Linux下Gcc 的编译过程

         在linux下开发难免会用到gcc编译。GCC（GNU Compiler Collection。GNU编译器套装），是由 GNU 开发的编程语言编译器。它是GNU编译器套装以GPL许可证所发行的自由软件，也是 GNU计划的关键部分。

使用GCC编译程序时,编译过程能够被细分为四个阶段:
◆ 预处理(Pre-Processing)
◆ 编译(Compiling)
◆ 汇编(Assembling)
◆ 链接(Linking)

1、预处理 对源码文件里的文件包括(include)、预编译语句(如宏定义define等)进行分析，编译选项为gcc -E *.c

#define DEBUG "debug"

int main()
{
  char *a = DEBUG;
  return 1;
}

经过上面的预处理后，能够看到DEBUG被替换成了提前定义的内容

# 1 "hello.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "hello.c"



int main()
{
  char *a = "debug";
  return 1;
}

2、编译 调用cc1进行编译，使用gcc -S选项就能够生成汇编代码，这个阶段依据输入文件生成以.o为后缀的目标文件。生成的汇编代码例如以下：

        .file   "hello.c"
        .section        .rodata
.LC0:
        .string "debug"
        .text
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movq    $.LC0, -8(%rbp)
        movl    $1, %eax
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (GNU) 4.4.6 20110731 (Red Hat 4.4.6-3)"
        .section        .note.GNU-stack,"",@progbits

3、汇编  汇编过程是针对汇编语言的步骤，调用as进行工作。一般来讲，.S为后缀的汇编语言源码文件和汇编、.s为后缀的汇编语言文件经过预编译和汇编之后都生成以.o为后缀的目标文件。此过程生成ELF格式的目标代码，使用gcc -c进行汇编

使用readelf -a hello.o能够看到具体的elf信息

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              REL (Relocatable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          0 (bytes into file)
  Start of section headers:          296 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           0 (bytes)
  Number of program headers:         0
  Size of section headers:           64 (bytes)
  Number of section headers:         13
  Section header string table index: 10

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00000040
       0000000000000013  0000000000000000  AX       0     0     4
  [ 2] .rela.text        RELA             0000000000000000  00000568
       0000000000000018  0000000000000018          11     1     8
  [ 3] .data             PROGBITS         0000000000000000  00000054
       0000000000000000  0000000000000000  WA       0     0     4
  [ 4] .bss              NOBITS           0000000000000000  00000054
       0000000000000000  0000000000000000  WA       0     0     4
  [ 5] .rodata           PROGBITS         0000000000000000  00000054
       0000000000000006  0000000000000000   A       0     0     1
  [ 6] .comment          PROGBITS         0000000000000000  0000005a
       000000000000002d  0000000000000001  MS       0     0     1
  [ 7] .note.GNU-stack   PROGBITS         0000000000000000  00000087
       0000000000000000  0000000000000000           0     0     1
  [ 8] .eh_frame         PROGBITS         0000000000000000  00000088
       0000000000000038  0000000000000000   A       0     0     8
  [ 9] .rela.eh_frame    RELA             0000000000000000  00000580
       0000000000000018  0000000000000018          11     8     8
  [10] .shstrtab         STRTAB           0000000000000000  000000c0
       0000000000000061  0000000000000000           0     0     1
  [11] .symtab           SYMTAB           0000000000000000  00000468
       00000000000000f0  0000000000000018          12     9     8
  [12] .strtab           STRTAB           0000000000000000  00000558
       000000000000000e  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings)
  I (info), L (link order), G (group), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

There are no section groups in this file.

There are no program headers in this file.

Relocation section '.rela.text' at offset 0x568 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000008  00050000000b R_X86_64_32S      0000000000000000 .rodata + 0

Relocation section '.rela.eh_frame' at offset 0x580 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000020  000200000002 R_X86_64_PC32     0000000000000000 .text + 0

There are no unwind sections in this file.

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS hello.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    8 
     8: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     9: 0000000000000000    19 FUNC    GLOBAL DEFAULT    1 main

4、链接 链接过程。生成可运行代码。链接分为两种，一种是静态链接，第二种是动态链接。

使用静态链接的优点是，依赖的动态链接库较少，对动态链接库的版本号不会非常敏感，具有较好的兼容性。缺点是生成的程序比較大。使用动态链接的优点是，生成的程序比較小，占用较少的内存。

gcc hello.o -o hello 就能够完毕最后的链接操作并生成可运行文件，至于怎样生成动态库和静态库，以及怎样链接动态库和静态库，以后会再作介绍。