C程序编译过程

• 1.1程序被其他程序翻译成不同的格式

　　1.hello.c

#include <stdio.h>
 
int main()
{
     printf("hello world\n");
}

　　2.编译过程

　　3.编译系统
　　　　预处理器、编译器、汇编器和链接器一起构成了编译系统

　　预处理阶段。预处理器（cpp）根据以字符#开通的命令，修改原始的C程序。比如hello.c中第1行的#include<stdio.h>命令告诉预处理器读取系统头文件stdio.h的内容，并把它直接插入到程序的文本中。结果就得到了另一个C程序，通常是以.i作为文件扩展名

　　预处理的过程主要处理包括以下过程：

• 将所有的#define删除，并且展开所有的宏定义
• 处理所有的条件预编译指令，比如#if #ifdef #elif #else #endif等
• 处理#include 预编译指令，将被包含的文件插入到该预编译指令的位置。
• 删除所有注释 “//”和”/* */”.
• 添加行号和文件标识，以便编译时产生调试用的行号及编译错误警告行号。
• 保留所有的#pragma编译器指令，因为编译器需要使用它们

　　通常使用以下命令来进行预处理：

　　gcc -E hello.c -o hello.i

　　参数-E表示只进行预处理 或者也可以使用以下指令完成预处理过程

　　cpp hello.c > hello.i      /*  cpp – The C Preprocessor  */

 #  "hello.c"
 #  "<built-in>"
 #  "<command-line>"
 #  "hello.c"
 #  "/usr/include/stdio.h"
 #  "/usr/include/stdio.h"
 #  "/usr/include/features.h"
 #  "/usr/include/features.h"
 #  "/usr/include/x86_64-linux-gnu/bits/predefs.h"
 #  "/usr/include/features.h"
 #  "/usr/include/features.h"
 #  "/usr/include/x86_64-linux-gnu/sys/cdefs.h"
 #  "/usr/include/x86_64-linux-gnu/sys/cdefs.h"
 #  "/usr/include/x86_64-linux-gnu/bits/wordsize.h"
 #  "/usr/include/x86_64-linux-gnu/sys/cdefs.h"
 #  "/usr/include/features.h"
 #  "/usr/include/features.h"
 #  "/usr/include/x86_64-linux-gnu/gnu/stubs.h"   
 
 #  "/usr/include/x86_64-linux-gnu/bits/wordsize.h"
 #  "/usr/include/x86_64-linux-gnu/gnu/stubs.h"   
 
 #  "/usr/include/x86_64-linux-gnu/gnu/stubs-64.h"
 #  "/usr/include/x86_64-linux-gnu/gnu/stubs.h"
 #  "/usr/include/features.h"
 #  "/usr/include/stdio.h"   
 
 #  "/usr/lib/gcc/x86_64-linux-gnu/4.6/include/stddef.h"
 #  "/usr/lib/gcc/x86_64-linux-gnu/4.6/include/stddef.h"
 typedef long unsigned int size_t;
 #  "/usr/include/stdio.h"   
 
 #  "/usr/include/x86_64-linux-gnu/bits/types.h"
 #  "/usr/include/x86_64-linux-gnu/bits/types.h"
 #  "/usr/include/x86_64-linux-gnu/bits/wordsize.h"
 #  "/usr/include/x86_64-linux-gnu/bits/types.h"   
 
 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;
 
 typedef signed long int __int64_t;
 typedef unsigned long int __uint64_t;
 
 typedef long int __quad_t;
 typedef unsigned long int __u_quad_t;
 #  "/usr/include/x86_64-linux-gnu/bits/types.h"
 #  "/usr/include/x86_64-linux-gnu/bits/typesizes.h"
 #  "/usr/include/x86_64-linux-gnu/bits/types.h"   
 
 typedef unsigned long int __dev_t;
 typedef unsigned int __uid_t;
 typedef unsigned int __gid_t;
 typedef unsigned long int __ino_t;
 typedef unsigned long int __ino64_t;
 typedef unsigned int __mode_t;
 typedef unsigned long int __nlink_t;
 typedef long int __off_t;
 typedef long int __off64_t;
 typedef int __pid_t;
 typedef struct { int __val[]; } __fsid_t;
 typedef long int __clock_t;
 typedef unsigned long int __rlim_t;
 typedef unsigned long int __rlim64_t;
 typedef unsigned int __id_t;
 typedef long int __time_t;
 typedef unsigned int __useconds_t;
 typedef long int __suseconds_t;
 
 typedef int __daddr_t;
 typedef long int __swblk_t;
 typedef int __key_t;
 
 typedef int __clockid_t;
 
 typedef void * __timer_t;
 
 typedef long int __blksize_t;
 
 typedef long int __blkcnt_t;
 typedef long int __blkcnt64_t;
 
 typedef unsigned long int __fsblkcnt_t;
 typedef unsigned long int __fsblkcnt64_t;
 
 typedef unsigned long int __fsfilcnt_t;
 typedef unsigned long int __fsfilcnt64_t;
 
 typedef long int __ssize_t;
 
 typedef __off64_t __loff_t;
 typedef __quad_t *__qaddr_t;
 typedef char *__caddr_t;
 
 typedef long int __intptr_t;
 
 typedef unsigned int __socklen_t;
 #  "/usr/include/stdio.h"
 #  "/usr/include/stdio.h"
 struct _IO_FILE;
 
 typedef struct _IO_FILE FILE;
 
 #  "/usr/include/stdio.h"
 typedef struct _IO_FILE __FILE;
 #  "/usr/include/stdio.h"
 #  "/usr/include/libio.h"
 #  "/usr/include/libio.h"
 #  "/usr/include/_G_config.h"
 #  "/usr/include/_G_config.h"
 #  "/usr/lib/gcc/x86_64-linux-gnu/4.6/include/stddef.h"
 #  "/usr/include/_G_config.h"   
 
 #  "/usr/include/wchar.h"
 #  "/usr/include/wchar.h"
 typedef struct
 {
   int __count;
   union
   {
 
     unsigned int __wch;
 
     char __wchb[];
   } __value;
 } __mbstate_t;
 #  "/usr/include/_G_config.h"   
 
 typedef struct
 {
   __off_t __pos;
   __mbstate_t __state;
 } _G_fpos_t;
 typedef struct
 {
   __off64_t __pos;
   __mbstate_t __state;
 } _G_fpos64_t;
 #  "/usr/include/_G_config.h"
 typedef int _G_int16_t __attribute__ ((__mode__ (__HI__)));
 typedef int _G_int32_t __attribute__ ((__mode__ (__SI__)));
 typedef unsigned int _G_uint16_t __attribute__ ((__mode__ (__HI__)));
 typedef unsigned int _G_uint32_t __attribute__ ((__mode__ (__SI__)));
 #  "/usr/include/libio.h"
 #  "/usr/include/libio.h"
 #  "/usr/lib/gcc/x86_64-linux-gnu/4.6/include/stdarg.h"
 #  "/usr/lib/gcc/x86_64-linux-gnu/4.6/include/stdarg.h"
 typedef __builtin_va_list __gnuc_va_list;
 #  "/usr/include/libio.h"
 #  "/usr/include/libio.h"
 struct _IO_jump_t; struct _IO_FILE;
 #  "/usr/include/libio.h"
 typedef void _IO_lock_t;
 
 struct _IO_marker {
   struct _IO_marker *_next;
   struct _IO_FILE *_sbuf;
 
   int _pos;
 #  "/usr/include/libio.h"
 };
 
 enum __codecvt_result
 {
   __codecvt_ok,
   __codecvt_partial,
   __codecvt_error,
   __codecvt_noconv
 };
 #  "/usr/include/libio.h"
 struct _IO_FILE {
   int _flags;
 
   char* _IO_read_ptr;
   char* _IO_read_end;
   char* _IO_read_base;
   char* _IO_write_base;
   char* _IO_write_ptr;
   char* _IO_write_end;
   char* _IO_buf_base;
   char* _IO_buf_end;
 
   char *_IO_save_base;
   char *_IO_backup_base;
   char *_IO_save_end;
 
   struct _IO_marker *_markers;
 
   struct _IO_FILE *_chain;
 
   int _fileno;
 
   int _flags2;
 
   __off_t _old_offset;
 
   unsigned short _cur_column;
   signed char _vtable_offset;
   char _shortbuf[];
 
   _IO_lock_t *_lock;
 #  "/usr/include/libio.h"
   __off64_t _offset;
 #  "/usr/include/libio.h"
   void *__pad1;
   void *__pad2;
   void *__pad3;
   void *__pad4;
   size_t __pad5;
 
   int _mode;
 
   char _unused2[ * sizeof (int) -  * sizeof (void *) - sizeof (size_t)];
 
 };
 
 typedef struct _IO_FILE _IO_FILE;
 
 struct _IO_FILE_plus;
 
 extern struct _IO_FILE_plus _IO_2_1_stdin_;
 extern struct _IO_FILE_plus _IO_2_1_stdout_;
 extern struct _IO_FILE_plus _IO_2_1_stderr_;
 #  "/usr/include/libio.h"
 typedef __ssize_t __io_read_fn (void *__cookie, char *__buf, size_t __nbytes);
 
 typedef __ssize_t __io_write_fn (void *__cookie, __const char *__buf,
      size_t __n);
 
 typedef int __io_seek_fn (void *__cookie, __off64_t *__pos, int __w);
 
 typedef int __io_close_fn (void *__cookie);
 #  "/usr/include/libio.h"
 extern int __underflow (_IO_FILE *);
 extern int __uflow (_IO_FILE *);
 extern int __overflow (_IO_FILE *, int);
 #  "/usr/include/libio.h"
 extern int _IO_getc (_IO_FILE *__fp);
 extern int _IO_putc (int __c, _IO_FILE *__fp);
 extern int _IO_feof (_IO_FILE *__fp) __attribute__ ((__nothrow__ , __leaf__));
 extern int _IO_ferror (_IO_FILE *__fp) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int _IO_peekc_locked (_IO_FILE *__fp);
 
 extern void _IO_flockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
 extern void _IO_funlockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
 extern int _IO_ftrylockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
 #  "/usr/include/libio.h"
 extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict,
    __gnuc_va_list, int *__restrict);
 extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict,
     __gnuc_va_list);
 extern __ssize_t _IO_padn (_IO_FILE *, int, __ssize_t);
 extern size_t _IO_sgetn (_IO_FILE *, void *, size_t);
 
 extern __off64_t _IO_seekoff (_IO_FILE *, __off64_t, int, int);
 extern __off64_t _IO_seekpos (_IO_FILE *, __off64_t, int);
 
 extern void _IO_free_backup_area (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__));
 #  "/usr/include/stdio.h"   
 
 typedef __gnuc_va_list va_list;
 #  "/usr/include/stdio.h"
 typedef __off_t off_t;
 #  "/usr/include/stdio.h"
 typedef __ssize_t ssize_t;
 
 typedef _G_fpos_t fpos_t;
 
 #  "/usr/include/stdio.h"
 #  "/usr/include/x86_64-linux-gnu/bits/stdio_lim.h"
 #  "/usr/include/stdio.h"   
 
 extern struct _IO_FILE *stdin;
 extern struct _IO_FILE *stdout;
 extern struct _IO_FILE *stderr;
 
 extern int remove (__const char *__filename) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int rename (__const char *__old, __const char *__new) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int renameat (int __oldfd, __const char *__old, int __newfd,
        __const char *__new) __attribute__ ((__nothrow__ , __leaf__));
 
 extern FILE *tmpfile (void) ;
 #  "/usr/include/stdio.h"
 extern char *tmpnam (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;
 #  "/usr/include/stdio.h"
 extern char *tempnam (__const char *__dir, __const char *__pfx)
      __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) ;
 
 extern int fclose (FILE *__stream);
 
 extern int fflush (FILE *__stream);
 
 #  "/usr/include/stdio.h"
 extern int fflush_unlocked (FILE *__stream);
 #  "/usr/include/stdio.h"  
 
 extern FILE *fopen (__const char *__restrict __filename,
       __const char *__restrict __modes) ;
 
 extern FILE *freopen (__const char *__restrict __filename,
         __const char *__restrict __modes,
         FILE *__restrict __stream) ;
 #  "/usr/include/stdio.h"  
 
 #  "/usr/include/stdio.h"
 extern FILE *fdopen (int __fd, __const char *__modes) __attribute__ ((__nothrow__ , __leaf__)) ;
 #  "/usr/include/stdio.h"
 extern FILE *fmemopen (void *__s, size_t __len, __const char *__modes)
   __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
       int __modes, size_t __n) __attribute__ ((__nothrow__ , __leaf__));
 
 extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
          size_t __size) __attribute__ ((__nothrow__ , __leaf__));
 
 extern void setlinebuf (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int fprintf (FILE *__restrict __stream,
       __const char *__restrict __format, ...);
 
 extern int printf (__const char *__restrict __format, ...);
 
 extern int sprintf (char *__restrict __s,
       __const char *__restrict __format, ...) __attribute__ ((__nothrow__));
 
 extern int vfprintf (FILE *__restrict __s, __const char *__restrict __format,
        __gnuc_va_list __arg);
 
 extern int vprintf (__const char *__restrict __format, __gnuc_va_list __arg);
 
 extern int vsprintf (char *__restrict __s, __const char *__restrict __format,
        __gnuc_va_list __arg) __attribute__ ((__nothrow__));
 
 extern int snprintf (char *__restrict __s, size_t __maxlen,
        __const char *__restrict __format, ...)
      __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, , )));
 
 extern int vsnprintf (char *__restrict __s, size_t __maxlen,
         __const char *__restrict __format, __gnuc_va_list __arg)
      __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, , )));
 
 #  "/usr/include/stdio.h"
 extern int vdprintf (int __fd, __const char *__restrict __fmt,
        __gnuc_va_list __arg)
      __attribute__ ((__format__ (__printf__, , )));
 extern int dprintf (int __fd, __const char *__restrict __fmt, ...)
      __attribute__ ((__format__ (__printf__, , )));
 
 extern int fscanf (FILE *__restrict __stream,
      __const char *__restrict __format, ...) ;
 
 extern int scanf (__const char *__restrict __format, ...) ;
 
 extern int sscanf (__const char *__restrict __s,
      __const char *__restrict __format, ...) __attribute__ ((__nothrow__ , __leaf__));
 #  "/usr/include/stdio.h"
 extern int fscanf (FILE *__restrict __stream, __const char *__restrict __format, ...) __asm__ ("" "__isoc99_fscanf")
 
                                ;
 extern int scanf (__const char *__restrict __format, ...) __asm__ ("" "__isoc99_scanf")
                               ;
 extern int sscanf (__const char *__restrict __s, __const char *__restrict __format, ...) __asm__ ("" "__isoc99_sscanf") __attribute__ ((__nothrow__ , __leaf__))
 
                       ;
 #  "/usr/include/stdio.h"  
 
 extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format,
       __gnuc_va_list __arg)
      __attribute__ ((__format__ (__scanf__, , ))) ;
 
 extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg)
      __attribute__ ((__format__ (__scanf__, , ))) ;
 
 extern int vsscanf (__const char *__restrict __s,
       __const char *__restrict __format, __gnuc_va_list __arg)
      __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__format__ (__scanf__, , )));
 #  "/usr/include/stdio.h"
 extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vfscanf")
 
      __attribute__ ((__format__ (__scanf__, , ))) ;
 extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")
 
      __attribute__ ((__format__ (__scanf__, , ))) ;
 extern int vsscanf (__const char *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vsscanf") __attribute__ ((__nothrow__ , __leaf__))
 
      __attribute__ ((__format__ (__scanf__, , )));
 #  "/usr/include/stdio.h"  
 
 extern int fgetc (FILE *__stream);
 extern int getc (FILE *__stream);
 
 extern int getchar (void);
 
 #  "/usr/include/stdio.h"
 extern int getc_unlocked (FILE *__stream);
 extern int getchar_unlocked (void);
 #  "/usr/include/stdio.h"
 extern int fgetc_unlocked (FILE *__stream);
 
 extern int fputc (int __c, FILE *__stream);
 extern int putc (int __c, FILE *__stream);
 
 extern int putchar (int __c);
 
 #  "/usr/include/stdio.h"
 extern int fputc_unlocked (int __c, FILE *__stream);
 
 extern int putc_unlocked (int __c, FILE *__stream);
 extern int putchar_unlocked (int __c);
 
 extern int getw (FILE *__stream);
 
 extern int putw (int __w, FILE *__stream);
 
 extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
      ;
 
 extern char *gets (char *__s) ;
 
 #  "/usr/include/stdio.h"
 extern __ssize_t __getdelim (char **__restrict __lineptr,
           size_t *__restrict __n, int __delimiter,
           FILE *__restrict __stream) ;
 extern __ssize_t getdelim (char **__restrict __lineptr,
         size_t *__restrict __n, int __delimiter,
         FILE *__restrict __stream) ;
 
 extern __ssize_t getline (char **__restrict __lineptr,
        size_t *__restrict __n,
        FILE *__restrict __stream) ;
 
 extern int fputs (__const char *__restrict __s, FILE *__restrict __stream);
 
 extern int puts (__const char *__s);
 
 extern int ungetc (int __c, FILE *__stream);
 
 extern size_t fread (void *__restrict __ptr, size_t __size,
        size_t __n, FILE *__restrict __stream) ;
 
 extern size_t fwrite (__const void *__restrict __ptr, size_t __size,
         size_t __n, FILE *__restrict __s);
 
 #  "/usr/include/stdio.h"
 extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,
          size_t __n, FILE *__restrict __stream) ;
 extern size_t fwrite_unlocked (__const void *__restrict __ptr, size_t __size,
           size_t __n, FILE *__restrict __stream);
 
 extern int fseek (FILE *__stream, long int __off, int __whence);
 
 extern long int ftell (FILE *__stream) ;
 
 extern void rewind (FILE *__stream);
 
 #  "/usr/include/stdio.h"
 extern int fseeko (FILE *__stream, __off_t __off, int __whence);
 
 extern __off_t ftello (FILE *__stream) ;
 #  "/usr/include/stdio.h"  
 
 extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);
 
 extern int fsetpos (FILE *__stream, __const fpos_t *__pos);
 #  "/usr/include/stdio.h"  
 
 #  "/usr/include/stdio.h"  
 
 extern void clearerr (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int feof (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern int ferror (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
 extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern void perror (__const char *__s);
 
 #  "/usr/include/x86_64-linux-gnu/bits/sys_errlist.h"
 #  "/usr/include/x86_64-linux-gnu/bits/sys_errlist.h"
 extern int sys_nerr;
 extern __const char *__const sys_errlist[];
 #  "/usr/include/stdio.h"   
 
 extern int fileno (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 #  "/usr/include/stdio.h"
 extern FILE *popen (__const char *__command, __const char *__modes) ;
 
 extern int pclose (FILE *__stream);
 
 extern char *ctermid (char *__s) __attribute__ ((__nothrow__ , __leaf__));
 #  "/usr/include/stdio.h"
 extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
 
 extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
 
 extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
 #  "/usr/include/stdio.h"  
 
 #  "hello.c" 
 
 int main()
 {
  printf("hello world!\n");
 }

hello.i

　　编译阶段。编译器（ccl）将文本文件hello.i翻译成文本文件hello.s，它包含一个汇编语言程序。汇编语言程序中的每条语句都以一种标准的文本格式确切地描述了一条低级机器语言指令。汇编语言是非常有用的，因为他为不同高级语言的不同编译器提供了通用的输出语言。例如，C编译器和Fortran编译器产生的输出文件用的都是一样的汇编语言。

　　编译过程就是把预处理完的文件进行一系列的词法分析，语法分析，语义分析及优化后生成相应的汇编代码。

　　gcc –S hello.i –o hello.s

     .file    "hello.c"
     .section    .rodata
 .LC0:
     .string    "hello world!"
     .text
     .globl    main
     .type    main, @function
 main:
 .LFB0:
     .cfi_startproc
     pushq    %rbp
     .cfi_def_cfa_offset
     .cfi_offset , -
     movq    %rsp, %rbp
     .cfi_def_cfa_register
     movl    $.LC0, %edi
     call    puts
     popq    %rbp
     .cfi_def_cfa ,
     ret
     .cfi_endproc
 .LFE0:
     .size    main, .-main
     .ident    "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
     .section    .note.GNU-stack,"",@progbits

hello.s

　　汇编阶段。汇编器（as）将hello.s翻译成机器语言指令，并把这些指令打包成一种叫做可充定位目标程序（relocatable object program）的格式，并将结果保存在目标文件hello.o中，hello.o文件是一个二进制文件，它的字节编码是机器语言指令而不是字符。如果我们在文本编辑器中打开hello.o文件，看到的将是一堆乱码。

　　汇编器是将汇编代码转变成机器可以执行的命令，每一个汇编语句几乎都对应一条机器指令。汇编相对于编译过程比较简单，根据汇编指令和机器指令的对照表一一翻译即可。

　　$ gcc –c hello.c –o hello.o

　　或者

　　$ as hello.s –o hello.co

　　由于hello.o的内容为机器码，不能以普通文本形式的查看

　　连接阶段。请注意，hello程序调用了printf函数，他是每个C编译器都会提供的标准C库中的一个函数。printf函数存在于一个名为printf.o程序中。链接器（ld）就负责处理这种合并。结果就得到hello文件，他是一个可执行目标文件（或者简称可执行文件），可以被加载到内存，由系统执行。

　　通过调用链接器ld来链接程序运行需要的一大堆目标文件，以及所依赖的其它库文件，最后生成可执行文件。

　　

　　参考：

　　书籍：《深入理解计算机系统》

　　博客：http://blog.csdn.net/koudaidai/article/details/8092647