4.5.3 How Directory Searches are Performed
When a prerequisite is found through directory search, regardless of type
(general or selective), 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:
-
If a target file does not exist at the path specified in the makefile,
directory search is performed.
-
If the directory search is successful, that path is kept and this file
is tentatively stored as the target.
-
All prerequisites of this target are examined using this same method.
-
After processing the prerequisites, the target may or may not need to be
rebuilt:
-
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.
-
If the target does need to be rebuilt (is out-of-date), the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 If an out-of-date target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.
