File: warn-on-use.h

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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