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Hierarchical processing makes cpp.req more susceptible to "once-only
header" optimization. To demonstrate the problem, I've implemented
some debugging facilities. Here is how <gtk/gtk.h> is processed.
$ cpp.req -vv /usr/include/gtk-2.0/gtk/gtk.h
[...]
Include gdk/gdk.h
+ Push /usr/include/gtk-2.0/gdk/gdk.h
Include gdk/gdkapplaunchcontext.h
+ Push /usr/include/gtk-2.0/gdk/gdkapplaunchcontext.h
Include gio/gio.h
! Push /usr/include/glib-2.0/gio/gio.h
Include gio/giotypes.h
Push /usr/include/glib-2.0/gio/giotypes.h
Include gio/gioenums.h
Push /usr/include/glib-2.0/gio/gioenums.h
Include glib-object.h
Push /usr/include/glib-2.0/glib-object.h
Include gobject/gbinding.h
Push /usr/include/glib-2.0/gobject/gbinding.h
Include glib.h
Push /usr/include/glib-2.0/glib.h
[...]
+ Push /usr/include/gtk-2.0/gtk/gtkdebug.h
Include glib.h
Pop
[...]
recovered glib.h -> /usr/include/glib-2.0/glib.h
recovered stdarg.h -> /usr/lib64/gcc/x86_64-alt-linux/4.5.3/include/stdarg.h
recovered time.h -> /usr/include/time.h
recovered glib-object.h -> /usr/include/glib-2.0/glib-object.h
In the output, "Include" lines annotate "#include" instructions which
are about to be processed by cpp; "Push" and "Pop" annotate actual
file operations performed by cpp. Technically, "Include" annotations
are enabled via -dI option which installs cb_include callback in
gcc/c-ppoutput.c; "Push" and "Pop" are triggered in the guts of the
libcpp library. The library has hardcoded optimization against repeated
inclusions. According to "info cpp", "It remembers when a header file
has a wrapper #ifndef. If a subsequent #include specifies that header,
and the macro in the #ifndef is still defined, it does not bother to
rescan the file at all." (See should_stack_file in libcpp/files.c.)
This means that, normally, each "Include" should be followed by a
corresponding "Push". However, due to "once-only header" optimization,
some includes are not followed by a push. This means that the file
has already been pushed, and it happens to use a wrapper #ifndef.
Note that, in the output, this is exactly the case with <glib2.h>.
Also note that, in the output, files internal to the package are marked
with "+" on the left. They are tracked down to the first non-packaged
file, which makes a dependency; these files are marked with "!". The
problem with <glib2.h> is then that it gets first included in an
external file. Later it is also included in an internal file, but
a "Push" is not triggered. And since the internal file is subordinate
to <gtk/gtk.h> and is not going to be processed on its own, the
dependency on <glib2.h> is lost.
To recover missing pushes, we have to associate each include with the
first-time push. In other words, we need a table which maintains a
(header -> filename) mapping; in the above example, the table will
contain (glib.h -> /usr/include/glib-2.0/glib.h). Using this table,
we can ascertain that each internal #include produced a result.
Now, this might still have some corner cases: includes with
non-canonical header names probably will not be recovered, and it is not
clear whether <foo.h> and "foo.h" should be processed differently.
It works well enough in simple cases, though.
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