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/* A C macro for emitting warnings if a function is used.
Copyright (C) 2010-2011 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
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 <>. */
/* _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.
This macro is 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. 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, which potentially defines
- adding this code to a header that wraps the system <stdio.h>:
#undef getline
_GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
"not universally present; use the gnulib module getline");
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):
static inline char ***rpl_environ (void) { return &environ; }
_GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
# undef environ
# define environ (*rpl_environ ())
#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)))
/* Verify the existence of the function. */
# define _GL_WARN_ON_USE(function, message) \
extern __typeof__ (function) function
# else /* Unsupported. */
# define _GL_WARN_ON_USE(function, message) \
_GL_WARN_EXTERN_C int _gl_warn_on_use
# 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. */
# 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)))
/* 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
/* _GL_WARN_EXTERN_C declaration;
performs the declaration with C linkage. */
# if defined __cplusplus
# define _GL_WARN_EXTERN_C extern "C"
# else
# define _GL_WARN_EXTERN_C extern
# endif