Introduction to the differences between compiler options of GCC dynamic library FPIC and FPIC

The author didn't pay special attention to fPIC and fpic The difference between , Recently aarch64 A problem encountered in compiling on the server , Let yourself make a good inquiry of some information , Perhaps ordinary developers are not easy to encounter it , I hope it will be of some help to readers who are destined .

1） When dynamic library is out , Merge libnettle.a The library encountered the following error prompt

/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

The key tips are
warning: too many GOT entries for -fpic, please recompile with -fPIC

Verified fpic and fPIC Compare

• The same thing ： Is to generate location independent code in the dynamic library . Through global offset table （GOT） Access all constant addresses . Dynamically load the program when the program starts GOT entry .

• Difference ： If the linked executable GOT The size exceeds the maximum value specified by the computer architecture , An error message will be reported in the compilation link , Tips -fpic It doesn't work ; So in this case , Need to use -fPIC recompile .GOT The size varies with the chip architecture ,SPARC Up for 8k, stay AArch64 Up for 28k（ What the author meets is it ,GOT Limit exceeded , So wrong reporting ）, stay m68k and RS / 6000 Up for 32k. and x86 There is no such restriction on .

Tips

In order to ensure the overall availability of the program during cross platform compilation , Usually, it is suggested to use fPIC

Reference source address

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

The original English text （ If the above address cannot be accessed , You can refer to the following original ）

-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.