autoconf AS_IF doesn't execute correct branch

问题

I'm having a bit of a mystery in a autoconf script, specifically AS_IF.

Here's the relevant code:

AC_CHECK_FUNCS([eventfd], [AC_DEFINE([NN_HAVE_EVENTFD])])
AC_CHECK_FUNCS([pipe], [AC_DEFINE([NN_HAVE_PIPE])])
AC_CHECK_FUNCS([pipe2], [
    AC_DEFINE([NN_HAVE_PIPE2])
    CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
])
AC_SEARCH_LIBS([socketpair], [], [AC_DEFINE([NN_HAVE_SOCKETPAIR])])

i.e. checking for their existance. These work correctly and define the correct symbols. In this case, only NN_HAVE_PIPE and NN_HAVE_SOCKETPAIR gets defined, which is correct as this is a HP-UX system.

Now to the mystery part. Later in the configure.ac, there's a conditional referring to these symbols:

echo "ac_cv_func_eventfd: $ac_cv_func_eventfd"
AS_IF([test x"$ac_cv_func_eventfd"=xyes], [
    AC_DEFINE([NN_USE_EVENTFD])], [
    AS_IF([test x"$ac_cv_func_pipe"=xyes], [
        AC_DEFINE([NN_USE_PIPE])], [
        AS_IF([test x"$ac_cv_func_socketpair"=xyes], [
            AC_DEFINE([NN_USE_SOCKETPAIR])], [
            AC_MSG_ERROR([No signaling supported])
        ])
    ])
])

Even though ac_cv_func_eventfd has value no (I added the echo line to make sure), NN_USE_EVENTFD gets defined anyway!

As the AS_IF macro is defined as such:

AS_IF (test1, [run-if-true1], ..., [run-if-false])

To me, the code seems quite correct, no? Is there anyone that can shed some light on this behavior?

Autoconf version is 2.67. The OS is HP-UX 11.31 ia64.

回答1:

Not quite correct. You must fix your AS_IF test from:

test x"$ac_cv_func_eventfd"=xyes

to

 test x"$ac_cv_func_eventfd" = xyes

Note the insertion of spaces around the =. Same goes for the other tests in the other AS_IFs.

To verify this you can try seeing this at the command line:

if test x"no"=xyes; then echo "yes"; else echo "no"; fi

vs.

if test x"no" = xyes; then echo "yes"; else echo "no"; fi

来源：https://stackoverflow.com/questions/18889558/autoconf-as-if-doesnt-execute-correct-branch