| 1 | /* A C macro for emitting warnings if a function is used. |
| 2 | Copyright (C) 2010-2020 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 |