1  /* A C macro for emitting warnings if a function is used.
  2     Copyright (C) 2010-2011 Free Software Foundation, Inc.
  3  
  4     This program is free software: you can redistribute it and/or modify it
  5     under the terms of the GNU General Public License as published
  6     by the Free Software Foundation; either version 3 of the License, or
  7     (at your option) any later version.
  8  
  9     This program is distributed in the hope that it will be useful,
 10     but WITHOUT ANY WARRANTY; without even the implied warranty of
 11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 12     General Public License for more details.
 13  
 14     You should have received a copy of the GNU General Public License
 15     along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 16  
 17  /* _GL_WARN_ON_USE (function, "literal string") issues a declaration
 18     for FUNCTION which will then trigger a compiler warning containing
 19     the text of "literal string" anywhere that function is called, if
 20     supported by the compiler.  If the compiler does not support this
 21     feature, the macro expands to an unused extern declaration.
 22  
 23     This macro is useful for marking a function as a potential
 24     portability trap, with the intent that "literal string" include
 25     instructions on the replacement function that should be used
 26     instead.  However, one of the reasons that a function is a
 27     portability trap is if it has the wrong signature.  Declaring
 28     FUNCTION with a different signature in C is a compilation error, so
 29     this macro must use the same type as any existing declaration so
 30     that programs that avoid the problematic FUNCTION do not fail to
 31     compile merely because they included a header that poisoned the
 32     function.  But this implies that _GL_WARN_ON_USE is only safe to
 33     use if FUNCTION is known to already have a declaration.  Use of
 34     this macro implies that there must not be any other macro hiding
 35     the declaration of FUNCTION; but undefining FUNCTION first is part
 36     of the poisoning process anyway (although for symbols that are
 37     provided only via a macro, the result is a compilation error rather
 38     than a warning containing "literal string").  Also note that in
 39     C++, it is only safe to use if FUNCTION has no overloads.
 40  
 41     For an example, it is possible to poison 'getline' by:
 42     - adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
 43       [getline]) in configure.ac, which potentially defines
 44       HAVE_RAW_DECL_GETLINE
 45     - adding this code to a header that wraps the system <stdio.h>:
 46       #undef getline
 47       #if HAVE_RAW_DECL_GETLINE
 48       _GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
 49         "not universally present; use the gnulib module getline");
 50       #endif
 51  
 52     It is not possible to directly poison global variables.  But it is
 53     possible to write a wrapper accessor function, and poison that
 54     (less common usage, like &environ, will cause a compilation error
 55     rather than issue the nice warning, but the end result of informing
 56     the developer about their portability problem is still achieved):
 57     #if HAVE_RAW_DECL_ENVIRON
 58     static inline char ***rpl_environ (void) { return &environ; }
 59     _GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
 60     # undef environ
 61     # define environ (*rpl_environ ())
 62     #endif
 63     */
 64  #ifndef _GL_WARN_ON_USE
 65  
 66  # if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
 67  /* A compiler attribute is available in gcc versions 4.3.0 and later.  */
 68  #  define _GL_WARN_ON_USE(function, message) \
 69  extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
 70  # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 71  /* Verify the existence of the function.  */
 72  #  define _GL_WARN_ON_USE(function, message) \
 73  extern __typeof__ (function) function
 74  # else /* Unsupported.  */
 75  #  define _GL_WARN_ON_USE(function, message) \
 76  _GL_WARN_EXTERN_C int _gl_warn_on_use
 77  # endif
 78  #endif
 79  
 80  /* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
 81     is like _GL_WARN_ON_USE (function, "string"), except that the function is
 82     declared with the given prototype, consisting of return type, parameters,
 83     and attributes.
 84     This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
 85     not work in this case.  */
 86  #ifndef _GL_WARN_ON_USE_CXX
 87  # if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
 88  #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 89  extern rettype function parameters_and_attributes \
 90       __attribute__ ((__warning__ (msg)))
 91  # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 92  /* Verify the existence of the function.  */
 93  #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 94  extern rettype function parameters_and_attributes
 95  # else /* Unsupported.  */
 96  #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 97  _GL_WARN_EXTERN_C int _gl_warn_on_use
 98  # endif
 99  #endif
100  
101  /* _GL_WARN_EXTERN_C declaration;
102     performs the declaration with C linkage.  */
103  #ifndef _GL_WARN_EXTERN_C
104  # if defined __cplusplus
105  #  define _GL_WARN_EXTERN_C extern "C"
106  # else
107  #  define _GL_WARN_EXTERN_C extern
108  # endif
109  #endif