[Libexif-devel] Need some help with adding support for extra canon makernotes

[Jef D. <jef...@ho...>]

Many makernote tags are already documented by the 'exiftool' project 
[1], but are not in the libexif library. I'm working on adding those 
canon makernotes to the libexif library. I already figured out how to 
add entries for already existing tags, but I'm having trouble adding new 
tags.

In mnote-canon-tag.h, I define the new tag:

MNOTE_CANON_TAG_FOCAL_LENGTH = 0x2

And in mnote-canon-tag.c, I add the necessary human readable names to 
both tables. And in mnote-canon-entry.c, I add a section to the function 
mnote_canon_entry_get_value. I copied more or less the code from section 
MNOTE_CANON_TAG_SETTINGS_1 as a starting point:

case MNOTE_CANON_TAG_FOCAL_LENGTH:
    CF (entry->format, EXIF_FORMAT_SHORT, val, maxlen);
    n = exif_get_short (data, entry->order) / 2;
    if (t >= n) return NULL;
    CC (entry->components, n, val, maxlen);
    vs = exif_get_short (entry->data + 2 + t * 2, entry->order);
    snprintf (buf, sizeof (buf), "0x%04x", vs);
    strncpy (val, buf, maxlen - strlen (val));
    break;

But when I run the exif commandline utility, an error is reported for 
the new tag:

Focal type          |Invalid number of components (4, expected 1).

What am I doing wrong here?

[1] http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/Canon.html