Applied Pokology

A blog about GNU poke
Back to index

Multi-line output in poke pretty-printers
[03-05-2020]

by Jose E. Marchesi

The ID3V1 tag format describes the format for the tags that are embeded in MP3 files, giving information about the song stored in the file, such as genre, the name of the artist, and so on. While hacking the id3v1 pickle today, I found a little dilemma on how to best present a pretty-printed version of a tag to the user.

This tag format is very rudimentary, and stores strings as arrays of characters of fixed sizes, which are not NULL-terminated:

deftype ID3V1_Tag =
  struct
  {
    char[3] id = ['T', 'A', 'G'];
    char[30] title;
    char[30] artist;
    char[30] album;
    char[4] year;

    union
    {
      /* ID3v1.1  */
      struct
      {
        char[28] comment;
        byte zero = 0;
        byte track : track != 0;
      } extended;
      /* ID3v1  */
      char[30] comment;
    } data;
    ...
  };

So, as you can imagine, opening a MP3 file and looking at the tag (which is located 128#B from the end of the file) is not very revealing at first sight:

(poke) ID3V1_Tag @ (iosize (get_ios) - 128#B)
ID3V1_Tag {
  id=[0x54UB,0x41UB,0x47UB],
  title=[0x30UB,0x31UB,0x20UB,0x2dUB,0x20UB,...],
  artist=[0x4aUB,0x6fUB,0x61UB,0x71UB,0x75UB,...],
  album=[0x4dUB,0x65UB,0x6eUB,0x74UB,0x69UB,...],
  year=[0x20UB,0x20UB,0x20UB,0x20UB],
  data=struct {
    extended=struct {
      comment=[0x20UB,0x20UB,0x20UB,0x20UB,0x20UB,...],
      zero=0x0UB,
      track=0x1UB
    }
  },
  genre=0xffUB
}

Fortunately, poke supports pretty printers. If a struct type has a method called _print, it will be used by poke as a pretty printer if the `pretty-print' option is set:

(poke) .set pretty-print yes

We have a convention already for the output emitted by pretty-printers: pretty printed values always start with #< and end with >. For example, a pretty-printed BPF register from a BPF instruction:

(poke) BPF_Insn @ ...
BPF_Insn = {
  ...
  regs=BPF_Regs {
     src=#<%r3>,
     dst=#<%r0>
  }
  ...
}

The #< > convention was originally adopted to make it explicit for the user that everything she sees between #< > is pretty-printed, and does _not_ necessarily reflect the physical structure of the data. Also some information may be missing. In order to get an exact and complete description of the data, the user should .set pretty-print and evaluate the value again at the prompt.

This works well for written representations that span just for a single line. But, in the case of the MP3 tag above, including all the information in a single line is annoying.

So I am suggesting to extend the convention: in case you want the pretty-printed representation to span for more than one line, you should place first the #< starting at the column 0, then the lines with the data, and finally > in its own line starting at column 0.

I used this convention for id3v1 tags, and this is the result:

(poke) .file eclipse.mp3
The current IOS is now `./eclipse.mp3'.
(poke) load id3v1
(poke) ID3V1_Tag @ (iosize (get_ios) - 128#B)
#<
  genre: 255
  title: 01 - Eclipse De Mar           
  artist: Joaqu�n Sabina                
  album: Mentiras Piadosas             
  year:     
  comment:                             
  track: 1
>

See the file pickles/id3v1.pk in the poke sources if you are curious about the implementation of this simple pickle, including its pretty-printer method.

Happy poking!

Follow up in the mailing list...

Jose E. Marchesi - http://jemarch.net/