convcmf.c

#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>

#pragma pack(push, 1)

typedef struct {
    char magic[6];
    unsigned short iInstrumentBlockOffset;
    unsigned short iMusicOffset;
    unsigned short iTicksPerQuarterNote;
    unsigned short iTicksPerSecond;
    unsigned short iTagOffsetTitle;
    unsigned short iTagOffsetComposer;
    unsigned short iTagOffsetRemarks;
    char iChannelsInUse[16];
    unsigned short iNumInstruments;
    unsigned short iTempo;
} CMFHEADER;

#define MIDI_SINGLE 0
#define MIDI_MULTIPLE 1
#define MIDI_PATTERN 2

typedef struct 
{
    char m_magic[4];
    unsigned int m_seclen;
    short m_format;
    short m_ntracks;
    short m_tickdiv;
} MidiHeader;

#pragma pack(pop)

int fsize(FILE *fp) {
    int prev = ftell(fp);
    fseek(fp, 0L, SEEK_END);
    
    int sz = ftell(fp);
    fseek(fp, prev, SEEK_SET);
    
    return sz;
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: convcmf <file>\n\n");
        return 1;
    }

    CMFHEADER cmf;
    char buffer[1024*1024];

    FILE *fcmf;
    fcmf = fopen(argv[1], "rb");
    int cmf_size = fsize(fcmf);
    
    // get header
    fread(&cmf, sizeof(CMFHEADER), 1, fcmf);
    
    // get music
    fseek(fcmf, cmf.iMusicOffset, SEEK_SET);
    fread(&buffer, 1, cmf_size - cmf.iMusicOffset, fcmf);
    fclose(fcmf);
    
    // write midi
    FILE *f = fopen("output.mid", "wb");
    
    MidiHeader header;
    memcpy(header.m_magic, "MThd", 4);
    header.m_seclen = ntohl(6);
    header.m_format = MIDI_SINGLE;
    header.m_ntracks = ntohs(1); // always one track in cmf
    header.m_tickdiv = ntohs(cmf.iTicksPerQuarterNote);    
    fwrite(&header, sizeof(MidiHeader), 1, f);
    
    // track    
    fwrite("MTrk", 4, 1, f);    
    char tempo_message[7] = {0x00, 0xFF, 0x51, 0x03, 0x00, 0x00, 0x00};
    int usecPerQuarterNote = ntohl((60.0/cmf.iTempo) * 1000000.0); // Quarter note = 60 / BPM
    memcpy(&tempo_message[4], (char*)&usecPerQuarterNote+1, 3); // why?
    
    int track_len = ntohl(cmf_size - cmf.iMusicOffset + sizeof(tempo_message) + 1);
    fwrite(&track_len, sizeof(int), 1, f);
    fwrite(&tempo_message, sizeof(tempo_message), 1, f);
    fwrite(&buffer, 1, cmf_size - cmf.iMusicOffset+1, f);
    
    fclose(f);
    
    printf("wrote output.mid. done.\n");
}