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 7.4 Nesting Packages
 ====================
 
 In the GNU Build System, packages can be nested to arbitrary depth.
 This means that a package can embed other packages with their own
 ‘configure’, ‘Makefile’s, etc.
 
    These other packages should just appear as subdirectories of their
 parent package.  They must be listed in ‘SUBDIRS’ like other ordinary
 directories.  However the subpackage’s ‘Makefile’s should be output by
 its own ‘configure’ script, not by the parent’s ‘configure’.  This is
 achieved using the ‘AC_CONFIG_SUBDIRS’ Autoconf macro (⇒
 AC_CONFIG_SUBDIRS (autoconf)Subdirectories.).
 
    Here is an example package for an ‘arm’ program that links with a
 ‘hand’ library that is a nested package in subdirectory ‘hand/’.
 
    ‘arm’’s ‘configure.ac’:
 
      AC_INIT([arm], [1.0])
      AC_CONFIG_AUX_DIR([.])
      AM_INIT_AUTOMAKE
      AC_PROG_CC
      AC_CONFIG_FILES([Makefile])
      # Call hand's ./configure script recursively.
      AC_CONFIG_SUBDIRS([hand])
      AC_OUTPUT
 
    ‘arm’’s ‘Makefile.am’:
 
      # Build the library in the hand subdirectory first.
      SUBDIRS = hand
 
      # Include hand's header when compiling this directory.
      AM_CPPFLAGS = -I$(srcdir)/hand
 
      bin_PROGRAMS = arm
      arm_SOURCES = arm.c
      # link with the hand library.
      arm_LDADD = hand/libhand.a
 
    Now here is ‘hand’’s ‘hand/configure.ac’:
 
      AC_INIT([hand], [1.2])
      AC_CONFIG_AUX_DIR([.])
      AM_INIT_AUTOMAKE
      AC_PROG_CC
      AM_PROG_AR
      AC_PROG_RANLIB
      AC_CONFIG_FILES([Makefile])
      AC_OUTPUT
 
 and its ‘hand/Makefile.am’:
 
      lib_LIBRARIES = libhand.a
      libhand_a_SOURCES = hand.c
 
    When ‘make dist’ is run from the top-level directory it will create
 an archive ‘arm-1.0.tar.gz’ that contains the ‘arm’ code as well as the
 ‘hand’ subdirectory.  This package can be built and installed like any
 ordinary package, with the usual ‘./configure && make && make install’
 sequence (the ‘hand’ subpackage will be built and installed by the
 process).
 
    When ‘make dist’ is run from the hand directory, it will create a
 self-contained ‘hand-1.2.tar.gz’ archive.  So although it appears to be
 embedded in another package, it can still be used separately.
 
    The purpose of the ‘AC_CONFIG_AUX_DIR([.])’ instruction is to force
 Automake and Autoconf to search for auxiliary scripts in the current
 directory.  For instance, this means that there will be two copies of
 ‘install-sh’: one in the top-level of the ‘arm’ package, and another one
 in the ‘hand/’ subdirectory for the ‘hand’ package.
 
    The historical default is to search for these auxiliary scripts in
 the parent directory and the grandparent directory.  So if the
 ‘AC_CONFIG_AUX_DIR([.])’ line was removed from ‘hand/configure.ac’, that
 subpackage would share the auxiliary script of the ‘arm’ package.  This
 may look like a gain in size (a few kilobytes), but more importantly, it
 is a loss of modularity as the ‘hand’ subpackage is no longer
 self-contained (‘make dist’ in the subdirectory will not work anymore).
 
    Packages that do not use Automake need more work to be integrated
 this way.  ⇒Third-Party Makefiles.
 

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