7.1 General Automake principles
Automake at its simplest turns a file called `Makefile.am' into a
GNU-compliant `Makefile.in' for use with `configure'. Each
`Makefile.am' is written according to make syntax; Automake
recognizes special macro and target names and generates code based on
these.
There are a few Automake rules which differ slightly from make
rules:
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Ordinary
make comments are passed through to the output, but
comments beginning with `##' are Automake comments and are not
passed through.
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Automake supports
include directives. These directives are not
passed through to the `Makefile.in', but instead are processed by
automake -- files included this way are treated as if they
were textually included in `Makefile.am' at that point. This can
be used to add boilerplate to each `Makefile.am' in a project via a
centrally-maintained file. The filename to include can start with
`$(top_srcdir)' to indicate that it should be found relative to the
top-most directory of the project; if it is a relative path or if it
starts with `$(srcdir)' then it is relative to the current
directory. For example, here is how you would reference boilerplate
code from the file `config/Make-rules' (where `config' is a
top-level directory in the project):
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include $(top_srcdir)/config/Make-rules
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Automake supports conditionals which are not passed directly through to
`Makefile.in'. This feature is discussed in 19. Advanced GNU Automake Usage.
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Automake supports macro assignment using `+='; these assignments
are translated by Automake into ordinary `=' assignments in
`Makefile.in'.
All macros and targets, including those which Automake does not
recognize, are passed through to the generated `Makefile.in' --
this is a powerful extension mechanism. Sometimes Automake will define
macros or targets internally. If these are also defined in
`Makefile.am' then the definition in `Makefile.am' takes
precedence. This feature provides an easy way to tailor specific parts
of the output in small ways.
Note, however, that it is a mistake to override parts of the generated
code that aren't documented (and thus `exported' by Automake).
Overrides like this stand a good chance of not working with future
Automake releases.
Automake also scans `configure.in'. Sometimes it uses the
information it discovers to generate extra code, and sometimes to
provide extra error checking. Automake also turns every AC_SUBST
into a `Makefile' variable. This is convenient in more ways than
one: not only does it mean that you can refer to these macros in
`Makefile.am' without extra work, but, since Automake scans
`configure.in' before it reads any `Makefile.am', it also
means that special variables and overrides Automake recognizes can be
defined once in `configure.in'.
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