File: make.info,  Node: Makefile Basics,  Next: Utilities in Makefiles,  Up: Makefile Conventions

16.1 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 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 -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 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'.