text: # visibility.m4 serial 6
  dnl Copyright (C) 2005, 2008, 2010-2020 Free Software Foundation, Inc.
  dnl This file is free software; the Free Software Foundation
  dnl gives unlimited permission to copy and/or distribute it,
  dnl with or without modifications, as long as this notice is preserved.
  
  dnl From Bruno Haible.
  
  dnl Tests whether the compiler supports the command-line option
  dnl -fvisibility=hidden and the function and variable attributes
  dnl __attribute__((__visibility__("hidden"))) and
  dnl __attribute__((__visibility__("default"))).
  dnl Does *not* test for __visibility__("protected") - which has tricky
  dnl semantics (see the 'vismain' test in glibc) and does not exist e.g. on
  dnl Mac OS X.
  dnl Does *not* test for __visibility__("internal") - which has processor
  dnl dependent semantics.
  dnl Does *not* test for #pragma GCC visibility push(hidden) - which is
  dnl "really only recommended for legacy code".
  dnl Set the variable CFLAG_VISIBILITY.
  dnl Defines and sets the variable HAVE_VISIBILITY.
  
  AC_DEFUN([gl_VISIBILITY],
  [
    AC_REQUIRE([AC_PROG_CC])
    CFLAG_VISIBILITY=
    HAVE_VISIBILITY=0
    if test -n "$GCC"; then
      dnl First, check whether -Werror can be added to the command line, or
      dnl whether it leads to an error because of some other option that the
      dnl user has put into $CC $CFLAGS $CPPFLAGS.
      AC_CACHE_CHECK([whether the -Werror option is usable],
        [gl_cv_cc_vis_werror],
        [gl_save_CFLAGS="$CFLAGS"
         CFLAGS="$CFLAGS -Werror"
         AC_COMPILE_IFELSE(
           [AC_LANG_PROGRAM([[]], [[]])],
           [gl_cv_cc_vis_werror=yes],
           [gl_cv_cc_vis_werror=no])
         CFLAGS="$gl_save_CFLAGS"
        ])
      dnl Now check whether visibility declarations are supported.
      AC_CACHE_CHECK([for simple visibility declarations],
        [gl_cv_cc_visibility],
        [gl_save_CFLAGS="$CFLAGS"
         CFLAGS="$CFLAGS -fvisibility=hidden"
         dnl We use the option -Werror and a function dummyfunc, because on some
         dnl platforms (Cygwin 1.7) the use of -fvisibility triggers a warning
         dnl "visibility attribute not supported in this configuration; ignored"
         dnl at the first function definition in every compilation unit, and we
         dnl don't want to use the option in this case.
         if test $gl_cv_cc_vis_werror = yes; then
           CFLAGS="$CFLAGS -Werror"
         fi
         AC_COMPILE_IFELSE(
           [AC_LANG_PROGRAM(
              [[extern __attribute__((__visibility__("hidden"))) int hiddenvar;
                extern __attribute__((__visibility__("default"))) int exportedvar;
                extern __attribute__((__visibility__("hidden"))) int hiddenfunc (void);
                extern __attribute__((__visibility__("default"))) int exportedfunc (void);
                void dummyfunc (void) {}
              ]],
              [[]])],
           [gl_cv_cc_visibility=yes],
           [gl_cv_cc_visibility=no])
         CFLAGS="$gl_save_CFLAGS"
        ])
      if test $gl_cv_cc_visibility = yes; then
        CFLAG_VISIBILITY="-fvisibility=hidden"
        HAVE_VISIBILITY=1
      fi
    fi
    AC_SUBST([CFLAG_VISIBILITY])
    AC_SUBST([HAVE_VISIBILITY])
    AC_DEFINE_UNQUOTED([HAVE_VISIBILITY], [$HAVE_VISIBILITY],
      [Define to 1 or 0, depending whether the compiler supports simple visibility declarations.])
  ])
  