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Diffstat (limited to 'woe32dll/export.h')
| -rw-r--r-- | woe32dll/export.h | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/woe32dll/export.h b/woe32dll/export.h new file mode 100644 index 0000000..3e8a21f --- /dev/null +++ b/woe32dll/export.h @@ -0,0 +1,106 @@ +/* Exporting symbols from Cygwin shared libraries. + Copyright (C) 2006 Free Software Foundation, Inc. + Written by Bruno Haible <bruno@clisp.org>, 2006. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 3 of the License, or + (at your option) any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program. If not, see <http://www.gnu.org/licenses/>. */ + +/* There are four ways to build shared libraries on Cygwin: + + - Export only functions, no variables. + This has the drawback of severely affecting the programming style in use. + It does not let the programmer use full ANSI C. It lets one platform + dictate the code style on all platforms. This is unacceptable. + + - Use the GNU ld --enable-auto-import option. It is the default on Cygwin + since July 2005. But it has three fatal drawbacks: + - It produces executables and shared libraries with relocations in the + .text segment, defeating the principles of virtual memory. + - For some constructs such as + extern int var; + int * const b = &var; + it creates an executable that will give an error at runtime, rather + than either a compile-time or link-time error or a working executable. + (This is with both gcc and g++.) Whereas this code, not relying on + auto-import: + extern __declspec (dllimport) int var; + int * const b = &var; + gives a compile-time error with gcc and works with g++. + - It doesn't work in some cases (references to a member field of an + exported struct variable, or to a particular element of an exported + array variable), requiring code modifications. Again one platform + dictates code modifications on all platforms. + + This is unacceptable. Therefore we disable this option, through the + woe32-dll.m4 autoconf macro. + + - Define a macro that expands to __declspec(dllexport) when building + the library and to __declspec(dllimport) when building code outside + the library, and use it in all header files of the library. + This is acceptable if + 1. the header files are unique to this library (not shared with + other packages), and + 2. the library sources are contained in one directory, making it easy + to define a -DBUILDING_LIBXYZ flag for the library. + Example: + #ifdef BUILDING_LIBASPRINTF + #define LIBASPRINTF_DLL_EXPORTED __declspec(dllexport) + #else + #define LIBASPRINTF_DLL_EXPORTED __declspec(dllimport) + #endif + + We use this technique for the libintl and the libasprintf libraries. + + - Define a macro that expands to __declspec(dllimport) always, and use + it in all header files of the library. Use an explicit export list for + the library. + This is acceptable if + 1. the programming language is not C++ (because the name mangling of + static struct/class fields and of variables in namespaces makes it + hard to maintain an export list). + The benefit of this approach is that the partitioning of the source files + into libraries (which source file goes into which library) does not + affect the source code; only the Makefiles reflect it. + The performance loss due to the unnecessary indirection for references + to variables from within the library defining the variable is acceptable. + + We use this technique for libgettextlib (because it contains many gnulib + modules) and for libgettextsrc (because this makes it easy to move source + code from an msg* program to libgettextsrc). The macro is called + DLL_VARIABLE. + + This file allows building an explicit export list. You can either + - specify the variables to be exported, and use the GNU ld option + --export-all-symbols to export all function names, or + - specify the variables and functions to be exported explicitly. + + Note: --export-all-symbols is the default when no other symbol is explicitly + exported. This means, the use of an explicit export on the variables has + the effect of no longer exporting the functions! - until the option + --export-all-symbols is used. */ + + /* IMP(x) is a symbol that contains the address of x. */ +#define IMP(x) _imp__##x + + /* Ensure that the variable x is exported from the library, and that a + pseudo-variable IMP(x) is available. */ +#define VARIABLE(x) \ + /* Export x without redefining x. This code was found by compiling a \ + snippet: \ + extern __declspec(dllexport) int x; int x = 42; */ \ + asm (".section .drectve\n"); \ + asm (".ascii \" -export:" #x ",data\"\n"); \ + asm (".data\n"); \ + /* Allocate a pseudo-variable IMP(x). */ \ + extern int x; \ + void * IMP(x) = &x; |