gcc程序编译的过程

一个C/C++文件要经过预处理(preprocessing)、编译(compilation)、汇编(assembly)和链接(linking)等4步才能变成可执行文件,通常使用“编译”统称这4个步骤。

  • 预处理(preprocessing)
    C/C++源文件中,以“#”开头的命令被称为预处理命令,如包含命令“#include”、宏定义命令“#define”、条件编译命令“#if”、“#ifdef”等。预处理就是将要包含(include)的文件插入原文件中、将宏定义展开、根据条件编译命令选择要使用的代码,最后将这些代码输出到一个“.i”文件中等待进一步处理。

  • 编译(compilation)
    编译就是把C/C++代码(比如上述的“.i”文件)翻译成汇编代码。

  • 汇编(assembly)
    汇编就是将第二步输出的汇编代码翻译成符合一定格式的机器代码,在linux系统上一般表现为ELF目标文件(OBJ文件)。“反汇编”是指将机器代码转换为汇编代码。

  • 链接(linking)
    链接就是将上步生成的OBJ文件和系统库的OBJ文件、库文件链接起来,最终生成可以在特定平台运行的可执行文件。

编译器利用这4个步骤中的一个或多个来处理输入文件,源文件的后缀名表示源文件所用的语言,后缀名控制着编译器的默认动作。
文件后缀名对应表:

后缀名 类型
.c c源程序
.h 预处理器文件
.cpp c++源程序
.i 预处理后的c文件
.ii 预处理后的c++文件
.s 汇编语言源程序
.o 目标文件(Object file)
.a 静态链接库文件(linux)
.so 动态链接库文件(linux)
.lib 静态链接库文件(windows)
.dll 动态链接库文件(windows)

gcc的使用方法:

gcc --help

Usage: gcc [options] file...
Options:
  -pass-exit-codes         Exit with highest error code from a phase.
  --help                   Display this information.
  --target-help            Display target specific command line options.
  --help={common|optimizers|params|target|warnings|[^]{joined|separate|undocumented}}[,...].
                           Display specific types of command line options.
  (Use '-v --help' to display command line options of sub-processes).
  --version                Display compiler version information.
  -dumpspecs               Display all of the built in spec strings.
  -dumpversion             Display the version of the compiler.
  -dumpmachine             Display the compiler's target processor.
  -print-search-dirs       Display the directories in the compiler's search path.
  -print-libgcc-file-name  Display the name of the compiler's companion library.
  -print-file-name=   Display the full path to library .
  -print-prog-name=  Display the full path to compiler component .
  -print-multiarch         Display the target's normalized GNU triplet, used as
                           a component in the library path.
  -print-multi-directory   Display the root directory for versions of libgcc.
  -print-multi-lib         Display the mapping between command line options and
                           multiple library search directories.
  -print-multi-os-directory Display the relative path to OS libraries.
  -print-sysroot           Display the target libraries directory.
  -print-sysroot-headers-suffix Display the sysroot suffix used to find headers.
  -Wa,            Pass comma-separated  on to the assembler.
  -Wp,            Pass comma-separated  on to the preprocessor.
  -Wl,            Pass comma-separated  on to the linker.
  -Xassembler         Pass  on to the assembler.
  -Xpreprocessor      Pass  on to the preprocessor.
  -Xlinker            Pass  on to the linker.
  -save-temps              Do not delete intermediate files.
  -save-temps=        Do not delete intermediate files.
  -no-canonical-prefixes   Do not canonicalize paths when building relative
                           prefixes to other gcc components.
  -pipe                    Use pipes rather than intermediate files.
  -time                    Time the execution of each subprocess.
  -specs=            Override built-in specs with the contents of .
  -std=          Assume that the input sources are for .
  --sysroot=    Use  as the root directory for headers
                           and libraries.
  -B            Add  to the compiler's search paths.
  -v                       Display the programs invoked by the compiler.
  -###                     Like -v but options quoted and commands not executed.
  -E                       Preprocess only; do not compile, assemble or link.
  -S                       Compile only; do not assemble or link.
  -c                       Compile and assemble, but do not link.
  -o                 Place the output into .
  -pie                     Create a position independent executable.
  -shared                  Create a shared library.
  -x             Specify the language of the following input files.
                           Permissible languages include: c c++ assembler none
                           'none' means revert to the default behavior of
                           guessing the language based on the file's extension.

Options starting with -g, -f, -m, -O, -W, or --param are automatically
 passed on to the various sub-processes invoked by gcc.  In order to pass
 other options on to these processes the -W options must be used.

For bug reporting instructions, please see:
.

常用选项:

选项 含义
-v 查看gcc编译器的版本,显示gcc执行时的详细过程
-E 只预处理,不编译、汇编、链接
-S 只编译,不汇编、链接
-c 编译和汇编,不链接
-o 指定输出文件名为file
-static 进行静态编译,即链接静态库,禁止使用动态库
-shared 1.可以生成动态库文件 2.进行动态编译,尽可能的链接动态库,只有没有动态库时才会链接同名的静态库(默认选项,可省略)
-Ldir 在库文件的搜索路径列表中添加dir目录
-lname 链接称为libname.a(静态库)或者libname.so(动态库)的库文件。若两个库都在,则根据编译方式(-static还是-shared)而进行链接。
-fPIC 生成使用相对地址的位置无关的目标代码(Position Independent Code)。

以一个实例来分析gcc程序编译的过程(helloworld.c):

#include 

#define TRUE 1
#define FALSE 0 

#define DEBUG_ENABLE

int main(void){
    int i = 0;
    if(i == TRUE){
        printf("hello\n");
    }else{
#ifdef DEBUG_ENABLE
        printf("i = %d\n",i);
#endif
        printf("hello world\n");
    }
    return 0;
}
1.预处理
gcc -E -o helloworld.i helloworld.c

打开helloworld.i文件(用sublime打开),
可以看到include的文件已插入原文件中,宏定义展开、条件编译命令已选择好代码:

434  __attribute__((__cdecl__)) __attribute__((__nothrow__)) int putw (int, FILE *);
435
436
437
438
439 
440 # 2 "helloworld.c" 2
441
442
443
444
445
446
447
448 # 8 "helloworld.c"
449 int main(void){
450 int i = 0;
451 if(i == 1){
452  printf("hello\n");
453 }else{
454
455  printf("i = %d\n",i);
456
457  printf("hello world\n");
458 }
459 return 0;
460 }
461
2.编译
gcc -S -o helloworld.s helloworld.i

编译生成的汇编代码内容如下(用sublime打开):

    .file   "helloworld.c"
    .def    ___main;    .scl    2;  .type   32; .endef
    .section .rdata,"dr"
LC0:
    .ascii "hello\0"
LC1:
    .ascii "i = %d\12\0"
LC2:
    .ascii "hello world\0"
    .text
    .globl  _main
    .def    _main;  .scl    2;  .type   32; .endef
_main:
LFB10:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $32, %esp
    call    ___main
    movl    $0, 28(%esp)
    cmpl    $1, 28(%esp)
    jne L2
    movl    $LC0, (%esp)
    call    _puts
    jmp L3
L2:
    movl    28(%esp), %eax
    movl    %eax, 4(%esp)
    movl    $LC1, (%esp)
    call    _printf
    movl    $LC2, (%esp)
    call    _puts
L3:
    movl    $0, %eax
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
LFE10:
    .ident  "GCC: (MinGW.org GCC-6.3.0-1) 6.3.0"
    .def    _puts;  .scl    2;  .type   32; .endef
    .def    _printf;    .scl    2;  .type   32; .endef
3.汇编
gcc -c -o helloworld.o helloworld.s

.o文件打开内容如下(用winhex打开):


gcc程序编译的过程_第1张图片
捕获.JPG
4.链接
gcc -o helloworld helloworld.o

最终生成helloworld.exe文件,执行(使用的是Notepad++里的控制台):

helloworld
helloworld
Process started (PID=15044) >>>
i = 0
hello world
<<< Process finished (PID=15044). (Exit code 0)

在编译过程中,除非使用了"-E"、"-S"、"-c"选项,或者编译器错误阻止了完整的过程,否则最后步骤总是链接。
例如:

gcc  helloworld.c
gcc -o helloworld helloworld.c

都是已经完成链接操作。

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