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f434ba03 | 1 | /* A C macro for emitting warnings if a function is used. |
5df4cba6 | 2 | Copyright (C) 2010-2020 Free Software Foundation, Inc. |
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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 | |
770d76d7 | 12 | General Public License for more details. |
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13 | |
14 | You should have received a copy of the GNU General Public License | |
c0c3707f | 15 | along with this program. If not, see <https://www.gnu.org/licenses/>. */ |
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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 | ||
c0c3707f CB |
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 | |
f434ba03 PA |
28 | portability trap, with the intent that "literal string" include |
29 | instructions on the replacement function that should be used | |
c0c3707f CB |
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. | |
f434ba03 PA |
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): | |
c0c3707f CB |
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 | |
f434ba03 PA |
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))) | |
c0c3707f CB |
88 | # define _GL_WARN_ON_USE_ATTRIBUTE(message) \ |
89 | __attribute__ ((__warning__ (message))) | |
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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 | |
c0c3707f | 94 | # define _GL_WARN_ON_USE_ATTRIBUTE(message) |
f434ba03 PA |
95 | # else /* Unsupported. */ |
96 | # define _GL_WARN_ON_USE(function, message) \ | |
97 | _GL_WARN_EXTERN_C int _gl_warn_on_use | |
c0c3707f | 98 | # define _GL_WARN_ON_USE_ATTRIBUTE(message) |
f434ba03 PA |
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 |