Lua 的 lprefix.h 只有短短 46 行，我花了不少时间去弄懂这个文件。

lprefix.h
/*
** $Id: lprefix.h,v 1.2 2014/12/29 16:54:13 roberto Exp $
** Definitions for Lua code that must come before any other header file
** See Copyright Notice in lua.h
*/

#ifndef lprefix_h
#define lprefix_h


/*
** Allows POSIX/XSI stuff
*/
#if !defined(LUA_USE_C89) /* { */

#if !defined(_XOPEN_SOURCE)
#define _XOPEN_SOURCE           600
#elif _XOPEN_SOURCE == 0
#undef _XOPEN_SOURCE  /* use -D_XOPEN_SOURCE=0 to undefine it */
#endif

/*
** Allows manipulation of large files in gcc and some other compilers
*/
#if !defined(LUA_32BITS) && !defined(_FILE_OFFSET_BITS)
#define _LARGEFILE_SOURCE       1
#define _FILE_OFFSET_BITS       64
#endif

#endif        /* } */


/*
** Windows stuff
*/
#if defined(_WIN32)   /* { */

#if !defined(_CRT_SECURE_NO_WARNINGS)
#define _CRT_SECURE_NO_WARNINGS  /* avoid warnings about ISO C functions */
#endif

#endif      /* } */

#endif

我一开始的问题是在第 25 行判断 LUA_USE_C89 宏，我没有找到地方对应到啊。lprefix 是需要在每个头文件引入之前被引入的，那哪里来的 LUA_USE_C89。最后才发现是在 src/Makefile 里面是有相关的设置的，我把代码放到 xcode 的时候，把所有不需要的文件都删了。c89 是 Lua 最后的选择了吧，我看一下 readme，里面推荐的尝试顺序：

If your platform is not listed, try the closest one or posix, generic, c89, in this order.

为了兼容性，Lua 也是很拼的。

然后，我们可以再看回相对应的 Feature-Test-Macros(功能测试宏)：

  • _XOPEN_SOURCE
If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.

As the unification of all Unices, functionality only available in BSD and SVID is also included.

If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available. The extra functions will make all functions available which are necessary for the X/Open Unix brand.

If the macro _XOPEN_SOURCE has the value 500 this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2.
  • _LARGEFILE_SOURCE
If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions fseeko and ftello are available. Without these functions the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).
  • _FILE_OFFSET_BITS
This macro determines which file system interface shall be used, one replacing the other. Whereas _LARGEFILE64_SOURCE makes the 64 bit interface available as an additional interface, _FILE_OFFSET_BITS allows the 64 bit interface to replace the old interface.

If _FILE_OFFSET_BITS is undefined, or if it is defined to the value 32, nothing changes. The 32 bit interface is used and types like off_t have a size of 32 bits on 32 bit systems.

If the macro is defined to the value 64, the large file interface replaces the old interface. I.e., the functions are not made available under different names (as they are with _LARGEFILE64_SOURCE). Instead the old function names now reference the new functions, e.g., a call to fseeko now indeed calls fseeko64.

This macro should only be selected if the system provides mechanisms for handling large files. On 64 bit systems this macro has no effect since the *64 functions are identical to the normal functions.

This macro was introduced as part of the Large File Support extension (LFS).