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Chapter 15: Makeﬁle Conventions 149

15 Makeﬁle Conventions

This chapter describes conventions for writing the Makeﬁles for GNU programs. Using Au-

tomake will help you write a Makeﬁle that follows these conventions. For more information

on portable Makeﬁles, see posix and Section “Portable Make” in Autoconf .

15.1 General Conventions for Makeﬁles

Every Makeﬁle should contain this line:

SHELL = /bin/sh

to avoid trouble on systems where the SHELL variable might be inherited from the environ-

ment. (This is never a problem with GNU make.)

Diﬀerent make programs have incompatible suﬃx lists and implicit rules, and this some-

times creates confusion or misbehavior. So it is a good idea to set the suﬃx list explicitly

using only the suﬃxes you need in the particular Makeﬁle, like this:

.SUFFIXES:

.SUFFIXES: .c .o

The ﬁrst line clears out the suﬃx list, the second introduces all suﬃxes which may be

subject to implicit rules in this Makeﬁle.

Don’t assume that . is in the path for command execution. When you need to run

programs that are a part of your package during the make, please make sure that it uses ./

if the program is built as part of the make or $(srcdir)/ if the ﬁle is an unchanging part

of the source code. Without one of these preﬁxes, the current search path is used.

The distinction between ./ (the build directory) and $(srcdir)/ (the source directory)

is important because users can build in a separate directory using the ‘--srcdir’ option to

configure. A rule of the form:

foo.1 : foo.man sedscript

sed -f sedscript foo.man > foo.1

will fail when the build directory is not the source directory, because foo.man and sedscript

are in the source directory.

When using GNU make, relying on ‘VPATH’ to ﬁnd the source ﬁle will work in the case

where there is a single dependency ﬁle, since the make automatic variable ‘$<’ will represent

the source ﬁle wherever it is. (Many versions of make set ‘$<’ only in implicit rules.) A

Makeﬁle target like

foo.o : bar.c

$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o

should instead be written as

foo.o : bar.c

$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@

in order to allow ‘VPATH’ to work correctly. When the target has multiple dependencies,

using an explicit ‘$(srcdir)’ is the easiest way to make the rule work well. For example,

the target above for foo.1 is best written as:

foo.1 : foo.man sedscript

sed -f $(srcdir)/sedscript $(srcdir)/foo.man > $@

GNU distributions usually contain some ﬁles which are not source ﬁles—for example, Info

ﬁles, and the output from Autoconf, Automake, Bison or Flex. Since these ﬁles normally

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