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Annotation of embedaddon/libiconv/srclib/warn-on-use.h, revision 1.1

1.1     ! misho       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