+ # (Note 1: This will need to be revisited if a system that Clang
+ # supports has POSIX threads in a separate library. This tends not
+ # to be the way of modern systems, but it's conceivable.)
+
+ # (Note 2: On some systems, notably Darwin, -pthread is not needed
+ # to get POSIX threads support; the API is always present and
+ # active. We could reasonably leave PTHREAD_CFLAGS empty. But
+ # -pthread does define _REENTRANT, and while the Darwin headers
+ # ignore this macro, third-party headers might not.)
+
+ PTHREAD_CFLAGS="-pthread"
+ PTHREAD_LIBS=
+
+ ax_pthread_ok=yes
+
+ # However, older versions of Clang make a point of warning the user
+ # that, in an invocation where only linking and no compilation is
+ # taking place, the -pthread option has no effect ("argument unused
+ # during compilation"). They expect -pthread to be passed in only
+ # when source code is being compiled.
+ #
+ # Problem is, this is at odds with the way Automake and most other
+ # C build frameworks function, which is that the same flags used in
+ # compilation (CFLAGS) are also used in linking. Many systems
+ # supported by AX_PTHREAD require exactly this for POSIX threads
+ # support, and in fact it is often not straightforward to specify a
+ # flag that is used only in the compilation phase and not in
+ # linking. Such a scenario is extremely rare in practice.
+ #
+ # Even though use of the -pthread flag in linking would only print
+ # a warning, this can be a nuisance for well-run software projects
+ # that build with -Werror. So if the active version of Clang has
+ # this misfeature, we search for an option to squash it.
+
+ AC_CACHE_CHECK([whether Clang needs flag to prevent "argument unused" warning when linking with -pthread],
+ [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG],
+ [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG=unknown
+ # Create an alternate version of $ac_link that compiles and
+ # links in two steps (.c -> .o, .o -> exe) instead of one
+ # (.c -> exe), because the warning occurs only in the second
+ # step
+ ax_pthread_save_ac_link="$ac_link"
+ ax_pthread_sed='s/conftest\.\$ac_ext/conftest.$ac_objext/g'
+ ax_pthread_link_step=`$as_echo "$ac_link" | sed "$ax_pthread_sed"`
+ ax_pthread_2step_ac_link="($ac_compile) && (echo ==== >&5) && ($ax_pthread_link_step)"
+ ax_pthread_save_CFLAGS="$CFLAGS"
+ for ax_pthread_try in '' -Qunused-arguments -Wno-unused-command-line-argument unknown; do
+ AS_IF([test "x$ax_pthread_try" = "xunknown"], [break])
+ CFLAGS="-Werror -Wunknown-warning-option $ax_pthread_try -pthread $ax_pthread_save_CFLAGS"
+ ac_link="$ax_pthread_save_ac_link"
+ AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+ [ac_link="$ax_pthread_2step_ac_link"
+ AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+ [break])
+ ])
+ done
+ ac_link="$ax_pthread_save_ac_link"
+ CFLAGS="$ax_pthread_save_CFLAGS"
+ AS_IF([test "x$ax_pthread_try" = "x"], [ax_pthread_try=no])
+ ax_cv_PTHREAD_CLANG_NO_WARN_FLAG="$ax_pthread_try"
+ ])
+
+ case "$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG" in
+ no | unknown) ;;
+ *) PTHREAD_CFLAGS="$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG $PTHREAD_CFLAGS" ;;
+ esac
+
+fi # $ax_pthread_clang = yes
+
+if test "x$ax_pthread_ok" = "xno"; then
+for ax_pthread_try_flag in $ax_pthread_flags; do
+
+ case $ax_pthread_try_flag in