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>> > > +extern int ped_disk_get_max_partition_num(const PedDisk* disk);

merely queries the code for the appropriate partition table type
and always returns a non-negative number.

> when it encounters an error.  All possitive numbers can be valide to
> express max_number_of_supported_partitions.  Including 0. So a negative
> number would seem natural for me to use in this case.
>
>  0: means that the label does no support partitions.
>  <0: means the number of partitions
>  >0: means something nasty has ocurred.