(automake-1.16.info)Default _SOURCES

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8.5 Default ‘_SOURCES’
======================

‘_SOURCES’ variables are used to specify source files of programs (Note:
A Program), libraries (Note: A Library), and Libtool libraries
(Note: A Shared Library).

   When no such variable is specified for a target, Automake will define
one itself.  The default is to compile a single C file whose base name
is the name of the target itself, with any extension replaced by
‘AM_DEFAULT_SOURCE_EXT’, which defaults to ‘.c’.

   For example if you have the following somewhere in your ‘Makefile.am’
with no corresponding ‘libfoo_a_SOURCES’:

     lib_LIBRARIES = libfoo.a sub/libc++.a

‘libfoo.a’ will be built using a default source file named ‘libfoo.c’,
and ‘sub/libc++.a’ will be built from ‘sub/libc++.c’.  (In older
versions ‘sub/libc++.a’ would be built from ‘sub_libc___a.c’, i.e., the
default source was the canonized name of the target, with ‘.c’ appended.
We believe the new behavior is more sensible, but for backward
compatibility ‘automake’ will use the old name if a file or a rule with
that name exists and ‘AM_DEFAULT_SOURCE_EXT’ is not used.)

   Default sources are mainly useful in test suites, when building many
test programs each from a single source.  For instance, in

     check_PROGRAMS = test1 test2 test3
     AM_DEFAULT_SOURCE_EXT = .cpp

‘test1’, ‘test2’, and ‘test3’ will be built from ‘test1.cpp’,
‘test2.cpp’, and ‘test3.cpp’.  Without the last line, they will be built
from ‘test1.c’, ‘test2.c’, and ‘test3.c’.

   Another case where this is convenient is building many Libtool
modules (‘moduleN.la’), each defined in its own file (‘moduleN.c’).

     AM_LDFLAGS = -module
     lib_LTLIBRARIES = module1.la module2.la module3.la

   Finally, there is one situation where this default source computation
needs to be avoided: when a target should not be built from sources.  We
already saw such an example in Note: true; this happens when all the
constituents of a target have already been compiled and just need to be
combined using a ‘_LDADD’ variable.  Then it is necessary to define an
empty ‘_SOURCES’ variable, so that ‘automake’ does not compute a
default.

     bin_PROGRAMS = target
     target_SOURCES =
     target_LDADD = libmain.a libmisc.a