是什么_LARGEFILE_SOURCE和_FILE_OFFSET_BITS = 64之间的区别？
据我所知， -D_FILE_OFFSET_BITS = 64 引起 off_t 是64位。那么，是什么 -D_LARGEFILE_SOURCE 做到这一点还没有被 -D_FILE_OFFSET_BITS = 64 做了什么？什么这些定义做什么呢？
I understand that -D_FILE_OFFSET_BITS=64 causes off_t to be 64bits. So what does -D_LARGEFILE_SOURCE do that isn't already done by -D_FILE_OFFSET_BITS=64? What do these definitions do exactly?
glibc的Feature测试宏文档状态：
The GLIBC Feature test macros documentation states:
_LARGEFILE_SOURCE 结果
如果该宏定义了一些额外的功能，可它纠正所有previous标准一些缺点。具体地，功能fseeko和ftello可用。如果没有这些功能的ISO C接口（fseek的，FTELL）和低级别的POSIX接口（lseek的）之间的差异会导致问题。
_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.
本宏介绍，作为大文件支持扩展（LFS）的一部分。
This macro was introduced as part of the Large File Support extension (LFS).
所以，宏观专门让 fseeko 和 ftello 可用。 _FILE_OFFSET_BITS 设置本身并不能使这些功能。
So that macro specifically makes fseeko and ftello available. _FILE_OFFSET_BITS settings alone don't make these functions available.
（注意，如果你使用C，与GCC默认的GNU话，你可能不需要明确定义 _LARGEFILE_SOURCE 。怎么做，如果你使用 -std = C99 的实例。）
(Note that if you're using a GNU dialect of C, the default with GCC, you might not need to explicitly define _LARGEFILE_SOURCE. You do if you use -std=c99 for instance.)