gnulib/import/warn-on-use.h

   1 /* A C macro for emitting warnings if a function is used.
   2    Copyright (C) 2010-2019 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 <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 __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
  91 /* Verify the existence of the function.  */
  92 #  define _GL_WARN_ON_USE(function, message) \
  93 extern __typeof__ (function) function
  94 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
  95 # else /* Unsupported.  */
  96 #  define _GL_WARN_ON_USE(function, message) \
  97 _GL_WARN_EXTERN_C int _gl_warn_on_use
  98 #  define _GL_WARN_ON_USE_ATTRIBUTE(message)
  99 # endif
 100 #endif
 101
 102 /* _GL_WARN_ON_USE_CXX (function, rettype, parameters_and_attributes, "string")
 103    is like _GL_WARN_ON_USE (function, "string"), except that the function is
 104    declared with the given prototype, consisting of return type, parameters,
 105    and attributes.
 106    This variant is useful for overloaded functions in C++. _GL_WARN_ON_USE does
 107    not work in this case.  */
 108 #ifndef _GL_WARN_ON_USE_CXX
 109 # if 4 < __GNUC__ || (__GNUC__ == 4 && 3 <= __GNUC_MINOR__)
 110 #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 111 extern rettype function parameters_and_attributes \
 112      __attribute__ ((__warning__ (msg)))
 113 # elif __GNUC__ >= 3 && GNULIB_STRICT_CHECKING
 114 /* Verify the existence of the function.  */
 115 #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 116 extern rettype function parameters_and_attributes
 117 # else /* Unsupported.  */
 118 #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
 119 _GL_WARN_EXTERN_C int _gl_warn_on_use
 120 # endif
 121 #endif
 122
 123 /* _GL_WARN_EXTERN_C declaration;
 124    performs the declaration with C linkage.  */
 125 #ifndef _GL_WARN_EXTERN_C
 126 # if defined __cplusplus
 127 #  define _GL_WARN_EXTERN_C extern "C"
 128 # else
 129 #  define _GL_WARN_EXTERN_C extern
 130 # endif
 131 #endif