How to check for the existence of a header of a package?

Question

In Autoconf, when you want to check for the existence of some header, you do:

AC_CHECK_HEADERS([foo.h])

Now, let's suppose that I have some package, sponge, of which pkg-config knows, and I want to find out if it has a header named spongefoo.h.

I can do the following:

PKG_CHECK_MODULES([SPONGE])
CPPFLAGS="$CPPFLAGS $SPONGE_CFLAGS"
AC_CHECK_HEADERS([spongefoo.h])

This would work, but the Autoconf/Automake documentation says (my additions in brackets):

"Sometimes package developers are tempted to set user variables such as 'CFLAGS' [and, I add, 'CPPFLAGS'] because it appears to make their job easier. However, the package itself should never set a user variable, [...] To get around this problem, Automake introduces an automake-specific shadow variable [named AM_CPPFLAGS]"

However, AC_CHECK_HEADERS() and other test macros don't know about Automake's shadow variables.

So how do I properly check that a package has some header?

(BTW, AC_CHECK_HEADER() gets a 4'th argument, "INCLUDES", but this is a verbatim '"#include"' text, not a '"-I..."' switch, so it doesn't help me much.)

Answer 1

The requirement to temporarily set a user variable like CPPFLAGS to some value and run a compilation test while preserving the original value is actually done quite often. This is usually accomplished in Autoconf using a pattern similar to the following:

ac_save_CPPFLAGS=$CPPFLAGS
CPPFLAGS="$CPPFLAGS $SPONGE_CFLAGS"
AC_CHECK_HEADERS([spongefoo.h])
CPPFLAGS=$ac_save_CPPFLAGS

Note that the ac_ shell variable prefix is informally reserved for Autoconf itself, you might want to use your own variable name like proj_save_CPPFLAGS.