g++链接及动态库和静态库浅析

制作动态库

1. myprint.h文件

   #pragma once 
   void myprint();
   

   和
   myprint.cc 文件

   #include 
   #include "myprint.h"
   void myprint() {
       printf("myprint\n");
   }
   

2. 使用命令制作动态库

   [test_so_a] :) mkdir so
   [test_so_a] :) g++ -fPIC -o myprint.o -c myprint.cc 
   [test_so_a] :) g++ -shared -o libmyprint.so myprint.o 
   [test_so_a] :) mv libmyprint.so so/
   [test_so_a] :) ls -al ./so/
   total 24
   drwxrwxr-x 2 pengchen pengchen  4096 Mar 13 19:06 .
   drwxrwxr-x 3 pengchen pengchen  4096 Mar 13 19:06 ..
   -rwxrwxr-x 1 pengchen pengchen 16208 Mar 13 19:06 libmyprint.so
   [test_so_a] :) 
   

3. 使用动态库main.cc

   
   #include "myprint.h"
   
   int main() {
       myprint();
   }
   

   生成可执行文件指令

   [test_so_a] :) g++ -E main.cc -o main_dynamic.i
   [test_so_a] :) g++ -S main_dynamic.i -o main_dynamic.s
   [test_so_a] :) g++ -c main_dynamic.s -o main_dynamic.o
   [test_so_a] :) g++ -o main_dynamic main_dynamic.o -L ./so/ -lmyprint
   [test_so_a] :) echo $LD_LIBRARY_PATH
   
   [test_so_a] :) ./main_dynamic 
   ./main_dynamic: error while loading shared libraries: libmyprint.so: cannot open shared object file: No such file or directory
   [test_so_a] :( export LD_LIBRARY_PATH=./so/:${LD_LIBRARY_PATH}
   [test_so_a] :) ./main_dynamic 
   myprint
   [test_so_a] :) 
   

   这里几个问题？
   1. 为什么在链接阶段需要添加-L（大些L）和-l（小写L，与大写的i意思完全不同）选项?
   2. 环境变量LD_LIBRARY_PATH的作用？
   3. 为什么没用-I（大写的i）也可以？

第一个问题

1. 为什么需要-L以“编译和链接阶段，指定链接器应该搜索哪些目录以找到库文件。”？反正动态库又不放到可执行文件里面？LD_LIBRARY_PATH不就行了？

   -L 选项在编译和链接阶段起到了关键作用，确保了链接器能正确解析符号、生成符号表和依赖信息。虽然动态库本身不嵌入到可执行文件中，但这些步骤对于生成一个能够正确运行的可执行文件是不可或缺的。而 LD_LIBRARY_PATH 则是在运行时阶段工作的，保证程序在执行时能找到这些库。两者的作用和使用阶段不同，不能互相替代。

   • 验证是否具有”正确解析符号“ 功能

     [test_so_a] :) nm -AC main_dynamic.o
     main_dynamic.o:                 U _GLOBAL_OFFSET_TABLE_
     main_dynamic.o:0000000000000000 T main
     main_dynamic.o:                 U myprint()
     [test_so_a] :) nm -AC main_dynamic
     main_dynamic:0000000000004010 B __bss_start
     main_dynamic:0000000000004010 b completed.8061
     main_dynamic:                 w __cxa_finalize@@GLIBC_2.2.5
     main_dynamic:0000000000004000 D __data_start
     main_dynamic:0000000000004000 W data_start
     main_dynamic:0000000000001090 t deregister_tm_clones
     main_dynamic:0000000000001100 t __do_global_dtors_aux
     main_dynamic:0000000000003db0 d __do_global_dtors_aux_fini_array_entry
     main_dynamic:0000000000004008 D __dso_handle
     main_dynamic:0000000000003db8 d _DYNAMIC
     main_dynamic:0000000000004010 D _edata
     main_dynamic:0000000000004018 B _end
     main_dynamic:00000000000011d8 T _fini
     main_dynamic:0000000000001140 t frame_dummy
     main_dynamic:0000000000003da8 d __frame_dummy_init_array_entry
     main_dynamic:000000000000214c r __FRAME_END__
     main_dynamic:0000000000003fb8 d _GLOBAL_OFFSET_TABLE_
     main_dynamic:                 w __gmon_start__
     main_dynamic:0000000000002004 r __GNU_EH_FRAME_HDR
     main_dynamic:0000000000001000 t _init
     main_dynamic:0000000000003db0 d __init_array_end
     main_dynamic:0000000000003da8 d __init_array_start
     main_dynamic:0000000000002000 R _IO_stdin_used
     main_dynamic:                 w _ITM_deregisterTMCloneTable
     main_dynamic:                 w _ITM_registerTMCloneTable
     main_dynamic:00000000000011d0 T __libc_csu_fini
     main_dynamic:0000000000001160 T __libc_csu_init
     main_dynamic:                 U __libc_start_main@@GLIBC_2.2.5
     main_dynamic:0000000000001149 T main
     main_dynamic:00000000000010c0 t register_tm_clones
     main_dynamic:0000000000001060 T _start
     main_dynamic:0000000000004010 D __TMC_END__
     main_dynamic:                 U myprint()
     

     发现不管是在目标文件还是可执行文件，myprint()都是未解析的符号错误（undefined reference），GPT说对于动态库是正常的。那拿静态库试一下

     [test_so_a] :) g++ -c myprint.cc -o myprint.o
     [test_so_a] :) ar rcs ./a/libmyprint.a myprint.o
     [test_so_a] :) g++ -o main_static -L ./a/ -lmyprint
     /usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
     (.text+0x24): undefined reference to `main'
     collect2: error: ld returned 1 exit status
     [test_so_a] :( g++ -o main_static main_static.o -L ./a/ -lmyprint
     [test_so_a] :) ./main_static 
     myprint
     [test_so_a] :) nm -AC main_static.o
     main_static.o:                 U _GLOBAL_OFFSET_TABLE_
     main_static.o:0000000000000000 T main
     main_static.o:                 U myprint()
     [test_so_a] :) nm -AC main_static
     main_static:0000000000004010 B __bss_start
     main_static:0000000000004010 b completed.8061
     main_static:                 w __cxa_finalize@@GLIBC_2.2.5
     main_static:0000000000004000 D __data_start
     main_static:0000000000004000 W data_start
     main_static:0000000000001090 t deregister_tm_clones
     main_static:0000000000001100 t __do_global_dtors_aux
     main_static:0000000000003dc0 d __do_global_dtors_aux_fini_array_entry
     main_static:0000000000004008 D __dso_handle
     main_static:0000000000003dc8 d _DYNAMIC
     main_static:0000000000004010 D _edata
     main_static:0000000000004018 B _end
     main_static:00000000000011f8 T _fini
     main_static:0000000000001140 t frame_dummy
     main_static:0000000000003db8 d __frame_dummy_init_array_entry
     main_static:000000000000217c r __FRAME_END__
     main_static:0000000000003fb8 d _GLOBAL_OFFSET_TABLE_
     main_static:                 w __gmon_start__
     main_static:000000000000200c r __GNU_EH_FRAME_HDR
     main_static:0000000000001000 t _init
     main_static:0000000000003dc0 d __init_array_end
     main_static:0000000000003db8 d __init_array_start
     main_static:0000000000002000 R _IO_stdin_used
     main_static:                 w _ITM_deregisterTMCloneTable
     main_static:                 w _ITM_registerTMCloneTable
     main_static:00000000000011f0 T __libc_csu_fini
     main_static:0000000000001180 T __libc_csu_init
     main_static:                 U __libc_start_main@@GLIBC_2.2.5
     main_static:0000000000001149 T main
     main_static:                 U puts@@GLIBC_2.2.5
     main_static:00000000000010c0 t register_tm_clones
     main_static:0000000000001060 T _start
     main_static:0000000000004010 D __TMC_END__
     main_static:000000000000115d T myprint()
     [test_so_a] :) 
     

     发现main_static:000000000000115d T myprint()。
   • 生成符号表
     其实上述已经为可执行文件main_dynamic和main_static生成了带有myprint的符号表，毕竟没有-L都无法生成文件。故不表。
   • 查看是否生成依赖信息

     [test_so_a] :( g++ -o main_dynamic main_dynamic.o
     /usr/bin/ld: main_dynamic.o: in function `main':
     main.cc:(.text+0x9): undefined reference to `myprint()'
     collect2: error: ld returned 1 exit status
     [test_so_a] :( g++ -o main_dynamic main_dynamic.o -L ./so -lmyprint
     [test_so_a] :) ldd main_dynamic 
             linux-vdso.so.1 (0x00007ffdb0769000)
             libmyprint.so => ./so/libmyprint.so (0x00007f0a63c44000)
             libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f0a63a47000)
             /lib64/ld-linux-x86-64.so.2 (0x00007f0a63c50000)
     [test_so_a] :) 
     

第二个问题

1. 环境变量LD_LIBRARY_PATH的作用？
   为可执行程序指定在运行时加载自定义动态库的搜索路径。对于标准库，其标准搜索路径是（这里不确定是不是这个命令）:

   [test_so_a] :) ldconfig -v 2>/dev/null | grep -v ^$'\t'
   /usr/lib/x86_64-linux-gnu/libfakeroot:
   /usr/local/lib:
   /lib/x86_64-linux-gnu:
   /lib:
   [test_so_a] :) cpp -v /dev/null -o /dev/null
   Using built-in specs.
   COLLECT_GCC=cpp
   OFFLOAD_TARGET_NAMES=nvptx-none:hsa
   OFFLOAD_TARGET_DEFAULT=1
   Target: x86_64-linux-gnu
   Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.4.0-1ubuntu1~20.04.2' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-9QDOt0/gcc-9-9.4.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
   Thread model: posix
   gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.2) 
   COLLECT_GCC_OPTIONS='-E' '-v' '-o' '/dev/null' '-mtune=generic' '-march=x86-64'
    /usr/lib/gcc/x86_64-linux-gnu/9/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu /dev/null -o /dev/null -mtune=generic -march=x86-64 -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection
   ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
   ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/9/include-fixed"
   ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/9/../../../../x86_64-linux-gnu/include"
   #include "..." search starts here:
   #include <...> search starts here:
    /usr/lib/gcc/x86_64-linux-gnu/9/include
    /usr/local/include
    /usr/include/x86_64-linux-gnu
    /usr/include
   End of search list.
   COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/
   LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/9/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/9/../../../:/lib/:/usr/lib/
   COLLECT_GCC_OPTIONS='-E' '-v' '-o' '/dev/null' '-mtune=generic' '-march=x86-64'
   [test_so_a] :) 
   

   这里

   [test_so_a] :) echo $LD_LIBRARY_PATH
   
   [test_so_a] :) ./main_dynamic 
   ./main_dynamic: error while loading shared libraries: libmyprint.so: cannot open shared object file: No such file or directory
   [test_so_a] :( export LD_LIBRARY_PATH=./so/:${LD_LIBRARY_PATH}
   [test_so_a] :) ./main_dynamic 
   myprint
   [test_so_a] :) 
   

   参考https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dynamic_or_shared.html
   https://www.zhihu.com/question/537366949

为什么没用-I（大写的i）也可以？

1. 参考

   You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
   For the quote form of the include directive, the directory of the current file is searched first.
   For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
   Directories specified with -I options are scanned in left-to-right order.
   Directories specified with -isystem options are scanned in left-to-right order.
   Standard system directories are scanned.
   Directories specified with -idirafter options are scanned in left-to-right order.
   参考
   https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html