warn-on-use.h

   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