1 /* Exporting symbols from Cygwin shared libraries.
2 Copyright (C) 2006, 2011-2020 Free Software Foundation, Inc.
3 Written by Bruno Haible <bruno@clisp.org>, 2006.
4
5 This program is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation; either version 3 of the License, or
8 (at your option) any later version.
9
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
14
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <https://www.gnu.org/licenses/>. */
17
18 /* There are four ways to build shared libraries on Cygwin:
19
20 - Export only functions, no variables.
21 This has the drawback of severely affecting the programming style in use.
22 It does not let the programmer use full ANSI C. It lets one platform
23 dictate the code style on all platforms. This is unacceptable.
24
25 - Use the GNU ld --enable-auto-import option. It is the default on Cygwin
26 since July 2005. But it has three fatal drawbacks:
27 - It produces executables and shared libraries with relocations in the
28 .text segment, defeating the principles of virtual memory.
29 - For some constructs such as
30 extern int var;
31 int * const b = &var;
32 it creates an executable that will give an error at runtime, rather
33 than either a compile-time or link-time error or a working executable.
34 (This is with both gcc and g++.) Whereas this code, not relying on
35 auto-import:
36 extern __declspec (dllimport) int var;
37 int * const b = &var;
38 gives a compile-time error with gcc and works with g++.
39 - It doesn't work in some cases (references to a member field of an
40 exported struct variable, or to a particular element of an exported
41 array variable), requiring code modifications. Again one platform
42 dictates code modifications on all platforms.
43
44 This is unacceptable. Therefore we disable this option, through the
45 woe32-dll.m4 autoconf macro.
46
47 - Define a macro that expands to __declspec(dllexport) when building
48 the library and to __declspec(dllimport) when building code outside
49 the library, and use it in all header files of the library.
50 This is acceptable if
51 1. the header files are unique to this library (not shared with
52 other packages), and
53 2. the library sources are contained in one directory, making it easy
54 to define a -DBUILDING_LIBXYZ flag for the library.
55 Example:
56 #ifdef BUILDING_LIBASPRINTF
57 #define LIBASPRINTF_DLL_EXPORTED __declspec(dllexport)
58 #else
59 #define LIBASPRINTF_DLL_EXPORTED __declspec(dllimport)
60 #endif
61
62 We use this technique for the libintl and the libasprintf libraries.
63
64 - Define a macro that expands to __declspec(dllimport) always, and use
65 it in all header files of the library. Use an explicit export list for
66 the library.
67 This is acceptable if
68 1. the programming language is not C++ (because the name mangling of
69 static struct/class fields and of variables in namespaces makes it
70 hard to maintain an export list).
71 The benefit of this approach is that the partitioning of the source files
72 into libraries (which source file goes into which library) does not
73 affect the source code; only the Makefiles reflect it.
74 The performance loss due to the unnecessary indirection for references
75 to variables from within the library defining the variable is acceptable.
76
77 We use this technique for libgettextlib (because it contains many gnulib
78 modules) and for libgettextsrc (because this makes it easy to move source
79 code from an msg* program to libgettextsrc). The macro is called
80 DLL_VARIABLE.
81
82 This file allows building an explicit export list. You can either
83 - specify the variables to be exported, and use the GNU ld option
84 --export-all-symbols to export all function names, or
85 - specify the variables and functions to be exported explicitly.
86
87 Note: --export-all-symbols is the default when no other symbol is explicitly
88 exported. This means, the use of an explicit export on the variables has
89 the effect of no longer exporting the functions! - until the option
90 --export-all-symbols is used.
91
92 See <https://haible.de/bruno/woe32dll.html> for more details. */
93
94 #if defined __GNUC__ /* GCC compiler, GNU toolchain */
95
96 /* IMP(x) is a symbol that contains the address of x. */
97 #if defined _WIN64 || defined _LP64
98 # define IMP(x) __imp_##x
99 #else
100 # define IMP(x) _imp__##x
101 #endif
102
103 /* Ensure that the variable x is exported from the library, and that a
104 pseudo-variable IMP(x) is available. */
105 # define VARIABLE(x) \
106 /* Export x without redefining x. This code was found by compiling a \
107 snippet: \
108 extern __declspec(dllexport) int x; int x = 42; */ \
109 asm (".section .drectve\n"); \
110 asm (".ascii \" -export:" #x ",data\"\n"); \
111 asm (".data\n"); \
112 /* Allocate a pseudo-variable IMP(x). */ \
113 extern int x; \
114 void * IMP(x) = &x;
115
116 #else /* non-GNU compiler, non-GNU toolchain */
117
118 # define VARIABLE(x) /* nothing */
119
120 #endif