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 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 <http://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 /* IMP(x) is a symbol that contains the address of x. */ 93#define IMP(x) _imp__##x 94 95 /* Ensure that the variable x is exported from the library, and that a 96 pseudo-variable IMP(x) is available. */ 97#define VARIABLE(x) \ 98 /* Export x without redefining x. This code was found by compiling a \ 99 snippet: \ 100 extern __declspec(dllexport) int x; int x = 42; */ \ 101 asm (".section .drectve\n"); \ 102 asm (".ascii \" -export:" #x ",data\"\n"); \ 103 asm (".data\n"); \ 104 /* Allocate a pseudo-variable IMP(x). */ \ 105 extern int x; \ 106 void * IMP(x) = &x; 107