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9.4 Built Sources
=================

Because Automake’s automatic dependency tracking works as a side-effect
of compilation (Note: Dependencies) there is a bootstrap issue: a
target should not be compiled before its dependencies are made, but
these dependencies are unknown until the target is first compiled.

   Ordinarily this is not a problem, because dependencies are
distributed sources: they preexist and do not need to be built.  Suppose
that ‘foo.c’ includes ‘foo.h’.  When it first compiles ‘foo.o’, ‘make’
only knows that ‘foo.o’ depends on ‘foo.c’.  As a side-effect of this
compilation ‘depcomp’ records the ‘foo.h’ dependency so that following
invocations of ‘make’ will honor it.  In these conditions, it’s clear
there is no problem: either ‘foo.o’ doesn’t exist and has to be built
(regardless of the dependencies), or accurate dependencies exist and
they can be used to decide whether ‘foo.o’ should be rebuilt.

   It’s a different story if ‘foo.h’ doesn’t exist by the first ‘make’
run.  For instance, there might be a rule to build ‘foo.h’.  This time
‘file.o’’s build will fail because the compiler can’t find ‘foo.h’.
‘make’ failed to trigger the rule to build ‘foo.h’ first by lack of
dependency information.

   The ‘BUILT_SOURCES’ variable is a workaround for this problem.  A
source file listed in ‘BUILT_SOURCES’ is made on ‘make all’ or ‘make
check’ (or even ‘make install’) before other targets are processed.
However, such a source file is not _compiled_ unless explicitly
requested by mentioning it in some other ‘_SOURCES’ variable.

   So, to conclude our introductory example, we could use ‘BUILT_SOURCES
= foo.h’ to ensure ‘foo.h’ gets built before any other target (including
‘foo.o’) during ‘make all’ or ‘make check’.

   ‘BUILT_SOURCES’ is actually a bit of a misnomer, as any file which
must be created early in the build process can be listed in this
variable.  Moreover, all built sources do not necessarily have to be
listed in ‘BUILT_SOURCES’.  For instance, a generated ‘.c’ file doesn’t
need to appear in ‘BUILT_SOURCES’ (unless it is included by another
source), because it’s a known dependency of the associated object.

   It might be important to emphasize that ‘BUILT_SOURCES’ is honored
only by ‘make all’, ‘make check’ and ‘make install’.  This means you
cannot build a specific target (e.g., ‘make foo’) in a clean tree if it
depends on a built source.  However it will succeed if you have run
‘make all’ earlier, because accurate dependencies are already available.

   The next section illustrates and discusses the handling of built
sources on a toy example.

Built Sources Example

Several ways to handle built sources.