how do I get necessary values of header of ID3v2?

Question

I'm trying to read header of ID3V2 of mp3 files. I can get/print ID3 and want to print out "version" and "subversion" which is type char, but I can't get what i need.

here is code:

    }
    .....
    fseek(file,0,SEEK_SET); 
    fread(&tag.TAG, 1, sizeof(tag),file); // tag is structure with elements of header

    if(strncmp(tag.TAG,"ID3", 3) == 0)
    {
        fread(&tag.version,1, sizeof(tag),file);
        fread(&tag.subversion,1, sizeof(tag),file);

    printf("ID3v2.%s.%s", tag.version, tag.subversion);
   }
}

A.

Why are you reading `sizeof(tag)` bytes? the fread call should probably look like `fread(&member, sizeof member, 1, file)`. **And test the return value of `fread()`!** — pmg, Jun 16 '11 at 18:52
Stop editing answers to reply to them; edit or leave a comment on your own question instead (and if/when you hit 50 rep you can leave comments on the answers as well). — ildjarn, Jun 16 '11 at 19:32

score 0 · Answer 1 · answered Jun 16 '11 at 18:53

0

Are you reading enough bytes? You are passing the address of tag.TAG, but supplying sizeof(tag) and not sizeof(tag.TAG).

answered Jun 16 '11 at 18:53

Vinicius Kamakura

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karlphillip · Answer 2 · 2011-06-16T19:11:31.813

0

That would be %c for printing a char and not %s (used for printing null-terminated char*):

printf("ID3v2.%c.%c", tag.version, tag.subversion);

Use %d if you want to see the byte as a number.

edited Jun 16 '11 at 19:11

answered Jun 16 '11 at 18:53

karlphillip

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actually, `%hhd`, for a `char` treated as an integer. – ninjalj Jun 16 '11 at 19:01

score 0 · Accepted Answer · answered Jun 16 '11 at 20:44

You should read only once the header. i.e. if you have

struct id3v2hdr {
    char TAG[3];
    unsigned char version;
    unsigned char subversion;
    ...
}

Your code would be:

fseek(file,0,SEEK_SET); 
fread(&tag.TAG, 1, sizeof(tag),file); // tag is structure with elements of header

if(strncmp(tag.TAG,"ID3", 3) == 0)
{
    printf("ID3v2.%hhd.%hhd", tag.version, tag.subversion);
}

Note that version and subversion are byte-sized integers, not printable characters, so you should use %hhu (%hhd if they are signed) as its format specification.

Also, the pointer to the first element of a struct, and the pointer to a struct compare equal, so changing your fread line to:

fread(&tag, 1, sizeof(tag),file); // tag is structure with elements of header

is unnecessary (tough it would show the intent much more clearly).

how do I get necessary values of header of ID3v2?

3 Answers3