Info: (automake-1.16) Sources

automake-1.16: Sources

 
 9.4 Built Sources
 =================
 
 Because Automake’s automatic dependency tracking works as a side-effect
 of compilation (⇒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 when ‘make all’, ‘make
 check’, ‘make install’, ‘make install-exec’ (or ‘make dist’) is run,
 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 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.
 
    To emphasize, ‘BUILT_SOURCES’ is honored only by ‘make all’, ‘make
 check’, ‘make install’, and ‘make install-exec’ (and ‘make dist’).  This
 means you cannot build an arbitrary 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.
automake-1.16: Sources

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