2.1 General Operation

Automake works by reading a `Makefile.am' and generating a `Makefile.in'. Certain variables and targets defined in the `Makefile.am' instruct Automake to generate more specialized code; for instance, a `bin_PROGRAMS' variable definition will cause targets for compiling and linking programs to be generated.

The variable definitions and targets in the `Makefile.am' are copied verbatim into the generated file. This allows you to add arbitrary code into the generated `Makefile.in'. For instance the Automake distribution includes a non-standard cvs-dist target, which the Automake maintainer uses to make distributions from his source control system.

Note that most GNU make extensions are not recognized by Automake. Using such extensions in a `Makefile.am' will lead to errors or confusing behavior.

A special exception is that the GNU make append operator, `+=', is supported. This operator appends its right hand argument to the variable specified on the left. Automake will translate the operator into an ordinary `=' operator; `+=' will thus work with any make program.

Automake tries to keep comments grouped with any adjoining targets or variable definitions.

A target defined in `Makefile.am' generally overrides any such target of a similar name that would be automatically generated by automake. Although this is a supported feature, it is generally best to avoid making use of it, as sometimes the generated rules are very particular.

Similarly, a variable defined in `Makefile.am' or AC_SUBST'ed from `configure.in' will override any definition of the variable that automake would ordinarily create. This feature is more often useful than the ability to override a target definition. Be warned that many of the variables generated by automake are considered to be for internal use only, and their names might change in future releases.

When examining a variable definition, Automake will recursively examine variables referenced in the definition. For example, if Automake is looking at the content of foo_SOURCES in this snippet

xs = a.c b.c foo_SOURCES = c.c $(xs)

it would use the files `a.c', `b.c', and `c.c' as the contents of foo_SOURCES.

Automake also allows a form of comment which is not copied into the output; all lines beginning with `##' (leading spaces allowed) are completely ignored by Automake.

It is customary to make the first line of `Makefile.am' read:

## Process this file with automake to produce Makefile.in