14.1 General Conventions for Makefiles
Every Makefile should contain this line:
to avoid trouble on systems where the SHELL variable might be
inherited from the environment. (This is never a problem with GNU
make.)
Different make programs have incompatible suffix lists and
implicit rules, and this sometimes 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 -e 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 -e $(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 appear in the source
directory, they should always appear in the source directory, not in the
build directory. So Makefile rules to update them should put the
updated files in the source directory.
However, if a file does not appear in the distribution, then the
Makefile should not put it in the source directory, because building a
program in ordinary circumstances should not modify the source directory
in any way.
Try to make the build and installation targets, at least (and all their
subtargets) work correctly with a parallel make.
