iphone - 操作 NSMutableData 中的字节问题 - Wave header 信息

Question

我正在尝试操作波形文件头。我已将文件加载到 NSMutableData 中，并尝试更改 wav 文件头信息。我的问题就在于:

[maindata replaceBytesInRange:range withBytes:((UInt32 *)tfSC1Length)];

我收到 EXC_BAD_ACCESS 错误。不明白为什么。有什么想法吗？

我使用的代码如下:

// calculate total file length (minus 8 for the header), chunk size, number of channels, sample rate and bits per sample
int tfLength        = ([maindata length] - 8);
int tfChannels      = 1;   // mono
int tfSampleRate    = 44100;
int tfDataLength    = ([maindata length] - 44);
int tfBitsPerSample = 16;

int tfSC1Length     = 16;
int tfAudioFormat   = 1;

//we are rebuilding the header for the wave files...

// chunk identifier
((char *)[maindata mutableBytes])[0] = 'R';
((char *)[maindata mutableBytes])[1] = 'I';
((char *)[maindata mutableBytes])[2] = 'F';
((char *)[maindata mutableBytes])[3] = 'F';

// size (less 8 bytes)
NSRange range;
range.location = 4;
range.length = 4;
[maindata replaceBytesInRange:range withBytes:(UInt32 *)tfLength];

// the file format
((char *)[maindata mutableBytes])[8]  = 'W';
((char *)[maindata mutableBytes])[9]  = 'A';
((char *)[maindata mutableBytes])[10] = 'V';
((char *)[maindata mutableBytes])[11] = 'E';

// subchunk1 ID
((char *)[maindata mutableBytes])[12] = 'f';
((char *)[maindata mutableBytes])[13] = 'm';
((char *)[maindata mutableBytes])[14] = 't';
((char *)[maindata mutableBytes])[15] = ' ';

// subchunk length
range.location = 16;
range.length   = 4;
[maindata replaceBytesInRange:range withBytes:((UInt32 *)tfSC1Length)];

// audio format
range.location = 20;
range.length   = 2;
[maindata replaceBytesInRange:range withBytes:((UInt16 *)tfAudioFormat)];

// number of channels
range.location = 22;
range.length   = 2;
[maindata replaceBytesInRange:range withBytes:((UInt16 *)tfChannels)];

// sample rate
range.location = 24;
range.length   = 4;
[maindata replaceBytesInRange:range withBytes:((UInt32 *)tfSampleRate)];

// byte rate
range.location = 28;
range.length   = 4;
[maindata replaceBytesInRange:range withBytes:((UInt32 *)(tfSampleRate * ((tfBitsPerSample * tfChannels) / 8)))];

// block align
range.location = 32;
range.length   = 2;
[maindata replaceBytesInRange:range withBytes:((UInt16 *)((tfBitsPerSample * tfChannels) / 8))];

// bits per sample
range.location = 34;
range.length   = 2; 
[maindata replaceBytesInRange:range withBytes:((UInt16 *)tfBitsPerSample)];

// adjust the length field of the wave file...
((char *)[maindata mutableBytes])[36] = 'd';
((char *)[maindata mutableBytes])[37] = 'a';
((char *)[maindata mutableBytes])[38] = 't';
((char *)[maindata mutableBytes])[39] = 'a';

// length of the audio data
range.location = 40;
range.length   = 4;
[maindata replaceBytesInRange:range withBytes:(UInt32 *)tfDataLength];

Answer 1

我建议使用结构体来引用 header :

typedef struct {
    UInt32      riffChunkID;        // Always RIFF, in big endian. Integer fields are little-ending.
    UInt32      fileLength;
    UInt32      waveFileID;     // 'WAVE' for Wave files.
    UInt32      formatChunkID;  // 'fmt '
    UInt32      formatChunkSize;
    SInt16      formatTag;          // Wave Format ID: see constants
    SInt16      channels;           // Number of Channels: 1=mono, 2=stereo
    SInt32      sampleRate;         // Sample Rate: samples per second
    SInt32      bytesPerSec;        // sampleRate * blockAlign
    SInt16      blockAlign;         // sample frame size = channels * sampleSize / 8
    SInt16      bitsPerSample;      // sampleSize (8 or 16), also two's-complement for 16-bit, offset for 8-bit
    UInt32      dataChunkID;        // 'data'
    UInt32      dataChunkSize;
} WaveHeader;

int tfChannels = 1; // mono
int tfSampleRate = 44100;
int tfBitsPerSample = 16;

WaveHeader *header = [maindata mutableBytes];

header->riffChunkID = CFSwapInt32HostToBig ('RIFF');
header->fileLength = CFSwapInt32HostToLittle ([maindata length] - 8);
header->waveFileID = CFSwapInt32HostToBig ('WAVE');

header->formatChunkID = CFSwapInt32HostToBig ('fmt ');
header->formatChunkSize = CFSwapInt32HostToLittle (16);
header->formatTag = CFSwapInt16HostToLittle (1);
header->channels = CFSwapInt16HostToLittle (tfChannels);
header->sampleRate = CFSwapInt32HostToLittle (tfSampleRate);
header->bytesPerSec = CFSwapInt32HostToLittle (tfSampleRate * tfBitsPerSample / 8 * tfChannels);
header->blockAlign = CFSwapInt16HostToLittle (tfBitsPerSample / 8 * tfChannels);
header->bitsPerSample = CFSwapInt16HostToLittle (tfBitsPerSample);
header->dataChunkID = CFSwapInt32HostToBig ('data');
header->dataChunkSize = CFSwapInt32HostToLittle ([maindata length] - 44);

短得多。希望这会有所帮助。