When you declare another tree as a tree dependency of your tree, you declare your dependency on the other tree by mentioning its name in the tree-deps of your tree's root Abuild.conf. In order for this dependency to work, abuild must know where to find the tree. Abuild finds items and trees in the same way: it traverses the build forest from the top down and creates a table mapping names to paths. If the tree your tree depends on is inside of your tree, this poses no problem. But what if it is an external tree that is not inside your tree? In this instance, you must place the external tree somewhere within your overall build area, such as in another subdirectory of the parent of your own tree's root. Then you must create an Abuild.conf file in that common parent directory that knows about the root directories of the two trees. This is illustrated in Figure 7.1, “Top-Level Abuild.conf”.

Figure 7.1. Top-Level Abuild.conf

Tree A declares a tree dependency on tree B. In order for A to find B, an Abuild.conf file that points to both trees' locations must be created in a common ancestor directory. The ovals show the contents of each directory's Abuild.conf files.

The tree named B has an Abuild.conf that that declares no tree dependencies. It is a self-contained tree. However, A's Abuild.conf file mentions B by name. How does A find B? When you start abuild, it walks up the tree to find the highest-level Abuild.conf (or the highest level one not referenced as a child of the next higher Abuild.conf) and traverses downward from there. In this case, the Abuild.conf in A's parent directory knows the locations of both A and B. In this way, abuild has figured out where to find B when A declares the tree dependency. This is illustrated with a concrete example below.