Re: [PATCH 1/3] libsupport: fix portability issues with the bthread.c

["Theodore Tso" <tytso@mit.edu>]

On Fri, Apr 03, 2026 at 08:15:35AM -0700, Darrick J. Wong wrote:
> > It's "autoheader".  Whenver you add or remove tests to configure.ac,
> > you need to run "autoconf ; autoheader".  If you don't run
> > "autoheader", then even if the autoconf's feature test enables some
> > new feature test, say HAVE_PR_SET_IO_FLUSHER, the feature won't
> > actually be enabled in the #ifdef.
> 
> Aha!  Thanks for that tip; I'll go fix my dev branch.

Just to give more context, there are two ways that configure scripts
can pass the feature definitions to programs.  One is via the command
line of the compiler.  The problem with that is if you have dozens of
options like -DHAVE_PR_SET_IO_FLUSHER for every single compile, it
bloats the MAKELOG files and makes it hard to read, and eventually you
can run into the command-line length limits.

The other way is via a config.h file which is then #included in each
source file.  But for *that* to work, the config.h.in file needs to
have a template for echo variable that can be #defined.  And that's
what autoheader takes care of for you.  Simpler projects that don't
use a config.h file and pass everything on the compiler's command line
don't need to use autoheader.

For a demonstration, try editing lib/config.h.in to remove the lines:

-/* Define to 1 if PR_SET_IO_FLUSHER is present */
-#undef HAVE_PR_SET_IO_FLUSHER

Then "rm lib/config.h" and run "./config.status", and examine
lib/config.h.  You will see that HAVE_PR_SET_IO_FLUSHER is no longer
defined in the config.h file.  Then run "autoheader", and then re-run
"./config.status", and you will see that HAVE_PR_SET_IO_FLUSHER is
properly defined.

Cheers,

					- Ted