118 lines
5.6 KiB
C
118 lines
5.6 KiB
C
/* Exporting symbols from Cygwin shared libraries.
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Copyright (C) 2006, 2011-2018 Free Software Foundation, Inc.
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Written by Bruno Haible <bruno@clisp.org>, 2006.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with this program; see the file COPYING.LIB. If not,
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see <https://www.gnu.org/licenses/>. */
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/* There are four ways to build shared libraries on Cygwin:
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- Export only functions, no variables.
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This has the drawback of severely affecting the programming style in use.
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It does not let the programmer use full ANSI C. It lets one platform
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dictate the code style on all platforms. This is unacceptable.
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- Use the GNU ld --enable-auto-import option. It is the default on Cygwin
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since July 2005. But it has three fatal drawbacks:
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- It produces executables and shared libraries with relocations in the
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.text segment, defeating the principles of virtual memory.
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- For some constructs such as
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extern int var;
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int * const b = &var;
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it creates an executable that will give an error at runtime, rather
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than either a compile-time or link-time error or a working executable.
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(This is with both gcc and g++.) Whereas this code, not relying on
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auto-import:
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extern __declspec (dllimport) int var;
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int * const b = &var;
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gives a compile-time error with gcc and works with g++.
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- It doesn't work in some cases (references to a member field of an
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exported struct variable, or to a particular element of an exported
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array variable), requiring code modifications. Again one platform
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dictates code modifications on all platforms.
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This is unacceptable. Therefore we disable this option, through the
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woe32-dll.m4 autoconf macro.
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- Define a macro that expands to __declspec(dllexport) when building
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the library and to __declspec(dllimport) when building code outside
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the library, and use it in all header files of the library.
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This is acceptable if
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1. the header files are unique to this library (not shared with
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other packages), and
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2. the library sources are contained in one directory, making it easy
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to define a -DBUILDING_LIBXYZ flag for the library.
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Example:
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#ifdef BUILDING_LIBASPRINTF
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#define LIBASPRINTF_DLL_EXPORTED __declspec(dllexport)
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#else
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#define LIBASPRINTF_DLL_EXPORTED __declspec(dllimport)
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#endif
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We use this technique for the libintl and the libasprintf libraries.
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- Define a macro that expands to __declspec(dllimport) always, and use
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it in all header files of the library. Use an explicit export list for
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the library.
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This is acceptable if
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1. the programming language is not C++ (because the name mangling of
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static struct/class fields and of variables in namespaces makes it
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hard to maintain an export list).
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The benefit of this approach is that the partitioning of the source files
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into libraries (which source file goes into which library) does not
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affect the source code; only the Makefiles reflect it.
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The performance loss due to the unnecessary indirection for references
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to variables from within the library defining the variable is acceptable.
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We use this technique for libgettextlib (because it contains many gnulib
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modules) and for libgettextsrc (because this makes it easy to move source
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code from an msg* program to libgettextsrc). The macro is called
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DLL_VARIABLE.
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This file allows building an explicit export list. You can either
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- specify the variables to be exported, and use the GNU ld option
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--export-all-symbols to export all function names, or
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- specify the variables and functions to be exported explicitly.
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Note: --export-all-symbols is the default when no other symbol is explicitly
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exported. This means, the use of an explicit export on the variables has
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the effect of no longer exporting the functions! - until the option
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--export-all-symbols is used.
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See <https://haible.de/bruno/woe32dll.html> for more details. */
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#if defined __GNUC__ /* GCC compiler, GNU toolchain */
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/* IMP(x) is a symbol that contains the address of x. */
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# define IMP(x) _imp__##x
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/* Ensure that the variable x is exported from the library, and that a
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pseudo-variable IMP(x) is available. */
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# define VARIABLE(x) \
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/* Export x without redefining x. This code was found by compiling a \
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snippet: \
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extern __declspec(dllexport) int x; int x = 42; */ \
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asm (".section .drectve\n"); \
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asm (".ascii \" -export:" #x ",data\"\n"); \
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asm (".data\n"); \
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/* Allocate a pseudo-variable IMP(x). */ \
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extern int x; \
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void * IMP(x) = &x;
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#else /* non-GNU compiler, non-GNU toolchain */
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# define VARIABLE(x) /* nothing */
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#endif
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