flac dev - Apr 2016 - FLAC__metadata_get_picture always return false

I am using a libFLAC8.dylib that I have compiled with the original Makefile of
flac 1.3.1 distribution, in a project with a custom flac.h header file which is
an amalgamation of all the flac header files.
I can get this pair ( flac.h & libFLAC8.dylib ) working fine to convert flac
file to other supported audio file formats both ways. It work also fine to get
Vorbis Comment and other metadata ( like "bits per sample" for
instance ).
But when I use the following code, FLAC__metadata_get_picture() always return
false :


    FLAC__StreamMetadata *smet= NULL;
    const char *filename="/[...]/musicfile.flac";
    for (FLAC__StreamMetadata_Picture_Type pic_type =
FLAC__STREAM_METADATA_PICTURE_TYPE_OTHER;
         pic_type < FLAC__STREAM_METADATA_PICTURE_TYPE_UNDEFINED; pic_type++)
    {
        
        if (FLAC__metadata_get_picture(filename, &smet, pic_type, NULL,
NULL,  (unsigned)(-1) ,  (unsigned)(-1),  (unsigned)(-1), (unsigned)(-1)) )
        {
            FLAC__StreamMetadata_Picture pict = smet->data.picture;
            if ( pict.data_length )
            {
                printf("found image %s : %s", pict.mime_type,
pict.description);
            } else  printf(« no data %s : %s\n", pict.mime_type,
pict.description);
            break;
        } else printf("get_picture %u returned false\n", pic_type );
    }

I have tried to use -1 instead of pic_type as 3rd argument on a single call to
FLAC__metadata_get_picture()
I have tried different flac files which all returns a valid image file when I
use the command line tool metaflac --export-picture-toThe command line tools had
been successfully compiled at the same time than the dylib library
( on Mac OS 10.11, intel x86_64 )

in my project FLAC__metadata_get_picture always return false !!!

Anyone has an idea what could be going wrong ?
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