Taglists

Grzegorz Kraszewski

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This article in other languages: Polish

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 also has a size of 32 bits. It may be an integer, or a pointer to any structure or object. Taglists are commonly used in the MorphOS API for passing a variable number of arguments, usually sets of attributes with their values. A few special key values are used for array termination, concatenation and item 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 self descriptive, every taglist, being just a plain C array, must have some kind of termination. It is very similar to the string null-termination idea. The termination is done with a TagItem having its ti_Tag field set to TAG_END (which happens to be defined as zero). The ti_Data value of the terminating TagItem is ignored, it is usually set to zero too. The illustration below shows a simple taglist:

This taglist may be created with the following code:

double x = 3.438872763e+17;
Object *obj = NewObject( /* ... */ );

struct TagItem mytaglist[] = {
  { Tag1, 2837 },
  { Tag2, (ULONG)&x },
  { Tag3, (ULONG)"The quick brown fox..." },
  { Tag4, (ULONG)obj },
  { TAG_END, 0 }
};

Passing Taglists to Functions

Building taglists as global or local variables is not very convenient. That is why almost every MorphOS API function getting a taglist as one of its arguments has two forms. The first one accepts a pointer to the taglist. The second one is a variadic args macro building the taglist on the program stack dynamically. Such function pairs are named according to one of the two conventions:

1. SomeFunctionA() takes a pointer to a taglist, SomeFunction() builds the taglist dynamically.
2. SomeFunctionTagList() takes a pointer to a taglist, SomeFunctionTags() builds the taglist dynamically.

Continuing the above example, one can pass the example taglist to SomeFunction() in two ways:

SomeFunctionTagList(mytaglist);
SomeFunctionTags(Tag1, 2837, Tag2, (ULONG)&x, Tag3, (ULONG)"The quick brown fox...", Tag4,
 (ULONG)obj, TAG_END);

Of course in the second case variable mytaglist need not be defined anywhere. Note that for every taglist-based function the taglist is the last argument. There may be some plain arguments before it. It is a common practice to omit ti_Data for the terminator (it is ignored anyway).