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28 GNU make

This is done with the automatic variables such as ‘$^’ (see Section 10.5.3 [Automatic

Variables], page 120). For instance, the value of ‘$^’ is a list of all the prerequisites of the

rule, including the names of the directories in which they were found, and the value of ‘$@’

is the target. Thus:

foo.o : foo.c

cc -c $(CFLAGS) $^ -o $@

(The variable CFLAGS exists so you can specify ﬂags for C compilation by implicit rules; we

use it here for consistency so it will aﬀect all C compilations uniformly; see Section 10.3

[Variables Used by Implicit Rules], page 115.)

Often the prerequisites include header ﬁles as well, which you do not want to mention

in the recipe. The automatic variable ‘$<’ is just the ﬁrst prerequisite:

VPATH = src:../headers

foo.o : foo.c defs.h hack.h

cc -c $(CFLAGS) $< -o $@

4.4.5 Directory Search and Implicit Rules

The search through the directories speciﬁed in VPATH or with vpath also happens during

consideration of implicit rules (see Chapter 10 [Using Implicit Rules], page 111).

For example, when a ﬁle foo.o has no explicit rule, make considers implicit rules, such

as the built-in rule to compile foo.c if that ﬁle exists. If such a ﬁle is lacking in the current

directory, the appropriate directories are searched for it. If foo.c exists (or is mentioned

in the makeﬁle) in any of the directories, the implicit rule for C compilation is applied.

The recipes of implicit rules normally use automatic variables as a matter of necessity;

consequently they will use the ﬁle names found by directory search with no extra eﬀort.

4.4.6 Directory Search for Link Libraries

Directory search applies in a special way to libraries used with the linker. This special

feature comes into play when you write a prerequisite whose name is of the form ‘-lname’.

(You can tell something strange is going on here because the prerequisite is normally the

name of a ﬁle, and the ﬁle name of a library generally looks like libname.a, not like

‘-lname’.)

When a prerequisite’s name has the form ‘-lname’, make handles it specially by searching

for the ﬁle libname.so, and, if it is not found, for the ﬁle libname.a in the current directory,

in directories speciﬁed by matching vpath search paths and the VPATH search path, and then

in the directories /lib, /usr/lib, and prefix/lib (normally /usr/local/lib, but MS-

DOS/MS-Windows versions of make behave as if preﬁx is deﬁned to be the root of the

DJGPP installation tree).

For example, if there is a /usr/lib/libcurses.a library on your system (and no

/usr/lib/libcurses.so ﬁle), then

foo : foo.c -lcurses

cc $^ -o $@

would cause the command ‘cc foo.c /usr/lib/libcurses.a -o foo’ to be executed when

foo is older than foo.c or than /usr/lib/libcurses.a.

Although the default set of ﬁles to be searched for is libname.so and libname.a, this

is customizable via the .LIBPATTERNS variable. Each word in the value of this variable is

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