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Chapter 4: Writing Rules 27

vpath %.c foo

vpath % blish

vpath %.c bar

will look for a ﬁle ending in ‘.c’ in foo, then blish, then bar, while

vpath %.c foo:bar

vpath % blish

will look for a ﬁle ending in ‘.c’ in foo, then bar, then blish.

4.4.3 How Directory Searches are Performed

When a prerequisite is found through directory search, regardless of type (general or se-

lective), the pathname located may not be the one that make actually provides you in the

prerequisite list. Sometimes the path discovered through directory search is thrown away.

The algorithm make uses to decide whether to keep or abandon a path found via directory

search is as follows:

1. If a target ﬁle does not exist at the path speciﬁed in the makeﬁle, directory search is

performed.

2. If the directory search is successful, that path is kept and this ﬁle is tentatively stored

as the target.

3. All prerequisites of this target are examined using this same method.

4. After processing the prerequisites, the target may or may not need to be rebuilt:

a. If the target does not need to be rebuilt, the path to the ﬁle found during directory

search is used for any prerequisite lists which contain this target. In short, if make

doesn’t need to rebuild the target then you use the path found via directory search.

b. If the target does need to be rebuilt (is out-of-date), the pathname found during

directory search is thrown away, and the target is rebuilt using the ﬁle name

speciﬁed in the makeﬁle. In short, if make must rebuild, then the target is rebuilt

locally, not in the directory found via directory search.

This algorithm may seem complex, but in practice it is quite often exactly what you

want.

Other versions of make use a simpler algorithm: if the ﬁle does not exist, and it is found

via directory search, then that pathname is always used whether or not the target needs

to be built. Thus, if the target is rebuilt it is created at the pathname discovered during

directory search.

If, in fact, this is the behavior you want for some or all of your directories, you can use

the GPATH variable to indicate this to make.

GPATH has the same syntax and format as VPATH (that is, a space- or colon-delimited list

of pathnames). If an out-of-date target is found by directory search in a directory that also

appears in GPATH, then that pathname is not thrown away. The target is rebuilt using the

expanded path.

4.4.4 Writing Recipes with Directory Search

When a prerequisite is found in another directory through directory search, this cannot

change the recipe of the rule; they will execute as written. Therefore, you must write the

recipe with care so that it will look for the prerequisite in the directory where make ﬁnds it.

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