Olivier Dion schreef op vr 08-04-2022 om 10:28 [-0400]:
>  and don't fully understand it.

lttn-ust probably has some header
/gnu/store/...-lttng-unst-VERSION/include/lttng.h or the like.
It would look something like:

  [...]
  #include <liburcu.h>
  int lttng_foo(urcu_stuff *bar);
  [..]

Then the 'patch-header-inclusion' phase detects the #include
<liburcu.h>, looks for include/liburcu.h in the package inputs, and
finds /gnu/store/...-liburcu-VERSION/include/liburcu.h.  It then
replaces liburcu.h by /gnu/store/...-liburcu-VERSION/include/liburcu.h:

  [...]
  #include </gnu/store/.../include/liburcu.h>
  int lttng_foo(urcu_stuff *bar);
  [...]

Now, suppose I build an application dependning on lttng-ust.  Then the
C compiler will ‘include’ 'lttng.h' in the {CROSS_,}C_INLUDE_PATH. 
Then it sees:

  #include </gnu/store/.../include/liburcu.h>

Now, as this is an absolute /gnu/store/... file name, the compiler
knows where to find it without looking into {CROSS_,}C_INCLUDE_PATH, so
it will find the header even though it might not be in
{CROSS_,}C_INCLUDE_PATH.

It's the same system as doing some 'substitute*' to bake in the
absolute file name of some executable into the compiled application to
avoid relying on PATH, except applied to C headers instead of
executables and {CROSS_,}C_CINCLUDE_PATH instead of PATH.

Greetings,
Maxime.