结构化C/C++ -读取WAV标头32位浮点-字节偏移错误

Question

我使用一个结构来读取具有58字节wav头的音频格式3 (=IEEE浮点数32位)的wav文件。问题:字节38以“事实”块开头。这似乎没问题。DwFactSize的偏移量应该是42。问题开始了！偏移量为44！首先，我想，问题是在填充比特。所以我尝试了uint，未签名的字符和attribute((packed)).这不会改变什么。

这是wav头的定义：

0-3 'RIFF'/'RIFX‘Little/大端 4-7 wRiffLength文件长度减去8字节的riff头 8-11‘波’ 12 - 15 'fmt‘ 16 - 19 wFmtSize格式块长度减去8字节头 20 - 21 wFormatTag识别PCM、ULAW等 22 - 23 wChannels 每秒24 -27个dwSamplesPerSecond采样 28 - 31 dwAvgBytesPerSec对于压缩格式来说是非常重要的。 32 - 33 wBlockAlign基本块大小 34 - 35 wBitsPerSample对于压缩格式来说是很重要的

(直到35字节与通常的44字节头一样)

36 - 37 wExtSize =0格式扩展的长度 38 - 41“事实” 42-45 dwFactSize =4事实块的长度减去8字节头 46 - 49 dwSamplesWritten实际写出的样本数 50 - 53“数据” 54-57 dwDataLength数据块长度减去8字节头

产出：

..。

直到这里..。正确..。

Fact4 38

DwFactSize 44

dwSamplesWritten 48

Data4 52

dwDataLength 56

快速破解以显示偏移问题：

#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;
}

要生成这样的32位浮点数，88200 Wav文件: sox input16_44100.wav -b 32 -e -e output32F_88200.wav rate -s -a -v -L 88200。

Answer 1

显然，Fact[]和DwFactSize之间有两个字节的填充。

我怀疑这是因为你在那里使用typedef。只需将其编写为常规的C++定义：struct __attribute__((packed)) WAV_HEADER { ...