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,编译过程可以被细分为四个阶段:
◆ 预处理(Pre-Processing)
◆ 编译(Compiling)
◆ 汇编(Assembling)
◆ 链接(Linking)
1、预处理 对源代码文件中的文件包含(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，可以看到DEBUG被替换成了预定义的内容
# 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选项就可以生成汇编代码，这个阶段根据输入文件生成以.o为后缀的目标文件。生成的汇编代码如下：
.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,"",@progbits3、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代码文件和汇编、.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可以看到详细的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。生成可执行代码。链接分为两种，一种是静态链接，另外一种是动态链接。使用静态链接的好处是，依赖的动态链接库较少，对动态链接库的版本不会很敏感，具有较好的兼容性；缺点是生成的程序比较大。使用动态链接的好处是，生成的程序比较小，占用较少的内存。
gcc hello.o -o hello 就可以完成最后的链接操作并生成可执行文件，至于如何生成动态库和静态库，以及如何链接动态库和静态库，以后会再作介绍。