【gcc編譯優(yōu)化系列】如何獲取gcc默認(rèn)的鏈接腳本

1 問(wèn)題來(lái)源

我們都知道在一般的嵌入式開(kāi)發(fā)中，使用gcc編譯固件的一般流程是，先把所有的.c文件和.s文件編譯成.o文件，然后把所有的.o文件鏈接成一個(gè)elf文件，最后由elf文件導(dǎo)出bin文件。 那么在鏈接成elf文件這一步中，我們需要使用到一個(gè)叫鏈接腳本的文件，這個(gè)文件主要描述了程序的ROM分布和RAM分布，它也直接決定了二進(jìn)制文件的內(nèi)容結(jié)構(gòu)。 那么問(wèn)題來(lái)了，當(dāng)我們手上沒(méi)有可用的鏈接腳本的時(shí)，我們?nèi)绻@取到默認(rèn)的鏈接腳本呢？

2 解決方案

2.1 測(cè)試平臺(tái)

Ubuntu20.04 X64環(huán)境 gcc version9.3.0(Ubuntu9.3.0-17ubuntu1~20.04)

2.2 示例代碼

一段很簡(jiǎn)單的hello world代碼：

#include 

int main(int argc, const char *argv[])
{
    printf("hello world ...\r\n");

    return 0;
}

我們直接使用gcc，不用任何參數(shù)就可以編譯可執(zhí)行文件：

test_gcc$ gcc test.c 
test_gcc$ 
test_gcc$ ls
a.out  pass_array2_test.c  test.c
test_gcc$ 
test_gcc$ ./a.out 
hello world ...

但是我并沒(méi)有看到任何鏈接腳本?。?因?yàn)槟鞘莋cc使用了默認(rèn)的鏈接腳本。

2.3 分析gcc的默認(rèn)參數(shù)

gcc有個(gè)-v選項(xiàng)可以看到看似簡(jiǎn)單的gcc test.c都發(fā)生了啥：

test_gcc$ gcc test.c -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.3.0-17ubuntu1~20.04' --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-HskZEa/gcc-9-9.3.0/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 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04) 
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/9/cc1 -quiet -v -imultiarch x86_64-linux-gnu test.c -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o /tmp/ccvW5Igg.s
GNU C17 (Ubuntu 9.3.0-17ubuntu1~20.04) version 9.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 9.3.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
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.
GNU C17 (Ubuntu 9.3.0-17ubuntu1~20.04) version 9.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 9.3.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: bbf13931d8de1abe14040c9909cb6969
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 as -v --64 -o /tmp/cc17VwSh.o /tmp/ccvW5Igg.s
GNU assembler version 2.34 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.34
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/gcc/x86_64-linux-gnu/9/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/9/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/9/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper -plugin-opt=-fresolution=/tmp/ccPxjLXh.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -z relro /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/9/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/9 -L/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/9/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/9/../../.. /tmp/cc17VwSh.o -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s --pop-state /usr/lib/gcc/x86_64-linux-gnu/9/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'

我們抓住核心，主要有三步： 第一步是使用/usr/lib/gcc/x86_64-linux-gnu/9/cc1編譯.c文件，生成/tmp/ccvW5Igg.s； 第二步是使用as編譯.s文件，生成/tmp/cc17VwSh.o； 第三步是使用/usr/lib/gcc/x86_64-linux-gnu/9/collect2鏈接.o文件，生成a.out。 從第三步我們可以看出，最終生成elf文件(a.out)是collect2，它其實(shí)也是ld命令最終調(diào)用的命令。

2.4 導(dǎo)出默認(rèn)鏈接腳本的方法

2.4.1 使用ld命令導(dǎo)出

ld --verbose

這樣就可以得到輸出：

test_gcc$ ld --verbose
GNU ld (GNU Binutils for Ubuntu) 2.34
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   elf_iamcu
   elf_l1om
   elf_k1om
   i386pep
   i386pe
using internal linker script:
==================================================
/* Script for -z combreloc -z separate-code */
/* Copyright (C) 2014-2020 Free Software Foundation, Inc.
   Copying and distribution of this script, with or without modification,
   are permitted in any medium without royalty provided the copyright
   notice and this notice are preserved.  */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
              "elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("=/usr/local/lib/x86_64-linux-gnu"); SEARCH_DIR("=/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu64"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib64"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib");
SECTIONS
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   }
  .tbss           : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }
  .preinit_array    :
  {
    PROVIDE_HIDDEN (__preinit_array_start = .);
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  }
  .init_array    :
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    PROVIDE_HIDDEN (__init_array_start = .);
    KEEP (*(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*)))
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    PROVIDE_HIDDEN (__init_array_end = .);
  }
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    KEEP (*(SORT_BY_INIT_PRIORITY(.fini_array.*) SORT_BY_INIT_PRIORITY(.dtors.*)))
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  .ctors          :
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       linker won't look for a file to match a
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       doesn't matter which directory crtbegin.o
       is in.  */
    KEEP (*crtbegin.o(.ctors))
    KEEP (*crtbegin?.o(.ctors))
    /* We don't want to include the .ctor section from
       the crtend.o file until after the sorted ctors.
       The .ctor section from the crtend file contains the
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    KEEP (*(.ctors))
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==================================================

由于鏈接腳本是有語(yǔ)法規(guī)則的，所以我們需要把============以及其前后的部分刪掉，保留中間部分，然后拷貝到一個(gè)文本文件，命名為xxx.lds即可；這個(gè)是它的常用后綴名。

2.4.2 使用gcc命令導(dǎo)出

gcc -Wl,--verbose

其實(shí)這條命令，與ld那條命令有異曲同工的作用，只不過(guò)gcc這條命令就是外面再套了一層。輸出如下：

test_gcc$ gcc -Wl,--verbose
GNU ld (GNU Binutils for Ubuntu) 2.34
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   elf_iamcu
   elf_l1om
   elf_k1om
   i386pep
   i386pe
using internal linker script:
==================================================
/* Script for -pie -z combreloc -z separate-code -z relro -z now */
/* Copyright (C) 2014-2020 Free Software Foundation, Inc.
   Copying and distribution of this script, with or without modification,
   are permitted in any medium without royalty provided the copyright
   notice and this notice are preserved.  */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
              "elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("=/usr/local/lib/x86_64-linux-gnu"); SEARCH_DIR("=/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu64"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib64"); SEARCH_DIR("=/usr/x86_64-linux-gnu/lib");
SECTIONS
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}


==================================================
/usr/bin/ld: mode elf_x86_64
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crti.o succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crti.o
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/crtbeginS.o succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/crtbeginS.o
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.so failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so succeeded
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
attempt to open libgcc_s.so.1 failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so.1 failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libgcc_s.so.1 succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libgcc_s.so.1
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.so failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libc.so failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libc.a failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libc.so succeeded
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libc.so
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libc.so
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libc.so
attempt to open /lib/x86_64-linux-gnu/libc.so.6 succeeded
/lib/x86_64-linux-gnu/libc.so.6
attempt to open /usr/lib/x86_64-linux-gnu/libc_nonshared.a succeeded
/usr/lib/x86_64-linux-gnu/libc_nonshared.a
(/usr/lib/x86_64-linux-gnu/libc_nonshared.a)elf-init.oS
attempt to open /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 succeeded
/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
/usr/lib/x86_64-linux-gnu/libc_nonshared.a
/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.so failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so succeeded
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
opened script file /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so
attempt to open libgcc_s.so.1 failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc_s.so.1 failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libgcc_s.so.1 succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libgcc_s.so.1
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.so failed
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/crtendS.o succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/crtendS.o
attempt to open /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crtn.o succeeded
/usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/crtn.o
ld-linux-x86-64.so.2 needed by /lib/x86_64-linux-gnu/libc.so.6
found ld-linux-x86-64.so.2 at /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
/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'
/usr/bin/ld: link errors found, deleting executable `a.out'
collect2: error: ld returned 1 exit status

跟上面一樣，找到========，去掉它，以及它前后的內(nèi)容，生成.lds文件即可。

2.4.3 如果你想偷個(gè)懶

如果你手上有該平臺(tái)調(diào)通過(guò)的鏈接腳本，拿過(guò)來(lái)改改是最好的方式，這樣不用自己去配置ROM/RAM空間，一般來(lái)說(shuō)，廠商提供的SDK肯定是帶了鏈接腳本的，注意合理使用即可。

3 更多分享

本項(xiàng)目的所有測(cè)試代碼和編譯腳本，均可以在我的github倉(cāng)庫(kù)01workstation中找到，歡迎指正問(wèn)題。

同時(shí)也非常歡迎關(guān)注我的CSDN主頁(yè)和專(zhuān)欄：

【http://yyds.recan-li.cn】

【C/C++語(yǔ)言編程專(zhuān)欄】

【GCC專(zhuān)欄】

【信息安全專(zhuān)欄】

有問(wèn)題的話(huà)，可以跟我討論，知無(wú)不答，謝謝大家。

審核編輯：湯梓紅