On 04/12/2016 09:05 AM, Nick Bowler wrote: > > But in your second instance, the first expansion of AC_CHECK_HEADER > expands AC_PROG_CPP inside an "if". The result is that no preprocessor > is checked in the "else" case. You can see this in the configure output, > the following line is only printed in the avr case: > > checking how to run the C preprocessor... avr-gcc -E > > There are several basic solutions: > > - First, you can just expand AC_PROG_CPP directly and unconditionally > before your if. This will ensure the macro is available in both cases. > > - Second is to rewrite your condition using AS_IF, which automatically > "hoists" the dependency AC_PROG_CPP (and any other dependencies) > outside of the if condition. For example: > > AS_IF([test x"$host" = x"avr"], > [AC_CHECK_HEADER([avr/io.h], [], > [AC_MSG_ERROR([missing header: avr/io.h])]) > AC_CHECK_HEADER([util/delay.h], [], > [AC_MSG_ERROR([missing header: util/delay.h])])], > > [AC_CHECK_HEADER([stdio.h], [], > [AC_MSG_ERROR([missing header: stdio.h])]) Also, checking for <stdio.h> is pointless these days. You can portably assume a C89 compiler (and these days, often a C99 compiler), which guarantees <stdio.h> is present. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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