m4_version_compare does not see variable - autoconf

Question

I want to test the version of some package (e.g PCRE) in a configure.in script, and define C++ variables in a Makefile accordingly. I thought that the autoconf function m4_version_compare would do the trick, but I can't get it to work. I am sure I am not using it correctly.
I have the following in configure.in:
AC_INIT([MyPackage], 0.4)
# Checks for common programs using default macros
AC_PROG_CC
PCREVERSION=`pcre-config --version`
AC_MSG_RESULT([Detected PCRE version ${PCREVERSION}])
PCRE_POST_8_0=m4_version_compare([PCREVERSION], [8.0])
AC_MSG_RESULT([PCRE version >= 8.0: ${PCRE_POST_8_0}])
AC_SUBST(PCRE_POST_8_0)
AC_OUTPUT(src/Makevars)
then autoconf + ./configure produces the following output:
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
Detected PCRE version 8.12
PCRE version >= 8.0: -1
configure: creating ./config.status
config.status: creating src/Makevars
What am I doing wrong? It seems that m4_version_compare sees the variable PCREVERSION as being 0. Should I define this variable in another way? How?
Thank you.
Renaud

Answer 1

You cannot use m4_version_compare to compare version numbers found at configure-time.
The m4_version_compare macro takes two strings that must be known at the time autoconf runs to build the configure script. Here you are comparing the string PCREVERSION to the string 8.0, and the former is reported to be lesser than the latter.
What you would like to use is in fact the value of the shell variable named PCREVERSION, i.e., $PCREVERSION, unfortunately this value is only known when ./configure runs, so that won't work.
You should make your own comparison using the shell or some other means. Personaly I would rely on AC_PREPROC_IFELSE to let the preprocessor do two tests at once: make sure the pcre.h file exists and that it is recent enough. Something like this might work (untested):
AC_PREPROC_IFELSE(
[AC_LANG_PROGRAM([[#include "pcre.h"
#if PCRE_MAJOR < 8
#error out of date
#endif]], [[]])],
[pcre8available=yes],
[pcre8available=no])

Answer 2

I got this working using AS_VERSION_COMPARE, which is run at runtime (unlike m4_version_compare, as pointed out by adl). So the code I eventually use for achieving this is:
AC_INIT([MyPackage], 0.4)
# Checks for common programs using default macros
AC_PROG_CC
PCREVERSION=`pcre-config --version`
CMPV="8.0"
AC_MSG_CHECKING([is PCRE version >= ${CMPV}])
AS_VERSION_COMPARE(${PCREVERSION}, ${CMPV}, [PCRE_POST_8_0=-1], [PCRE_POST_8_0=0],[PCRE_POST_8_0=1])
AS_IF([test "${PCRE_POST_8_0}" != "-1"], AC_MSG_RESULT([yes]), AC_MSG_RESULT([no]))
AC_SUBST(PCRE_POST_8_0)
AC_OUTPUT(src/Makevars)
I think it is nice because it is generic and will work with any program that provides a command to retrieve its version.
But I will keep in mind adl's trick for the case where the version is not directly accessible from the shell. Thanks!