release/src/router/libiconv/woe32dll/export.h

   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;