程序一定要从 main 函数开始运行吗？

int main() {
    printf("程序喵\n");
    return 0;
}
gcc -c test

objdump -r test.o
test.o:     file format elf64-x86-64
RELOCATION RECORDS FOR [.text]:
OFFSET           TYPE              VALUE
0000000000000007 R_X86_64_PC32     .rodata-0x0000000000000004
000000000000000c R_X86_64_PLT32    puts-0x0000000000000004
RELOCATION RECORDS FOR [.eh_frame]:
OFFSET           TYPE              VALUE
0000000000000020 R_X86_64_PC32     .text

nm -u test.o
                 U _GLOBAL_OFFSET_TABLE_
                 U puts

~/test$ gcc -c -fno-builtin testlink.cc -o test.o
~/test$ nm test.o
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 T main
                 U printf

-ffunction-sections
-fdata-sections

const char* str = "hello";
void print() {
    asm("movl $13,%%edx \n\t"
        "movl str,%%ecx \n\t"
        "movl $0,%%ebx \n\t"
        "movl $4,%%eax \n\t"
        "int $0x80 \n\t"
        :
        :"r"(str):"edx", "ecx", "ebx");
}
void exit() {
    asm("movl $42,%ebx \n\t"
        "movl $1,%eax \n\t"
        "int $0x80 \n\t");
}
void nomain() {
    print();
    exit();
}

gcc -c -fno-builtin test.cc

~/test$ nm -a test.o
0000000000000000 b .bss
0000000000000000 n .comment
0000000000000000 d .data
0000000000000000 d .data.rel.local
0000000000000000 r .eh_frame
0000000000000000 n .note.GNU-stack
0000000000000000 r .rodata
0000000000000000 t .text
0000000000000026 T _Z4exitv
0000000000000000 T _Z5printv
0000000000000039 T _Z6nomainv
0000000000000000 D str
0000000000000000 a test.cc

~/test$ gcc -c -fno-builtin test.c -o test.o
~/test$ nm -a test.o
0000000000000000 b .bss
0000000000000000 n .comment
0000000000000000 d .data
0000000000000000 d .data.rel.local
0000000000000000 r .eh_frame
0000000000000000 n .note.GNU-stack
0000000000000000 r .rodata
0000000000000000 t .text
0000000000000026 T exit
0000000000000039 T nomain
0000000000000000 T print
0000000000000000 D str
0000000000000000 a test.c

~/test$ ld -static -e nomain -o test test.o
~/test$ ./test
hello

$ ld -verbose
GNU ld (GNU Binutils for Ubuntu) 2.30
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   elf_iamcu
   i386linux
   elf_l1om
   elf_k1om
   i386pep
   i386pe
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
/* Copyright (C) 2014-2018 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
{
  /* Read-only sections, merged into text segment: */
  PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000)); . = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;
 
  .init           :
  {
    KEEP (*(SORT_NONE(.init)))
  }
  .plt            : { *(.plt) *(.iplt) }
  .plt.got        : { *(.plt.got) }
  .plt.sec        : { *(.plt.sec) }
  .text           :
  {
    *(.text.unlikely .text.*_unlikely .text.unlikely.*)
    *(.text.exit .text.exit.*)
    *(.text.startup .text.startup.*)
    *(.text.hot .text.hot.*)
    *(.text .stub .text.* .gnu.linkonce.t.*)
    /* .gnu.warning sections are handled specially by elf32.em.  */
    *(.gnu.warning)
  }
  .fini           :
  {
    KEEP (*(SORT_NONE(.fini)))
  }
  .rodata         : { *(.rodata .rodata.* .gnu.linkonce.r.*) }
  /DISCARD/ : { *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }
}

ENTRY(nomain)
SECTIONS
{
    . = 0x8048000 + SIZEOF_HEADERS;
    tinytext : { *(.text) *(.data) *(.rodata) }
    /DISCARD/ : { *(.comment) }
}

~/test$ ld -static -T test.lds -e nomain -o test test.o
~/test$ ./test
hello

~/test$ readelf -S test
~/test$ There are 6 section headers, starting at offset 0x482a0:
Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .eh_frame         PROGBITS         00000000080480b0  000480b0
       0000000000000078  0000000000000000   A       0     0     8
  [ 2] tinytext          PROGBITS         0000000008048128  00048128
       0000000000000066  0000000000000000 WAX       0     0     8
  [ 3] .shstrtab         STRTAB           0000000000000000  0004826e
       000000000000002e  0000000000000000           0     0     1
  [ 4] .symtab           SYMTAB           0000000000000000  00048190
       00000000000000c0  0000000000000018           5     4     8
  [ 5] .strtab           STRTAB           0000000000000000  00048250
       000000000000001e  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

ar rcs libxxx.a xx1.o xx2.o 打包静态链接库
ar -t libc.a 查看静态链接库里都有什么目标文件
ar -x libc.a 会解压所有的目标文件到当前目录
gcc --verbose 可以查看整个编译链接步骤

objdump -i 查看本机目标架构
objdump -f 显示文件头信息
objdump -d 反汇编程序
objdump -t 显示符号表入口，每个目标文件都有什么符号
objdump -r 显示文件的重定位入口，重定位表
objdump -x 显示所有可用的头信息，等于-a -f -h -r -t
objdump -H 帮助

readelf -h 列出文件头
readelf -S 列出每个段
readelf -r 列出重定位表
readelf -d 列出动态段

nm -a 显示所有的符号
nm -D 显示动态符号
nm -u 仅显示没有定义的外部符号
nm -defined-only 仅显示定义的符号