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/* A C macro for emitting warnings if a function is used.
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Copyright (C) 2010-2013 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published
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by the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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/* _GL_WARN_ON_USE (function, "literal string") issues a declaration
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for FUNCTION which will then trigger a compiler warning containing
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the text of "literal string" anywhere that function is called, if
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supported by the compiler. If the compiler does not support this
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feature, the macro expands to an unused extern declaration.
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This macro is useful for marking a function as a potential
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portability trap, with the intent that "literal string" include
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instructions on the replacement function that should be used
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instead. However, one of the reasons that a function is a
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portability trap is if it has the wrong signature. Declaring
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FUNCTION with a different signature in C is a compilation error, so
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this macro must use the same type as any existing declaration so
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that programs that avoid the problematic FUNCTION do not fail to
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compile merely because they included a header that poisoned the
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function. But this implies that _GL_WARN_ON_USE is only safe to
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use if FUNCTION is known to already have a declaration. Use of
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this macro implies that there must not be any other macro hiding
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the declaration of FUNCTION; but undefining FUNCTION first is part
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of the poisoning process anyway (although for symbols that are
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provided only via a macro, the result is a compilation error rather
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than a warning containing "literal string"). Also note that in
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C++, it is only safe to use if FUNCTION has no overloads.
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For an example, it is possible to poison 'getline' by:
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- adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
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[getline]) in configure.ac, which potentially defines
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- adding this code to a header that wraps the system <stdio.h>:
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#if HAVE_RAW_DECL_GETLINE
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_GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
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"not universally present; use the gnulib module getline");
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It is not possible to directly poison global variables. But it is
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possible to write a wrapper accessor function, and poison that
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(less common usage, like &environ, will cause a compilation error
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rather than issue the nice warning, but the end result of informing
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the developer about their portability problem is still achieved):
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#if HAVE_RAW_DECL_ENVIRON
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static char ***rpl_environ (void) { return &environ; }
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_GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
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# define environ (*rpl_environ ())
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#ifndef _GL_WARN_ON_USE
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# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
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/* A compiler attribute is available in gcc versions 4.3.0 and later. */
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# define _GL_WARN_ON_USE(function, message) \
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extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
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# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
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/* Verify the existence of the function. */
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# define _GL_WARN_ON_USE(function, message) \
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extern __typeof__ (function) function
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# else /* Unsupported. */
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# define _GL_WARN_ON_USE(function, message) \
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_GL_WARN_EXTERN_C int _gl_warn_on_use
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/* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
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is like _GL_WARN_ON_USE (function, "string"), except that the function is
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declared with the given prototype, consisting of return type, parameters,
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This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
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not work in this case. */
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#ifndef _GL_WARN_ON_USE_CXX
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# if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
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# define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
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extern rettype function parameters_and_attributes \
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__attribute__ ((__warning__ (msg)))
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# elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
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/* Verify the existence of the function. */
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# define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
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extern rettype function parameters_and_attributes
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# else /* Unsupported. */
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# define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
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_GL_WARN_EXTERN_C int _gl_warn_on_use
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/* _GL_WARN_EXTERN_C declaration;
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performs the declaration with C linkage. */
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#ifndef _GL_WARN_EXTERN_C
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# if defined __cplusplus
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# define _GL_WARN_EXTERN_C extern "C"
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# define _GL_WARN_EXTERN_C extern
added
removed
build-aux/snippet/warn-on-use.h
