`-fpic`和`-fPIC` gcc参数有什么区别？

Question

我已经阅读了gcc手册页，但我仍然不能理解-fpic和-fPIC之间的区别。有人能用一种简单明了的方式解释一下吗？

相关问题：

• What does -fPIC mean when building a shared library?
• What, if any, are the implications of compiling objects with gcc -fPIC flag if they get used in executables?

Answer 1

http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

使用-fPIC或-fpic生成与位置无关的代码。是否使用-fPIC或-fpic生成位置无关代码取决于目标。-fPIC选项总是有效的，但可能会产生比-fpic更大的代码(记住，这是因为PIC处于更大的情况下，所以它可能会产生更多的代码)。使用-fpic选项通常会生成更小、更快的代码，但会有平台相关的限制，例如全局可见符号的数量或代码的大小。当你创建共享库时，链接器会告诉你它是否合适。当有疑问时，我选择-fPIC，因为它总是有效的。

Answer 2

从Gcc manual page

在为共享库生成代码时，-fpic表示-M小数据，-fPIC表示-M大数据。

其中：

 -msmall-data
 -mlarge-data
       When -mexplicit-relocs is in effect, static data is accessed via
       gp-relative relocations.  When -msmall-data is used, objects 8
       bytes long or smaller are placed in a small data area (the
       ".sdata" and ".sbss" sections) and are accessed via 16-bit
       relocations off of the $gp register.  This limits the size of the
       small data area to 64KB, but allows the variables to be directly
       accessed via a single instruction.

       The default is -mlarge-data.  With this option the data area is
       limited to just below 2GB.  Programs that require more than 2GB
       of data must use "malloc" or "mmap" to allocate the data in the
       heap instead of in the program's data segment.

       When generating code for shared libraries, -fpic implies
       -msmall-data and -fPIC implies -mlarge-data.