结合"hello world"探讨gcc编译程序的过程

结合"hello world"探讨gcc编译程序的过程

1. gcc简介

gcc/g++是GNU工程的C和C++编译器。都要用四个步骤中的一个或多个处理输入文件。

· 预处理(preprocessing)

· 编译(compilation)

· 汇编(assembly)

· 连接(linking)

源文件后缀名标识源文件的语言，但是对编译器来说，后缀名控制着缺省设定。

gcc命令的一般格式为：gcc [选项] 要编译的文件 [选项] [目标文件]

gcc的详细内容，可参考gcc manual。

2. gcc的输出选项

gcc输出选项，-o后指定输出文件名，gcc解释如下。

`-o FILE'

Place output in file FILE. This applies regardless to whatever sort of output
is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If `-o' is not specified, the default is to put an executable file in `a.out',
the object file for `SOURCE.SUFFIX' in `SOURCE.o', its assembler file in `SOURCE.s', a precompiled header file in `SOURCE.SUFFIX.gch', and all preprocessed C source on standard output.

指定输出文件为FILE，该选项不考虑GCC产生什么输出，无论是可执行文件、目标文件、汇编文件还是预处理后的C代码。

如果没有使用`-o'选项，默认的输出结果是:可执行文件为a.out，source.suffix的目标文件是source.o，汇编文件是source.s，而预处理后的C代码送往标准输出。

3. "hello wolrd"的处理过程

本文以"hello world"程序，探讨GCC编译程序的过程。

hello.c文件内容如下。

#include <stdio.h>

int main(int argc, char **argv)

{

printf("Hello World! via %x\n", printf);

return 0;

}

3.1 预处理(preprocessing)

在该阶段，编译器将C源代码中的包含的头文件如stdio.h编译进来，可以使用gcc的选项”-E”进行查看预处理结果。

Gcc manual中对-E选项的解释如下。

`-E'

Stop after the preprocessing stage; do not run the compiler proper. The output
is in the form of preprocessed source code, which is sent to the standard output.

Input files which don't require preprocessing are ignored.

预处理后停止，不进行编译。预处理后的代码送往标准输出。GCC忽略任何不需要预处理的输入文件。

格式：gcc -E hello.c -o hello.i

作用：对hello.c进行预处理输出为hello.i文件

Hello.c被预处理后的hello.i文件。

#
1 "hello.c"

#
1 "<built-in>"

#
1 "<command line>"

#
1 "hello.c"

#
1 "/usr/include/stdio.h" 1 3 4

#
28 "/usr/include/stdio.h" 3 4

#
1 "/usr/include/features.h" 1 3 4

#
335 "/usr/include/features.h" 3 4

#
1 "/usr/include/sys/cdefs.h" 1 3 4

#
360 "/usr/include/sys/cdefs.h" 3 4

#
1 "/usr/include/bits/wordsize.h" 1 3 4

#
361 "/usr/include/sys/cdefs.h" 2 3 4

#
336 "/usr/include/features.h" 2 3 4

#
359 "/usr/include/features.h" 3 4

#
1 "/usr/include/gnu/stubs.h" 1 3 4

#
1 "/usr/include/bits/wordsize.h" 1 3 4

#
5 "/usr/include/gnu/stubs.h" 2 3 4

#
1 "/usr/include/gnu/stubs-32.h" 1 3 4

#
8 "/usr/include/gnu/stubs.h" 2 3 4

#
360 "/usr/include/features.h" 2 3 4

#
29 "/usr/include/stdio.h" 2 3 4

#
1 "/usr/lib/gcc/i386-redhat-linux/4.1.2/include/stddef.h" 1 3 4

#
214 "/usr/lib/gcc/i386-redhat-linux/4.1.2/include/stddef.h" 3 4

typedef
unsigned int size_t;

#
35 "/usr/include/stdio.h" 2 3 4

#
1 "/usr/include/bits/types.h" 1 3 4

#
28 "/usr/include/bits/types.h" 3 4

#
1 "/usr/include/bits/wordsize.h" 1 3 4

#
29 "/usr/include/bits/types.h" 2 3 4

typedef
unsigned char __u_char;

typedef
unsigned short int __u_short;

typedef
unsigned int __u_int;

typedef
unsigned long int __u_long;

typedef
signed char __int8_t;

typedef
unsigned char __uint8_t;

typedef
signed short int __int16_t;

typedef
unsigned short int __uint16_t;

typedef
signed int __int32_t;

typedef
unsigned int __uint32_t;

__extension__
typedef signed long long int __int64_t;

__extension__
typedef unsigned long long int __uint64_t;

__extension__
typedef long long int __quad_t;

__extension__
typedef unsigned long long int __u_quad_t;

#
131 "/usr/include/bits/types.h" 3 4

#
1 "/usr/include/bits/typesizes.h" 1 3 4

#
132 "/usr/include/bits/types.h" 2 3 4

...

3.2 编译(compilation)

在这个阶段中，Gcc首先要检查代码的规范性、是否有语法错误等，以确定代码的实际要做的工作，在检查无误后，Gcc把代码翻译成汇编语言。可以使用”-S”选项来进行查看，该选项只进行编译而不进行汇编，生成汇编代码。

Gcc manual中对-S选项的解释如下。

`-S'

Stop after the stage of compilation proper; do not assemble. The output is in
the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the
suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ignored.

编译后即停止，不进行汇编。对于每个输入的非汇编语言文件，输出文件是汇编语言文件。

缺省情况下，GCC通过用`.o'替换源文件名后缀`.c',`.i'等以产生目标文件名。可以使用-o选项指定选择其他名字。GCC忽略任何不需要编译的输入文件。

格式：gcc –S hello.i –o hello.s

作用：将预处理输出文件hello.i汇编成hello.s文件

Hello.i被汇编后的hello.s文件。

.file "hello.c"

.section .rodata

.LC0:

.string "Hello World! via %x\n"

.text

.globl main

.type main, @function

main:

leal 4(%esp), %ecx

andl $-16, %esp

pushl -4(%ecx)

pushl %ebp

movl %esp, %ebp

pushl %ecx

subl $20, %esp

movl $printf, 4(%esp)

movl $.LC0, (%esp)

call printf

movl $0, %eax

addl $20, %esp

popl %ecx

popl %ebp

leal -4(%ecx), %esp

ret

.size main, .-main

.ident "GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)"

.section .note.GNU-stack,"",@progbits

3.3 汇编(assembly)

汇编阶段是把编译阶段生成的.s文件转成二进制目标代码。

gcc manual中对选项-C的解释如下。

`-c'

Compile or assemble the source files, but
do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix
`.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compilation or assembly, are ignored.

编译或汇编源文件，但不连接，编译器输出对应于源文件的目标文件。

缺省情况下，GCC通过用'.o'替换源文件名后缀`.c',
`.i', `.s'等等以产生目标文件名。可以使用-o选项指定选择其他名字。GCC忽略-c选项后面任何无法识别的不需要编译或汇编的输入文件。

gcc manual中对选项-V的解释如下。

`-v'

Print (on standard error output) the commands executed to run the stages of compilation.
Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

(在标准错误输出)显示执行编译阶段的命令，同时显示编译器驱动程序、预处理器、编译器的版本号。

格式：gcc –c hello.s –o hello.o

作用：将编译输出的.s文件汇编成二进制目标代码.o文件

我们可以通过objdump程序来查看hello.o的内容。

# objdump -d hello.o

hello.o: file format elf32-i386

Disassembly of section .text:

00000000 <main>:

0: 8d 4c 24 04 lea 0x4(%esp),%ecx

4: 83 e4 f0 and $0xfffffff0,%esp

7: ff 71 fc pushl -0x4(%ecx)

a: 55 push %ebp

b: 89 e5 mov %esp,%ebp

d: 51 push %ecx

e: 83 ec 14 sub $0x14,%esp

11: c7 44 24 04 00 00 00 movl $0x0,0x4(%esp)

18: 00

19: c7 04 24 00 00 00 00 movl $0x0,(%esp)

20: e8 fc ff ff ff call 21 <main+0x21>

25: b8 00 00 00 00 mov $0x0,%eax

2a: 83 c4 14 add $0x14,%esp

2d: 59 pop %ecx

2e: 5d pop %ebp

2f: 8d 61 fc lea -0x4(%ecx),%esp

32: c3 ret

3.4 链接(linking)

编译成功后，就进入链接。

格式：gcc hello.o -o hello.exe

或者gcc hello.o -o hello

作用：将编译输出的.o文件链接为最终可执行文件。

同样，我们也可以通过objdump程序来查看链接后的输出hello文件的内容。

hello: file format elf32-i386

Disassembly
of section .init:

08048298 <_init>:

8048298: 55 push %ebp

8048299: 89 e5 mov %esp,%ebp

804829b: 83 ec 08 sub $0x8,%esp

804829e: e8 71 00 00 00 call 8048314 <call_gmon_start>

80482a3: e8 f8 00 00 00 call 80483a0 <frame_dummy>

80482a8: e8 d3 01 00 00 call 8048480 <__do_global_ctors_aux>

80482ad: c9 leave

80482ae: c3 ret

Disassembly of section .plt:

080482b0 <__gmon_start__@plt-0x10>:

80482b0: ff 35 40 96 04 08 pushl 0x8049640

80482b6: ff 25 44 96 04 08 jmp *0x8049644

80482bc: 00 00 add %al,(%eax)

...

080482c0 <__gmon_start__@plt>:

80482c0: ff 25 48 96 04 08 jmp *0x8049648

80482c6: 68 00 00 00 00 push $0x0

80482cb: e9 e0 ff ff ff jmp 80482b0 <_init+0x18>

080482d0 <__libc_start_main@plt>:

80482d0: ff 25 4c 96 04 08 jmp *0x804964c

80482d6: 68 08 00 00 00 push $0x8

80482db: e9 d0 ff ff ff jmp 80482b0 <_init+0x18>

080482e0 <printf@plt>:

80482e0: ff 25 50 96 04 08 jmp *0x8049650

80482e6: 68 10 00 00 00 push $0x10

80482eb: e9 c0 ff ff ff jmp 80482b0 <_init+0x18>

...

080483c4 <main>:

80483c4: 8d 4c 24 04 lea 0x4(%esp),%ecx

80483c8: 83 e4 f0 and $0xfffffff0,%esp

80483cb: ff 71 fc pushl -0x4(%ecx)

80483ce: 55 push %ebp

80483cf: 89 e5 mov %esp,%ebp

80483d1: 51 push %ecx

80483d2: 83 ec 14 sub $0x14,%esp

80483d5: c7 44 24 04 e0 82 04 movl $0x80482e0,0x4(%esp)

80483dc: 08

80483dd: c7 04 24 d0 84 04 08 movl $0x80484d0,(%esp)

80483e4: e8 f7 fe ff ff call 80482e0 <printf@plt>

80483e9: b8 00 00 00 00 mov $0x0,%eax

80483ee: 83 c4 14 add $0x14,%esp

80483f1: 59 pop %ecx

80483f2: 5d pop %ebp

80483f3: 8d 61 fc lea -0x4(%ecx),%esp

80483f6: c3 ret

80483f7: 90 nop

...

4. 小结

编译程序的4个过程：

预处理：gcc
-E hello.c -o hello.i

编译：gcc
–S hello.i –o hello.s

汇编：gcc –c hello.s –o hello.o

链接：gcc hello.o -o hello