Taglists

Grzegorz Kraszewski

A taglist is an array of "key-value" pairs. The key is always a 32-bit integer number and is called a tag. The value has also size of 32 bits. It may be an integer, or pointer to any structure or object. Taglists are commonly used in the MorphOS API for passing variable number of arguments, usually sets of attributes with their values. A few special key values are used for array termination, concatenation and skipping. A set of functions in the utility.library may be used for taglist traversing, filtering, searching, copying etc.

Every pair in a taglist is a TagItem structure, defined in <utility/tagitem.h>:

struct TagItem
{
  ULONG ti_Tag;
  ULONG ti_Data;
};

To be selfdescriptive, every taglist, being just a plain C array, must have some kind of termination. It is very similar to string null-termination idea. The termination is done with a TagItem having its ti_Tag set to TAG_END (which happens to be defined as zero). ti_Data value of the terminating TagItem is ignored, it is usually set to zero too. The drawing below shows some simple taglist: