1/* Exporting symbols from Cygwin shared libraries.
2   Copyright (C) 2006 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 2, or (at your option)
8   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, write to the Free Software Foundation,
17   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
18
19/* There are four ways to build shared libraries on Cygwin:
20
21   - Export only functions, no variables.
22     This has the drawback of severely affecting the programming style in use.
23     It does not let the programmer use full ANSI C.  It lets one platform
24     dictate the code style on all platforms.  This is unacceptable.
25
26   - Use the GNU ld --enable-auto-import option.  It is the default on Cygwin
27     since July 2005. But it three fatal drawbacks:
28       - It produces executables and shared libraries with relocations in the
29         .text segment, defeating the principles of virtual memory.
30       - For some constructs such as
31             extern int var;
32             int * const b = &var;
33         it creates an executable that will give an error at runtime, rather
34         than either a compile-time or link-time error or a working executable.
35         (This is with both gcc and g++.) Whereas this code, not relying on
36         auto-import:
37             extern __declspec (dllimport) int var;
38             int * const b = &var;
39         gives a compile-time error with gcc and works with g++.
40       - It doesn't work in some cases (references to a member field of an
41         exported struct variable, or to a particular element of an exported
42         array variable), requiring code modifications.  Again one platform
43         dictates code modifications on all platforms.
44
45     This is unacceptable.  Therefore we disable this option, through the
46     woe32-dll.m4 autoconf macro.
47
48   - Define a macro that expands to  __declspec(dllexport)  when building
49     the library and to  __declspec(dllimport)  when building code outside
50     the library, and use it in all header files of the library.
51     This is acceptable if
52       1. the header files are unique to this library (not shared with
53          other packages), and
54       2. the library sources are contained in one directory, making it easy
55          to define a -DBUILDING_LIBXYZ flag for the library.
56     Example:
57         #ifdef BUILDING_LIBASPRINTF
58         #define LIBASPRINTF_DLL_EXPORTED __declspec(dllexport)
59         #else
60         #define LIBASPRINTF_DLL_EXPORTED __declspec(dllimport)
61         #endif
62
63     We use this technique for the libintl and the libasprintf libraries.
64
65   - Define a macro that expands to  __declspec(dllimport)  always, and use
66     it in all header files of the library.  Use an explicit export list for
67     the library.
68     This is acceptable if
69       1. the programming language is not C++ (because the name mangling of
70          static struct/class fields and of variables in namespaces makes it
71          hard to maintain an export list).
72     The benefit of this approach is that the partitioning of the source files
73     into libraries (which source file goes into which library) does not
74     affect the source code; only the Makefiles reflect it.
75     The performance loss due to the unnecessary indirection for references
76     to variables from within the library defining the variable is acceptable.
77
78     We use this technique for libgettextlib (because it contains many gnulib
79     modules) and for libgettextsrc (because this makes it easy to move source
80     code from an msg* program to libgettextsrc).  The macro is called
81     DLL_VARIABLE.
82
83   This file allows building an explicit export list.  You can either
84     - specify the variables to be exported, and use the GNU ld option
85       --export-all-symbols to export all function names, or
86     - specify the variables and functions to be exported explicitly.
87
88   Note: --export-all-symbols is the default when no other symbol is explicitly
89   exported.  This means, the use of an explicit export on the variables has
90   the effect of no longer exporting the functions! - until the option
91   --export-all-symbols is used.  */
92
93 /* IMP(x) is a symbol that contains the address of x.  */
94#define IMP(x) _imp__##x
95
96 /* Ensure that the variable x is exported from the library, and that a
97    pseudo-variable IMP(x) is available.  */
98#define VARIABLE(x) \
99 /* Export x without redefining x.  This code was found by compiling a	\
100    snippet:								\
101      extern __declspec(dllexport) int x; int x = 42;  */		\
102 asm (".section .drectve\n");						\
103 asm (".ascii \" -export:" #x ",data\"\n");				\
104 asm (".data\n");							\
105 /* Allocate a pseudo-variable IMP(x).  */				\
106 extern int x;								\
107 void * IMP(x) = &x;
108