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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 Lesser 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 | |
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