root/maint/gnulib/lib/warn-on-use.h

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   1 /* A C macro for emitting warnings if a function is used.
   2    Copyright (C) 2010-2021 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 Lesser General Public License as published
   6    by the Free Software Foundation; either version 2 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    Lesser General Public License for more details.
  13 
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://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    _GL_WARN_ON_USE_ATTRIBUTE ("literal string") expands to the
  24    attribute used in _GL_WARN_ON_USE.  If the compiler does not support
  25    this feature, it expands to empty.
  26 
  27    These macros are useful for marking a function as a potential
  28    portability trap, with the intent that "literal string" include
  29    instructions on the replacement function that should be used
  30    instead.
  31    _GL_WARN_ON_USE is for functions with 'extern' linkage.
  32    _GL_WARN_ON_USE_ATTRIBUTE is for functions with 'static' or 'inline'
  33    linkage.
  34 
  35    However, one of the reasons that a function is a portability trap is
  36    if it has the wrong signature.  Declaring FUNCTION with a different
  37    signature in C is a compilation error, so this macro must use the
  38    same type as any existing declaration so that programs that avoid
  39    the problematic FUNCTION do not fail to compile merely because they
  40    included a header that poisoned the function.  But this implies that
  41    _GL_WARN_ON_USE is only safe to use if FUNCTION is known to already
  42    have a declaration.  Use of this macro implies that there must not
  43    be any other macro hiding the declaration of FUNCTION; but
  44    undefining FUNCTION first is part of the poisoning process anyway
  45    (although for symbols that are provided only via a macro, the result
  46    is a compilation error rather than a warning containing
  47    "literal string").  Also note that in C++, it is only safe to use if
  48    FUNCTION has no overloads.
  49 
  50    For an example, it is possible to poison 'getline' by:
  51    - adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
  52      [getline]) in configure.ac, which potentially defines
  53      HAVE_RAW_DECL_GETLINE
  54    - adding this code to a header that wraps the system <stdio.h>:
  55      #undef getline
  56      #if HAVE_RAW_DECL_GETLINE
  57      _GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
  58        "not universally present; use the gnulib module getline");
  59      #endif
  60 
  61    It is not possible to directly poison global variables.  But it is
  62    possible to write a wrapper accessor function, and poison that
  63    (less common usage, like &environ, will cause a compilation error
  64    rather than issue the nice warning, but the end result of informing
  65    the developer about their portability problem is still achieved):
  66      #if HAVE_RAW_DECL_ENVIRON
  67      static char ***
  68      rpl_environ (void) { return &environ; }
  69      _GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
  70      # undef environ
  71      # define environ (*rpl_environ ())
  72      #endif
  73    or better (avoiding contradictory use of 'static' and 'extern'):
  74      #if HAVE_RAW_DECL_ENVIRON
  75      static char ***
  76      _GL_WARN_ON_USE_ATTRIBUTE ("environ is not always properly declared")
  77      rpl_environ (void) { return &environ; }
  78      # undef environ
  79      # define environ (*rpl_environ ())
  80      #endif
  81    */
  82 #ifndef _GL_WARN_ON_USE
  83 
  84 # if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
  85 /* A compiler attribute is available in gcc versions 4.3.0 and later.  */
  86 #  define _GL_WARN_ON_USE(function, message) \
  87 extern __typeof__ (function) function __attribute__ ((__warning__ (message)))
  88 #  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
  89   __attribute__ ((__warning__ (message)))
  90 # elif __clang_major__ >= 4
  91 /* Another compiler attribute is available in clang.  */
  92 #  define _GL_WARN_ON_USE(function, message) \
  93 extern __typeof__ (function) function \
  94   __attribute__ ((__diagnose_if__ (1, message, "warning")))
  95 #  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
  96   __attribute__ ((__diagnose_if__ (1, message, "warning")))
  97 # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
  98 /* Verify the existence of the function.  */
  99 #  define _GL_WARN_ON_USE(function, message) \
 100 extern __typeof__ (function) function
 101 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
 102 # else /* Unsupported.  */
 103 #  define _GL_WARN_ON_USE(function, message) \
 104 _GL_WARN_EXTERN_C int _gl_warn_on_use
 105 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
 106 # endif
 107 #endif
 108 
 109 /* _GL_WARN_ON_USE_CXX (function, rettype_gcc, rettype_clang, parameters_and_attributes, "message")
 110    is like _GL_WARN_ON_USE (function, "message"), except that in C++ mode the
 111    function is declared with the given prototype, consisting of return type,
 112    parameters, and attributes.
 113    This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
 114    not work in this case.  */
 115 #ifndef _GL_WARN_ON_USE_CXX
 116 # if !defined __cplusplus
 117 #  define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 118      _GL_WARN_ON_USE (function, msg)
 119 # else
 120 #  if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
 121 /* A compiler attribute is available in gcc versions 4.3.0 and later.  */
 122 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 123 extern rettype_gcc function parameters_and_attributes \
 124   __attribute__ ((__warning__ (msg)))
 125 #  elif __clang_major__ >= 4
 126 /* Another compiler attribute is available in clang.  */
 127 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 128 extern rettype_clang function parameters_and_attributes \
 129   __attribute__ ((__diagnose_if__ (1, msg, "warning")))
 130 #  elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 131 /* Verify the existence of the function.  */
 132 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 133 extern rettype_gcc function parameters_and_attributes
 134 #  else /* Unsupported.  */
 135 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 136 _GL_WARN_EXTERN_C int _gl_warn_on_use
 137 #  endif
 138 # endif
 139 #endif
 140 
 141 /* _GL_WARN_EXTERN_C declaration;
 142    performs the declaration with C linkage.  */
 143 #ifndef _GL_WARN_EXTERN_C
 144 # if defined __cplusplus
 145 #  define _GL_WARN_EXTERN_C extern "C"
 146 # else
 147 #  define _GL_WARN_EXTERN_C extern
 148 # endif
 149 #endif
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