gcc动态库fPIC和fpic编译选项差异介绍

作者以前并没有特别留意fPIC和fpic之间的差别，最近在aarch64服务器上编译遇到的一个问题，让自己好好的查询了一些资料，或许平常的开发人员并不容易遇到它，但愿对有缘的读者有一定帮助。

1）出动态库时，合并libnettle.a库遇到下面错误提示

/data/compilerdir/bin/opensdk/linux64/lib/libnettle.a(fat-arm64.o): in function `fat_init': /data2/compilerdir/nettle/fat-arm64.c:198:(.text.startup+0x184): relocation truncated to fit: R_AARCH64_LD64_GOTPAGE_LO15 against symbol `_nettle_aes192_decrypt_c' defined in .text section in /data/compilerdir/bin/opensdk/linux64/lib/libnettle.a(aes192-decrypt.o)
/usr/bin/ld: /data/compilerdir/nettle/fat-arm64.c:198: warning: too many GOT entries for -fpic, please recompile with -fPIC
collect2: error: ld returned 1 exit status
/data2/compilerdir/bin/libhyscoder.so] Error 1

其中关键提示为
warning: too many GOT entries for -fpic, please recompile with -fPIC

经查证 fpic 和 fPIC 比较

• 相同点：都是为了在动态库中生成位置无关的代码。通过全局偏移表（GOT）访问所有常量地址。程序启动时动态加载程序解析GOT条目。

• 不同点：如果链接的可执行文件的GOT大小超过计算机架构特定的最大值，则会在编译链接时报错误消息，提示 -fpic 不起作用；因此在这种情况下，需要使用 -fPIC 重新编译。GOT大小因芯片架构的不同而大小不一样，SPARC上为8k，在AArch64上为28k（笔者遇到的就是它，GOT超出了限制，所以报错），在m68k和RS / 6000上为32k。而x86上没有此限制。

提示

为了保障程序在跨平台编译时整体可用，通常情况下建议都用fPIC

参考来源地址

https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options

英文原文（如果上面地址不能访问，可以参考下面原文）

-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)

Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1.

-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.

Position-independent code requires special support, and therefore works only on certain machines.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2.