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+/* A C macro for emitting warnings if a function is used.
+   Copyright (C) 2010-2020 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published
+   by the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* _GL_WARN_ON_USE (function, "literal string") issues a declaration
+   for FUNCTION which will then trigger a compiler warning containing
+   the text of "literal string" anywhere that function is called, if
+   supported by the compiler.  If the compiler does not support this
+   feature, the macro expands to an unused extern declaration.
+
+   _GL_WARN_ON_USE_ATTRIBUTE ("literal string") expands to the
+   attribute used in _GL_WARN_ON_USE.  If the compiler does not support
+   this feature, it expands to empty.
+
+   These macros are useful for marking a function as a potential
+   portability trap, with the intent that "literal string" include
+   instructions on the replacement function that should be used
+   instead.
+   _GL_WARN_ON_USE is for functions with 'extern' linkage.
+   _GL_WARN_ON_USE_ATTRIBUTE is for functions with 'static' or 'inline'
+   linkage.
+
+   However, one of the reasons that a function is a portability trap is
+   if it has the wrong signature.  Declaring FUNCTION with a different
+   signature in C is a compilation error, so this macro must use the
+   same type as any existing declaration so that programs that avoid
+   the problematic FUNCTION do not fail to compile merely because they
+   included a header that poisoned the function.  But this implies that
+   _GL_WARN_ON_USE is only safe to use if FUNCTION is known to already
+   have a declaration.  Use of this macro implies that there must not
+   be any other macro hiding the declaration of FUNCTION; but
+   undefining FUNCTION first is part of the poisoning process anyway
+   (although for symbols that are provided only via a macro, the result
+   is a compilation error rather than a warning containing
+   "literal string").  Also note that in C++, it is only safe to use if
+   FUNCTION has no overloads.
+
+   For an example, it is possible to poison 'getline' by:
+   - adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
+     [getline]) in configure.ac, which potentially defines
+     HAVE_RAW_DECL_GETLINE
+   - adding this code to a header that wraps the system <stdio.h>:
+     #undef getline
+     #if HAVE_RAW_DECL_GETLINE
+     _GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
+       "not universally present; use the gnulib module getline");
+     #endif
+
+   It is not possible to directly poison global variables.  But it is
+   possible to write a wrapper accessor function, and poison that
+   (less common usage, like &environ, will cause a compilation error
+   rather than issue the nice warning, but the end result of informing
+   the developer about their portability problem is still achieved):
+     #if HAVE_RAW_DECL_ENVIRON
+     static char ***
+     rpl_environ (void) { return &environ; }
+     _GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
+     # undef environ
+     # define environ (*rpl_environ ())
+     #endif
+   or better (avoiding contradictory use of 'static' and 'extern'):
+     #if HAVE_RAW_DECL_ENVIRON
+     static char ***
+     _GL_WARN_ON_USE_ATTRIBUTE ("environ is not always properly declared")
+     rpl_environ (void) { return &environ; }
+     # undef environ
+     # define environ (*rpl_environ ())
+     #endif
+   */
+#ifndef _GL_WARN_ON_USE
+
+# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
+/* A compiler attribute is available in gcc versions 4.3.0 and later.  */
+#  define _GL_WARN_ON_USE(function, message) \
+extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
+#  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
+  __attribute__ ((__warning__ (message)))
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+/* Verify the existence of the function.  */
+#  define _GL_WARN_ON_USE(function, message) \
+extern __typeof__ (function) function
+#  define _GL_WARN_ON_USE_ATTRIBUTE(message)
+# else /* Unsupported.  */
+#  define _GL_WARN_ON_USE(function, message) \
+_GL_WARN_EXTERN_C int _gl_warn_on_use
+#  define _GL_WARN_ON_USE_ATTRIBUTE(message)
+# endif
+#endif
+
+/* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
+   is like _GL_WARN_ON_USE (function, "string"), except that the function is
+   declared with the given prototype, consisting of return type, parameters,
+   and attributes.
+   This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
+   not work in this case.  */
+#ifndef _GL_WARN_ON_USE_CXX
+# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+extern rettype function parameters_and_attributes \
+     __attribute__ ((__warning__ (msg)))
+# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
+/* Verify the existence of the function.  */
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+extern rettype function parameters_and_attributes
+# else /* Unsupported.  */
+#  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
+_GL_WARN_EXTERN_C int _gl_warn_on_use
+# endif
+#endif
+
+/* _GL_WARN_EXTERN_C declaration;
+   performs the declaration with C linkage.  */
+#ifndef _GL_WARN_EXTERN_C
+# if defined __cplusplus
+#  define _GL_WARN_EXTERN_C extern "C"
+# else
+#  define _GL_WARN_EXTERN_C extern
+# endif
+#endif