Problem reading wav header in zig, finding irrelevant values

Question

Im trying to read the header of a wav file ( wav file that works with media players) in my program, but i only find invalid values ( for sample rate, number of channels ect)

pub const WavObject = struct {
    riff_identifier: *[4]u8, // Chunk identifier: "RIFF"
    riff_chunk_size: u32, // Chunk size (file size - 8 bytes)
    riff_format: *[4]u8, // Format identifier: "WAVE"
    fmt_identifier: *[4]u8, // Format subchunk identifier: "fmt "
    fmt_subchunk_size: u32, // Subchunk size (16 for PCM)
    fmt_audio_format: u16, // Audio format (1 for PCM)
    fmt_num_channels: u16, // Number of channels (1 for mono, 2 for stereo)
    fmt_sample_rate: u32, // Sample rate (samples per second)
    fmt_byte_rate: u32, // Byte rate (sample rate * block size)
    fmt_block_size: u16, // Block size (channels * bits per sample  8)
    fmt_bits_per_sample: u16, //Bits per sample (8 or 16)
    data_identifier: *[4]u8, // Data subchunk identifier: "data"
    data_size: u32, // Data subchunk size (size of the raw audio data)

    
    pub fn deserializeHeader(wavBytes: []u8, allocator: std.mem.Allocator) !*WavObject {
        var wavObj = try allocator.create(WavObject);
        if (wavBytes.len < 44) return error.InvalidWav;

        const riffChunku32: u32 = undefined;
        const subChunkSize: u32 = undefined;
        const audioFormat: u16 = undefined;
        const numChannels: u16 = undefined;
        const sampleRate: u32 = undefined;
        const byteRate: u32 = undefined;
        const blockSize: u16 = undefined;
        const bitsPerSample: u16 = undefined;
        const dataSize: u32 = undefined;

        // Parse individual fields from the wavBytes array
        std.mem.writeInt(u32, wavBytes[4..8], riffChunku32, .little);
        std.mem.writeInt(u32, wavBytes[16..20], subChunkSize, .little);
        std.mem.writeInt(u16, wavBytes[20..22], audioFormat, .little);
        std.mem.writeInt(u16, wavBytes[22..24], numChannels, .little);
        std.mem.writeInt(u32, wavBytes[24..28], sampleRate, .little);
        std.mem.writeInt(u32, wavBytes[28..32], byteRate, .little);
        std.mem.writeInt(u16, wavBytes[32..34], blockSize, .little);
        std.mem.writeInt(u16, wavBytes[34..36], bitsPerSample, .little);
        std.mem.writeInt(u32, wavBytes[40..44], dataSize, .little);

        wavObj.riff_identifier = wavBytes[0..4];
        wavObj.riff_chunk_size = riffChunku32;
        wavObj.riff_format = wavBytes[8..12];
        wavObj.fmt_identifier = wavBytes[12..16];
        wavObj.fmt_subchunk_size = subChunkSize;
        wavObj.fmt_audio_format = audioFormat;
        wavObj.fmt_num_channels = numChannels;
        wavObj.fmt_sample_rate = sampleRate;
        wavObj.fmt_byte_rate = byteRate;
        wavObj.fmt_block_size = blockSize;
        wavObj.fmt_bits_per_sample = bitsPerSample;
        wavObj.data_identifier = wavBytes[36..40];
        wavObj.data_size = dataSize;
        return wavObj;
    }
    // test code 
    test "basic play wav" {
    const start = std.time.microTimestamp();
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    // reads header and prints values
    const buffer = try allocator.alloc(u8, 2048 * 100000);
    const file = try std.fs.cwd().openFile("./samples/hip_hop_kick.wav", .{ .mode = std.fs.File.OpenMode.read_only });
    const reader = file.reader();
    _ = try reader.read(buffer);
    const wavObj = try wav.WavObject.deserializeHeader(buffer[0..44], allocator);
    wavObj.PrintHeader();
    std.debug.print("{d}", .{buffer[0..44]});
    const end = std.time.microTimestamp();

    std.debug.print("time {d} \n", .{end - start});
}

Is there a reason you are using *[4]u8 for riff_identifier, riff_format, fmt_identifier, and data_identifier? Why not just use [4]u8? Also, deserializeHeader seems to allocate the WavObject on the heap for no reason. You could return it by value. — prokop
– prokop, Commented Dec 16, 2024 at 14:30

sigod · Accepted Answer · 2024-12-16 13:07:46Z

1

    // Parse individual fields from the wavBytes array
   std.mem.writeInt(u32, wavBytes[4..8], riffChunku32, .little);
   ...

You're using writeInt, which writes to memory, not reads from it. Try using readInt.

If you change wavBytes to []const u8 type in deserializeHeader, you'll get a compiler error highlighting your mistake.

answered Dec 16, 2024 at 13:07

sigod

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