lib/warn-on-use.h

   1 /* A C macro for emitting warnings if a function is used.
   2    Copyright (C) 2010-2024 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    _GL_WARN_ON_USE should not be used more than once for a given function
  36    in a given compilation unit (because this may generate a warning even
  37    if the function is never called).
  38
  39    However, one of the reasons that a function is a portability trap is
  40    if it has the wrong signature.  Declaring FUNCTION with a different
  41    signature in C is a compilation error, so this macro must use the
  42    same type as any existing declaration so that programs that avoid
  43    the problematic FUNCTION do not fail to compile merely because they
  44    included a header that poisoned the function.  But this implies that
  45    _GL_WARN_ON_USE is only safe to use if FUNCTION is known to already
  46    have a declaration.  Use of this macro implies that there must not
  47    be any other macro hiding the declaration of FUNCTION; but
  48    undefining FUNCTION first is part of the poisoning process anyway
  49    (although for symbols that are provided only via a macro, the result
  50    is a compilation error rather than a warning containing
  51    "literal string").  Also note that in C++, it is only safe to use if
  52    FUNCTION has no overloads.
  53
  54    For an example, it is possible to poison 'getline' by:
  55    - adding a call to gl_WARN_ON_USE_PREPARE([[#include <stdio.h>]],
  56      [getline]) in configure.ac, which potentially defines
  57      HAVE_RAW_DECL_GETLINE
  58    - adding this code to a header that wraps the system <stdio.h>:
  59      #undef getline
  60      #if HAVE_RAW_DECL_GETLINE
  61      _GL_WARN_ON_USE (getline, "getline is required by POSIX 2008, but"
  62        "not universally present; use the gnulib module getline");
  63      #endif
  64
  65    It is not possible to directly poison global variables.  But it is
  66    possible to write a wrapper accessor function, and poison that
  67    (less common usage, like &environ, will cause a compilation error
  68    rather than issue the nice warning, but the end result of informing
  69    the developer about their portability problem is still achieved):
  70      #if HAVE_RAW_DECL_ENVIRON
  71      static char ***
  72      rpl_environ (void) { return &environ; }
  73      _GL_WARN_ON_USE (rpl_environ, "environ is not always properly declared");
  74      # undef environ
  75      # define environ (*rpl_environ ())
  76      #endif
  77    or better (avoiding contradictory use of 'static' and 'extern'):
  78      #if HAVE_RAW_DECL_ENVIRON
  79      static char ***
  80      _GL_WARN_ON_USE_ATTRIBUTE ("environ is not always properly declared")
  81      rpl_environ (void) { return &environ; }
  82      # undef environ
  83      # define environ (*rpl_environ ())
  84      #endif
  85    */
  86 #ifndef _GL_WARN_ON_USE
  87
  88 # if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
  89 /* A compiler attribute is available in gcc versions 4.3.0 and later.  */
  90 #  define _GL_WARN_ON_USE(function, message) \
  91 _GL_WARN_EXTERN_C __typeof__ (function) function __attribute__ ((__warning__ (message)))
  92 #  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
  93   __attribute__ ((__warning__ (message)))
  94 # elif __clang_major__ >= 4
  95 /* Another compiler attribute is available in clang.  */
  96 #  define _GL_WARN_ON_USE(function, message) \
  97 _GL_WARN_EXTERN_C __typeof__ (function) function \
  98   __attribute__ ((__diagnose_if__ (1, message, "warning")))
  99 #  define _GL_WARN_ON_USE_ATTRIBUTE(message) \
 100   __attribute__ ((__diagnose_if__ (1, message, "warning")))
 101 # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 102 /* Verify the existence of the function.  */
 103 #  define _GL_WARN_ON_USE(function, message) \
 104 _GL_WARN_EXTERN_C __typeof__ (function) function
 105 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
 106 # else /* Unsupported.  */
 107 #  define _GL_WARN_ON_USE(function, message) \
 108 _GL_WARN_EXTERN_C int _gl_warn_on_use
 109 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
 110 # endif
 111 #endif
 112
 113 /* _GL_WARN_ON_USE_CXX (function, rettype_gcc, rettype_clang, parameters_and_attributes, "message")
 114    is like _GL_WARN_ON_USE (function, "message"), except that in C++ mode the
 115    function is declared with the given prototype, consisting of return type,
 116    parameters, and attributes.
 117    This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
 118    not work in this case.  */
 119 #ifndef _GL_WARN_ON_USE_CXX
 120 # if !defined __cplusplus
 121 #  define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 122      _GL_WARN_ON_USE (function, msg)
 123 # else
 124 #  if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
 125 /* A compiler attribute is available in gcc versions 4.3.0 and later.  */
 126 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 127 extern rettype_gcc function parameters_and_attributes \
 128   __attribute__ ((__warning__ (msg)))
 129 #  elif __clang_major__ >= 4
 130 /* Another compiler attribute is available in clang.  */
 131 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 132 extern rettype_clang function parameters_and_attributes \
 133   __attribute__ ((__diagnose_if__ (1, msg, "warning")))
 134 #  elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 135 /* Verify the existence of the function.  */
 136 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 137 extern rettype_gcc function parameters_and_attributes
 138 #  else /* Unsupported.  */
 139 #   define _GL_WARN_ON_USE_CXX(function,rettype_gcc,rettype_clang,parameters_and_attributes,msg) \
 140 _GL_WARN_EXTERN_C int _gl_warn_on_use
 141 #  endif
 142 # endif
 143 #endif
 144
 145 /* _GL_WARN_EXTERN_C declaration;
 146    performs the declaration with C linkage.  */
 147 #ifndef _GL_WARN_EXTERN_C
 148 # if defined __cplusplus
 149 #  define _GL_WARN_EXTERN_C extern "C"
 150 # else
 151 #  define _GL_WARN_EXTERN_C extern
 152 # endif
 153 #endif