Roland Ver. 4.5 Technical Information PDF herunterladen (Seite 39)

Chapter 4: Writing Rules 29

a pattern string. When a prerequisite like ‘-lname’ is seen, make will replace the percent

in each pattern in the list with name and perform the above directory searches using each

library ﬁle name.

The default value for .LIBPATTERNS is ‘lib%.so lib%.a’, which provides the default

behavior described above.

You can turn oﬀ link library expansion completely by setting this variable to an empty

value.

4.5 Phony Targets

A phony target is one that is not really the name of a ﬁle; rather it is just a name for a

recipe to be executed when you make an explicit request. There are two reasons to use a

phony target: to avoid a conﬂict with a ﬁle of the same name, and to improve performance.

If you write a rule whose recipe will not create the target ﬁle, the recipe will be executed

every time the target comes up for remaking. Here is an example:

clean:

rm *.o temp

Because the rm command does not create a ﬁle named clean, probably no such ﬁle will

ever exist. Therefore, the rm command will be executed every time you say ‘make clean’.

In this example, the clean target will not work properly if a ﬁle named clean is ever

created in this directory. Since it has no prerequisites, clean would always be considered

up to date and its recipe would not be executed. To avoid this problem you can explicitly

declare the target to be phony by making it a prerequisite of the special target .PHONY (see

Section 4.8 [Special Built-in Target Names], page 32) as follows:

.PHONY: clean

clean:

rm *.o temp

Once this is done, ‘make clean’ will run the recipe regardless of whether there is a ﬁle

named clean.

Phony targets are also useful in conjunction with recursive invocations of make (see

Section 5.7 [Recursive Use of make], page 50). In this situation the makeﬁle will often

contain a variable which lists a number of sub-directories to be built. A simplistic way to

handle this is to deﬁne one rule with a recipe that loops over the sub-directories, like this:

SUBDIRS = foo bar baz

subdirs:

for dir in $(SUBDIRS); do \

$(MAKE) -C $$dir; \

done

There are problems with this method, however. First, any error detected in a sub-make

is ignored by this rule, so it will continue to build the rest of the directories even when one

fails. This can be overcome by adding shell commands to note the error and exit, but then

it will do so even if make is invoked with the -k option, which is unfortunate. Second, and

perhaps more importantly, you cannot take advantage of make’s ability to build targets in

parallel (see Section 5.4 [Parallel Execution], page 47), since there is only one rule.

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 211 212

Kommentare zu diesen Handbüchern

Keine Kommentare

Roland Ver. 4.5 Technical Information Seite 39

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Drucken & Scannen Roland Ver. 4.5