<  std::endl;
std::cout << "\ndwSamplesWritten " << offsetof(WAV_HEADER, dwSamplesWritten) <<  std::endl;
std::cout << "\nData[4] " << offsetof(WAV_HEADER, Data) <<  std::endl;
std::cout << "\ndwDataLength " << offsetof(WAV_HEADER, dwDataLength) <<  std::endl;


return 0;
}

int getFileSize(FILE *inFile)
{
int fileSize = 0;
fseek(inFile,0,SEEK_END);
fileSize=ftell(inFile);
fseek(inFile,0,SEEK_SET);
return fileSize;
}

To generate such a 32 Bit Float, 88200 Wav-file: sox input16_44100.wav -b 32 -e float output32F_88200.wav rate -s -a -v -L 88200

Answer 1

There's 2 bytes of padding between Fact[] and DwFactSize , obviously.

I suspect it's because you're using a typedef there. Just write it as a regular C++ definition: struct __attribute__((packed)) WAV_HEADER { ...