# YBlog - Fun with wav
Keywords: wav, C, format, programming

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# Fun with wav

tl;dr: Played to process a `wav` file. `C` was easier and cleaner than Ruby.

edit: I wanted this program to work only on one specific machine (a x86 on a 32 bit Ubuntu). Therefore I didn’t had any portability consideration. This is only a hack.

I had to compute the sum of the absolute values of data of a `.wav` file. For efficiency (and fun) reasons, I had chosen `C` language.

I didn’t programmed in `C` for a long time. From my memory it was a pain to read and write to files. But in the end I was really impressed by the code I get. It was really clean. This is even more impressive knowing I used mostly low level functions.

A `wav` file has an header containing many metadata. This header was optimized to take as few space as possible. The header is then a block of packed bytes.

- The 4th first bytes must contains `RIFF` in ASCII,
- the following 4th Bytes is an 32 bits integer giving the size of the file minus 8, etc…

Surprisingly, I believe that reading this kind of file is easier in `C` than in most higher level language. Proof: I only have to search on the web the complete header format and write it in a struct.

    struct wavfile
    {
        char        id[4];          // should always contain "RIFF"
        int     totallength;    // total file length minus 8
        char        wavefmt[8];     // should be "WAVEfmt "
        int     format;         // 16 for PCM format
        short     pcm;            // 1 for PCM format
        short     channels;       // channels
        int     frequency;      // sampling frequency
        int     bytes_per_second;
        short     bytes_by_capture;
        short     bits_per_sample;
        char        data[4];        // should always contain "data"
        int     bytes_in_data;
    };

To read this kind of data in Ruby, I certainly had to write a block of code for each element in the struct. But in `C` I simply written:

    fread(&header,sizeof(header),1,wav)

Only one step to fill my data structure. Magic!

Then, get an int value coded on two Bytes is also not a natural operation for high level language. In `C`, to read a sequence of 2 Bytes numbers I only had to write:

    short value=0;
    while( fread(&value,sizeof(value),1,wav) ) {
        // do something with value
    }

Finally I ended with the following code. Remark I know the wav format (16 bit / 48000Hz):

    #include <stdio.h>
    #include <stdlib.h>
    #include <stdint.h>

    struct wavfile
    {
        char        id[4];          // should always contain "RIFF"
        int     totallength;    // total file length minus 8
        char        wavefmt[8];     // should be "WAVEfmt "
        int     format;         // 16 for PCM format
        short     pcm;            // 1 for PCM format
        short     channels;       // channels
        int     frequency;      // sampling frequency
        int     bytes_per_second;
        short     bytes_by_capture;
        short     bits_per_sample;
        char        data[4];        // should always contain "data"
        int     bytes_in_data;
    };

    int main(int argc, char *argv[]) {
        char *filename=argv[1];
        FILE *wav = fopen(filename,"rb");
        struct wavfile header;

        if ( wav == NULL ) {
            fprintf(stderr,"Can't open input file %s", filename);
            exit(1);
        }

        // read header
        if ( fread(&header,sizeof(header),1,wav) < 1 )
        {
            fprintf(stderr,"Can't read file header\n");
            exit(1);
        }
        if (    header.id[0] != 'R'
             || header.id[1] != 'I'
             || header.id[2] != 'F'
             || header.id[3] != 'F' ) {
            fprintf(stderr,"ERROR: Not wav format\n");
            exit(1);
        }

        fprintf(stderr,"wav format\n");

        // read data
        long sum=0;
        short value=0;
        while( fread(&value,sizeof(value),1,wav) ) {
            // fprintf(stderr,"%d\n", value);
            if (value<0) { value=-value; }
            sum += value;
        }
        printf("%ld\n",sum);
        exit(0);
    }

Of course it is only a hack. But we can see how easy and clean it should be to improve. As I say often: the right tool for your need instead of the same tool for all your needs. Because here `C` is clearly far superior than Ruby to handle this simple tasks.

I am curious to know if somebody know a nice way to do this with Ruby or Python.

edit: for compatibility reasons (64bit machines) used `int16_t` instead of `short` and `int` instead of `int`.

Edit (2): after most consideration about portability I made an hopefully more portable version. But I must confess this task was a bit tedious. The code remain as readable as before. But I had to use some compiler specific declaration to force the structure to be packed:

    __attribute__((__packed__))

Therefore this implementation should for big and little endian architecture. However, it must be compiled with `gcc`. The new code make more tests but still don’t use `mmap`. Here it is:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h> // for memcmp
    #include <stdint.h> // for int16_t and int32_t

    struct wavfile
    {
        char    id[4];          // should always contain "RIFF"
        int32_t totallength;    // total file length minus 8
        char    wavefmt[8];     // should be "WAVEfmt "
        int32_t format;         // 16 for PCM format
        int16_t pcm;            // 1 for PCM format
        int16_t channels;       // channels
        int32_t frequency;      // sampling frequency
        int32_t bytes_per_second;
        int16_t bytes_by_capture;
        int16_t bits_per_sample;
        char    data[4];        // should always contain "data"
        int32_t bytes_in_data;
    } __attribute__((__packed__));

    int is_big_endian(void) {
        union {
            uint32_t i;
            char c[4];
        } bint = {0x01000000};
        return bint.c[0]==1;
    }

    int main(int argc, char *argv[]) {
        char *filename=argv[1];
        FILE *wav = fopen(filename,"rb");
        struct wavfile header;

        if ( wav == NULL ) {
            fprintf(stderr,"Can't open input file %s\n", filename);
            exit(1);
        }

        // read header
        if ( fread(&header,sizeof(header),1,wav) < 1 ) {
            fprintf(stderr,"Can't read input file header %s\n", filename);
            exit(1);
        }

        // if wav file isn't the same endianness than the current environment
        // we quit
        if ( is_big_endian() ) {
            if (   memcmp( header.id,"RIFX", 4) != 0 ) {
                fprintf(stderr,"ERROR: %s is not a big endian wav file\n", filename);
                exit(1);
            }
        } else {
            if (   memcmp( header.id,"RIFF", 4) != 0 ) {
                fprintf(stderr,"ERROR: %s is not a little endian wav file\n", filename);
                exit(1);
            }
        }

        if (   memcmp( header.wavefmt, "WAVEfmt ", 8) != 0
            || memcmp( header.data, "data", 4) != 0
                ) {
            fprintf(stderr,"ERROR: Not wav format\n");
            exit(1);
        }
        if (header.format != 16) {
            fprintf(stderr,"\nERROR: not 16 bit wav format.");
            exit(1);
        }
        fprintf(stderr,"format: %d bits", header.format);
        if (header.format == 16) {
            fprintf(stderr,", PCM");
        } else {
            fprintf(stderr,", not PCM (%d)", header.format);
        }
        if (header.pcm == 1) {
            fprintf(stderr, " uncompressed" );
        } else {
            fprintf(stderr, " compressed" );
        }
        fprintf(stderr,", channel %d", header.pcm);
        fprintf(stderr,", freq %d", header.frequency );
        fprintf(stderr,", %d bytes per sec", header.bytes_per_second );
        fprintf(stderr,", %d bytes by capture", header.bytes_by_capture );
        fprintf(stderr,", %d bits per sample", header.bytes_by_capture );
        fprintf(stderr,"\n" );

        if ( memcmp( header.data, "data", 4) != 0 ) {
            fprintf(stderr,"ERROR: Prrroblem?\n");
            exit(1);
        }
        fprintf(stderr,"wav format\n");

        // read data
        long long sum=0;
        int16_t value;
        int i=0;
        fprintf(stderr,"---\n", value);
        while( fread(&value,sizeof(value),1,wav) ) {
            if (value<0) { value=-value; }
            sum += value;
        }
        printf("%lld\n",sum);
        exit(0);
    }

Edit(3): On reddit Bogdanp proposed a Python version:

    #!/usr/bin/env python
    from struct import calcsize, unpack
    from sys import argv, exit

    def word_iter(f):
        while True:
            _bytes = f.read(2)

        if len(_bytes) != 2:
            raise StopIteration

        yield unpack("=h", _bytes)[0]

    try:
        with open(argv[1], "rb") as f:
            wav = "=4ci8cihhiihh4ci"
            wav_size = calcsize(wav)
            metadata = unpack(wav, f.read(wav_size))

            if "".join(metadata[:4]) != "RIFF":
                print "error: not wav file."
                exit(1)

            print sum(abs(word) for word in word_iter(f))
    except IOError:
        print "error: can't open input file '%s'." % argv[1]
        exit(1)

and luikore proposed an impressive Ruby version:

    data = ARGF.read
     keys = %w[id totallength wavefmt format
           pcm channels frequency bytes_per_second
             bytes_by_capture bits_per_sample
               data bytes_in_data sum
     ]
     values = data.unpack 'Z4 i Z8 i s s i i s s Z4 i s*'
     sum = values.drop(12).map(&:abs).inject(:+)
     keys.zip(values.take(12) << sum) {|k, v|
           puts "#{k.ljust 17}: #{v}"
     }

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Published on 2010-10-14
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