程序员的自我修养---读书笔记

二.

1.预处理（preprocessing）->编译（compilation）->汇编（assembly）->链接（linking）

2.预编译 gcc -E hello.c -o hello.i

   Hello.i中的内容：

 

   # 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

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

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

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

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

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

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

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

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

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

# 378 "/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

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

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











# 1 "/usr/lib/gcc/i486-linux-gnu/4.4.3/include/stddef.h" 1 3 4

# 211 "/usr/lib/gcc/i486-linux-gnu/4.4.3/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





__extension__ typedef __u_quad_t __dev_t;

__extension__ typedef unsigned int __uid_t;

__extension__ typedef unsigned int __gid_t;

__extension__ typedef unsigned long int __ino_t;

__extension__ typedef __u_quad_t __ino64_t;

__extension__ typedef unsigned int __mode_t;

__extension__ typedef unsigned int __nlink_t;

__extension__ typedef long int __off_t;

__extension__ typedef __quad_t __off64_t;

__extension__ typedef int __pid_t;

__extension__ typedef struct { int __val[2]; } __fsid_t;

__extension__ typedef long int __clock_t;

__extension__ typedef unsigned long int __rlim_t;

__extension__ typedef __u_quad_t __rlim64_t;

__extension__ typedef unsigned int __id_t;

__extension__ typedef long int __time_t;

__extension__ typedef unsigned int __useconds_t;

__extension__ typedef long int __suseconds_t;



__extension__ typedef int __daddr_t;

__extension__ typedef long int __swblk_t;

__extension__ typedef int __key_t;





__extension__ typedef int __clockid_t;





__extension__ typedef void * __timer_t;





__extension__ typedef long int __blksize_t;









__extension__ typedef long int __blkcnt_t;

__extension__ typedef __quad_t __blkcnt64_t;





__extension__ typedef unsigned long int __fsblkcnt_t;

__extension__ typedef __u_quad_t __fsblkcnt64_t;





__extension__ typedef unsigned long int __fsfilcnt_t;

__extension__ typedef __u_quad_t __fsfilcnt64_t;



__extension__ typedef int __ssize_t;







typedef __off64_t __loff_t;

typedef __quad_t *__qaddr_t;

typedef char *__caddr_t;





__extension__ typedef int __intptr_t;





__extension__ typedef unsigned int __socklen_t;

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

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

struct _IO_FILE;







typedef struct _IO_FILE FILE;











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

typedef struct _IO_FILE __FILE;

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

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

# 32 "/usr/include/libio.h" 3 4

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

# 15 "/usr/include/_G_config.h" 3 4

# 1 "/usr/lib/gcc/i486-linux-gnu/4.4.3/include/stddef.h" 1 3 4

# 16 "/usr/include/_G_config.h" 2 3 4









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

# 83 "/usr/include/wchar.h" 3 4

typedef struct

{

  int __count;

  union

  {



    unsigned int __wch;







    char __wchb[4];

  } __value;

} __mbstate_t;

# 21 "/usr/include/_G_config.h" 2 3 4



typedef struct

{

  __off_t __pos;

  __mbstate_t __state;

} _G_fpos_t;

typedef struct

{

  __off64_t __pos;

  __mbstate_t __state;

} _G_fpos64_t;

# 53 "/usr/include/_G_config.h" 3 4

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__)));

# 33 "/usr/include/libio.h" 2 3 4

# 53 "/usr/include/libio.h" 3 4

# 1 "/usr/lib/gcc/i486-linux-gnu/4.4.3/include/stdarg.h" 1 3 4

# 40 "/usr/lib/gcc/i486-linux-gnu/4.4.3/include/stdarg.h" 3 4

typedef __builtin_va_list __gnuc_va_list;

# 54 "/usr/include/libio.h" 2 3 4

# 170 "/usr/include/libio.h" 3 4

struct _IO_jump_t; struct _IO_FILE;

# 180 "/usr/include/libio.h" 3 4

typedef void _IO_lock_t;











struct _IO_marker {

  struct _IO_marker *_next;

  struct _IO_FILE *_sbuf;







  int _pos;

# 203 "/usr/include/libio.h" 3 4

};





enum __codecvt_result

{

  __codecvt_ok,

  __codecvt_partial,

  __codecvt_error,

  __codecvt_noconv

};

# 271 "/usr/include/libio.h" 3 4

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[1];







  _IO_lock_t *_lock;

# 319 "/usr/include/libio.h" 3 4

  __off64_t _offset;

# 328 "/usr/include/libio.h" 3 4

  void *__pad1;

  void *__pad2;

  void *__pad3;

  void *__pad4;

  size_t __pad5;



  int _mode;



  char _unused2[15 * sizeof (int) - 4 * 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_;

# 364 "/usr/include/libio.h" 3 4

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);

# 416 "/usr/include/libio.h" 3 4

extern int __underflow (_IO_FILE *);

extern int __uflow (_IO_FILE *);

extern int __overflow (_IO_FILE *, int);

# 460 "/usr/include/libio.h" 3 4

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__));

extern int _IO_ferror (_IO_FILE *__fp) __attribute__ ((__nothrow__));



extern int _IO_peekc_locked (_IO_FILE *__fp);











extern void _IO_flockfile (_IO_FILE *) __attribute__ ((__nothrow__));

extern void _IO_funlockfile (_IO_FILE *) __attribute__ ((__nothrow__));

extern int _IO_ftrylockfile (_IO_FILE *) __attribute__ ((__nothrow__));

# 490 "/usr/include/libio.h" 3 4

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__));

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

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





typedef _G_fpos_t fpos_t;









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

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

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







extern struct _IO_FILE *stdin;

extern struct _IO_FILE *stdout;

extern struct _IO_FILE *stderr;















extern int remove (__const char *__filename) __attribute__ ((__nothrow__));



extern int rename (__const char *__old, __const char *__new) __attribute__ ((__nothrow__));









extern int renameat (int __oldfd, __const char *__old, int __newfd,

       __const char *__new) __attribute__ ((__nothrow__));

















extern FILE *tmpfile (void) ;

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

extern char *tmpnam (char *__s) __attribute__ ((__nothrow__)) ;











extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__)) ;

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

extern char *tempnam (__const char *__dir, __const char *__pfx)

     __attribute__ ((__nothrow__)) __attribute__ ((__malloc__)) ;

















extern int fclose (FILE *__stream);









extern int fflush (FILE *__stream);



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

extern int fflush_unlocked (FILE *__stream);

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













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) ;

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



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

extern FILE *fdopen (int __fd, __const char *__modes) __attribute__ ((__nothrow__)) ;

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

extern FILE *fmemopen (void *__s, size_t __len, __const char *__modes)

  __attribute__ ((__nothrow__)) ;









extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__)) ;













extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __attribute__ ((__nothrow__));







extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,

      int __modes, size_t __n) __attribute__ ((__nothrow__));











extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,

         size_t __size) __attribute__ ((__nothrow__));





extern void setlinebuf (FILE *__stream) __attribute__ ((__nothrow__));

















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__, 3, 4)));



extern int vsnprintf (char *__restrict __s, size_t __maxlen,

        __const char *__restrict __format, __gnuc_va_list __arg)

     __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 0)));



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

extern int vdprintf (int __fd, __const char *__restrict __fmt,

       __gnuc_va_list __arg)

     __attribute__ ((__format__ (__printf__, 2, 0)));

extern int dprintf (int __fd, __const char *__restrict __fmt, ...)

     __attribute__ ((__format__ (__printf__, 2, 3)));

















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__));

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

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__));

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

















extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format,

      __gnuc_va_list __arg)

     __attribute__ ((__format__ (__scanf__, 2, 0))) ;











extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg)

     __attribute__ ((__format__ (__scanf__, 1, 0))) ;





extern int vsscanf (__const char *__restrict __s,

      __const char *__restrict __format, __gnuc_va_list __arg)

     __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__scanf__, 2, 0)));

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

extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vfscanf")







     __attribute__ ((__format__ (__scanf__, 2, 0))) ;

extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")



     __attribute__ ((__format__ (__scanf__, 1, 0))) ;

extern int vsscanf (__const char *__restrict __s, __const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vsscanf")







     __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__scanf__, 2, 0)));

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



















extern int fgetc (FILE *__stream);

extern int getc (FILE *__stream);











extern int getchar (void);



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

extern int getc_unlocked (FILE *__stream);

extern int getchar_unlocked (void);

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

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);



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

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) ;



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

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);



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

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);



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

extern int fseeko (FILE *__stream, __off_t __off, int __whence);









extern __off_t ftello (FILE *__stream) ;

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













extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);









extern int fsetpos (FILE *__stream, __const fpos_t *__pos);

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



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





extern void clearerr (FILE *__stream) __attribute__ ((__nothrow__));



extern int feof (FILE *__stream) __attribute__ ((__nothrow__)) ;



extern int ferror (FILE *__stream) __attribute__ ((__nothrow__)) ;









extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__));

extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

















extern void perror (__const char *__s);













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

# 27 "/usr/include/bits/sys_errlist.h" 3 4

extern int sys_nerr;

extern __const char *__const sys_errlist[];

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









extern int fileno (FILE *__stream) __attribute__ ((__nothrow__)) ;









extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__)) ;

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

extern FILE *popen (__const char *__command, __const char *__modes) ;











extern int pclose (FILE *__stream);











extern char *ctermid (char *__s) __attribute__ ((__nothrow__));

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

extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__));







extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__)) ;





extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__));

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



# 2 "Hello.c" 2

int main (int argc, char **argv)

{

printf("Hello World!\n");

return 0;

}

3.编译    gcc -S hello.i -o hello.s

   Hello.s 中的内容：

   .file "Hello.c"

   .section .rodata

.LC0:

   .string "Hello World!"

   .text

.globl main

   .type main, @function

main:

   pushl %ebp

   movl  %esp, %ebp

   andl  $-16, %esp

   subl  $16, %esp

   movl  $.LC0, (%esp)

   call  puts

   movl  $0, %eax

   leave

   ret

   .size main, .-main

   .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"

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



4.汇编

   gcc -c hello.s -o hello.o

或    as hello.s -o hello.o



5.链接

   编译器：扫描->语法分析->语义分析->源代码优化->代码生成->目标代码优化



*【有限状态机】算法

* lex 词法分析器

语法分析器（Grammar Parser） ->产生语法树（Syntax Tree）就是以表达式为节点的树

*上下文无关语法（Context-free Grammar）





语法分析工具：yacc（Yet Another Compiler Compiler）



语义分析：

语义分析器（Semantic Analyzer）



静态语义

动态语义







中间语言生成：

源代码优化器（Source Code Optimizer）





中间代码可以将编译器分为 前端和 后端：



模块之间的通信：（1）函数调用（2）变量的访问

两者都可以归结为 模块间符号的引用



2.4 静态链接

ELF文件类型：可重定位文件，可执行文件，共享目标文件，核心转储文件

Relocatable File

Executable File

Shared Object File

Core Dump File



SimpleSection.c



int printf(const char* format, ...);

int global_init_var = 84;

int global_uninit_var;



void func1( int i )

{

   printf("%d\n", i);

}



int main(void)

{

   static int static_var = 85;

   static int static_var2;



   int a = 1;

   int b;



   func1( static_var + static_var2 + a + b);



   return a;

}



gcc -c SimpleSection.c



objdump -h SimpleSection.o 用来查看object内部的结构



* readelf 是专门针对ELF格式文件的解析器

* size命令可以用来查看ELF文件的代码段、数据段和BSS段的长度



ELF文件格式结构：

***

ELF Header

***

.text

***

.data

***

.bss

***

...other sections

***

Section header table

***

String Tables

Symbol Tables

....



整个链接过程正是基于符号来完成的



* nm 命令 可以用来查看*.o中的符号

$ nm SimpleSection.o





*函数签名（Function Signature）



*c++filt 可以用来解析被修饰过的名称

$ c++filt _ZN1N1C4funcEi





#ifdef __cplusplus

extern “C”{

#endif



void *memset (void *, int, size_t);

#ifdef __cplusplus

}

#endif



*弱符号 弱引用 强符号 强引用



如果一个程序被设计成可以支持单线程或多线程的模式，就可以通过弱引用的方法来判断当前的程序是链接到了单线程的Glibc库还是多线程的Glibc库（是否在编译时有-lpthread选项）



#include <stdio.h>

#include <pthread.h>



int pthread_create(

      pthread_t *,

      const pthread_attr_t*,

      void * (*)(void*),

      void *)

__attribute__ ((weak));



int main(){

   if(pthread_create){

      printf("This is multi-thread version\n");

      // run the multi-thread version

      // main_multi_thread()

   }else{

      printf("This is single-thread version\n");

   }



}



$ gcc pthread.c -o pt

$ gcc pthread.c -lpthread -o pt





*在linux下我们可以用 strip 命令去掉调试信息！



四.静态链接

4.1 空间与地址分配





4.2符号解析与重定位

* objdump -d a.o 可以查看a.o的代码反汇编结果

* objdump -r a.o 可以查看a.o的重定位表



4.3COMMON块

4.4C++相关问题

*函数级别链接

4.5静态库链接

* ar -t libc.a  ar工具查看静态库



e.g:



$ gcc -c -fno-builtin hello.c

//默认情况GCC会自作聪明的将”printf”替换成“puts”以提高运行速度

$ ar -x libc.a

//将libc.a 中的所有目标文件解压到当前目录

$ld hello.o printf.o

//链接却失败了！！！！！原因是缺少2个外部符号的定义





$ gcc  -static  --verbose -fno-builtin hello.c



--verbose 可以将编译链接的中间所有过程打印出来



cc1 GCC的c语言编译器

as GNU的汇编器

collect2 ld连接器的一个包装



4.6链接库控制



*.c



char * str = "Hello World!\n";



void print()

{

   asm( "movl $13, %%edx \n\t"

        "movl %0, %%ecx \n\t"

        "movl $0, %%ebx \n\t"

        "movl $4, %%eax \n\t"

        "int $0x80 \n\t"

        ::"r"(str):"edx","ecx","ebx");

}

void exit()

{

   asm("movl $42, %ebx \n\t"

        "movl $1, %eax \n\t"

        "int $0x80 \n\t");

}



void nomain()

{

   print();

   exit();

}



*.lds



ENTRY(nomain)

   SECTIONS

{

   . = 0x08048000 + SIZEOF_HEADERS;

tinytext : { *(.text) *(.data) *(.rodata)}

         /DISCARD/ : {*(.comment)}

}



gcc -c -fno-builtin *.c



ld -static -T *.lds -o *.c *.o







4.7BFD库

本章小结



六.可执行文件的装载和执行



*页映射

6.3.1 进程的建立

（1）创建虚拟地址空间

（2）读取可执行文件头，并且建立虚拟空间与可执行文件的映射关系

*虚拟内存区域 （VMA Virtual Memory Area）

（3）将CPU指令寄存器设置成可执行文件的入口，启动执行



段的权限：

（1）以代码段为代表的权限为可读可执行的段

（2）以数据段和BSS段为代表的权限为可读可写的段

（3）以只读数据段为代表的权限为只读的段



对于权限相同的段，我们可以合并映射（可以减少页面的碎片，节省内存空间）



从section角度看ELF文件就是链接视图（Linking View）

从segment的角度看ELF文件就是执行视图（Execution View）



fly@Baby-o_o:~/Code/HelloWorld$ readelf -S SectionMapping.elf

There are 27 section headers, starting at offset 0x7d84c:



Section Headers:

  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al

  [ 0]                   NULL            00000000 000000 000000 00      0   0  0

  [ 1] .note.ABI-tag     NOTE            080480f4 0000f4 000020 00   A  0   0  4

  [ 2] .note.gnu.build-i NOTE            08048114 000114 000024 00   A  0   0  4

  [ 3] .init             PROGBITS        08048138 000138 000030 00  AX  0   0  4

  [ 4] .text             PROGBITS        08048170 000170 05eb3c 00  AX  0   0 16

  [ 5] __libc_freeres_fn PROGBITS        080a6cb0 05ecb0 000b57 00  AX  0   0 16

  [ 6] .fini             PROGBITS        080a7808 05f808 00001c 00  AX  0   0  4

  [ 7] .rodata           PROGBITS        080a7840 05f840 018650 00   A  0   0 32

  [ 8] __libc_subfreeres PROGBITS        080bfe90 077e90 000030 00   A  0   0  4

  [ 9] __libc_atexit     PROGBITS        080bfec0 077ec0 000004 00   A  0   0  4

  [10] .eh_frame         PROGBITS        080bfec4 077ec4 004dc0 00   A  0   0  4

  [11] .gcc_except_table PROGBITS        080c4c84 07cc84 00011a 00   A  0   0  1

  [12] .tdata            PROGBITS        080c5f98 07cf98 000010 00 WAT  0   0  4

  [13] .tbss             NOBITS          080c5fa8 07cfa8 000018 00 WAT  0   0  4

  [14] .ctors            PROGBITS        080c5fa8 07cfa8 000008 00  WA  0   0  4

  [15] .dtors            PROGBITS        080c5fb0 07cfb0 00000c 00  WA  0   0  4

  [16] .jcr              PROGBITS        080c5fbc 07cfbc 000004 00  WA  0   0  4

  [17] .data.rel.ro      PROGBITS        080c5fc0 07cfc0 00002c 00  WA  0   0  4

  [18] .got              PROGBITS        080c5fec 07cfec 000008 04  WA  0   0  4

  [19] .got.plt          PROGBITS        080c5ff4 07cff4 00000c 04  WA  0   0  4

  [20] .data             PROGBITS        080c6000 07d000 000720 00  WA  0   0 32

  [21] .bss              NOBITS          080c6720 07d720 001b5c 00  WA  0   0 32

  [22] __libc_freeres_pt NOBITS          080c827c 07d720 000018 00  WA  0   0  4

  [23] .comment          PROGBITS        00000000 07d720 000023 01  MS  0   0  1

  [24] .shstrtab         STRTAB          00000000 07d743 000107 00      0   0  1

  [25] .symtab           SYMTAB          00000000 07dc84 008200 10     26 967  4

  [26] .strtab           STRTAB          00000000 085e84 007397 00      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)



fly@Baby-o_o:~/Code/HelloWorld$ readelf -l SectionMapping.elf



Elf file type is EXEC (Executable file)

Entry point 0x8048170

There are 6 program headers, starting at offset 52



Program Headers:

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align

  LOAD           0x000000 0x08048000 0x08048000 0x7cd9e 0x7cd9e R E 0x1000

  LOAD           0x07cf98 0x080c5f98 0x080c5f98 0x00788 0x022fc RW  0x1000

  NOTE           0x0000f4 0x080480f4 0x080480f4 0x00044 0x00044 R   0x4

  TLS            0x07cf98 0x080c5f98 0x080c5f98 0x00010 0x00028 R   0x4

  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4

  GNU_RELRO      0x07cf98 0x080c5f98 0x080c5f98 0x00068 0x00068 R   0x1



Section to Segment mapping:

  Segment Sections...

   00     .note.ABI-tag .note.gnu.build-id .init .text __libc_freeres_fn .fini .rodata __libc_subfreeres __libc_atexit .eh_frame .gcc_except_table

   01     .tdata .ctors .dtors .jcr .data.rel.ro .got .got.plt .data .bss __libc_freeres_ptrs

   02     .note.ABI-tag .note.gnu.build-id

   03     .tdata .tbss

   04   

   05     .tdata .ctors .dtors .jcr .data.rel.ro .got .got.plt





我们可以通过查看 “/proc”来查看进程的虚拟空间分布：

$ ./SectionMapping.elf &

[1]21963

$cat /proc/21963/maps



fly@Baby-o_o:~/Code/HelloWorld$ ./SectionMapping.elf &

[1] 3869

fly@Baby-o_o:~/Code/HelloWorld$ cat /proc/3869/maps

08048000-080c5000 r-xp 00000000 08:01 18874525   /home/fly/Code/HelloWorld/SectionMapping.elf

080c5000-080c7000 rwxp 0007c000 08:01 18874525   /home/fly/Code/HelloWorld/SectionMapping.elf

080c7000-080c9000 rwxp 00000000 00:00 0

08ee4000-08f06000 rwxp 00000000 00:00 0          [heap]

b7746000-b7747000 r-xp 00000000 00:00 0          [vdso]

bfc5b000-bfc70000 rw-p 00000000 00:00 0          [stack]



*匿名虚拟内存区域（Anonymous Virtual Memory  Area）



p192

6.5 linux内核装载ELF过程简介

（1）用户层面：bash进程->fork（）->execve()->执行elf->bash进程等待其结束

execve()被定义在unistd.h 原型：

int execve(const char *filename, char *const argv[], char *const envp[]);



*p174



七.动态链接



静态库的缺点：

（1）浪费内存空间，不同的程序调用相同的静态库，会在内存中存在两个相同的静态库的副本

（2）影响程序的更新、部署、和发布

一旦有一个模块需要更新，整个程序就要重新链接，发布给客户



为了解决上述静态库的缺点，动态库应运而生，把链接这个工程推迟到了运行时进行（动态链接 Dynamic Linking 的基本思想）



这样做的好处不仅仅是节省内存，还可以减少物理页面的换入换出，增加CPU缓存的命中率（因为不同进程间的数据和指令访问都集中在了同一个共享模块上）



缺点：当新旧模块接口不兼容的时候，导致原有的程序无法运行！！！



*延迟绑定



Example: SimpleDynamicalLinking



a.c

#include "Lib.h"



int main(){

   foobar(1);

   return 0;

}



b.c



#include "Lib.h"



int main(){

   foobar(2);

   return 0;

}



Lib.c

#include <stdio.h>



void foobar(int i){



   printf("Printing from Lib.so %d\n", i);

}



Lib.h



#ifndef LIB_H

#define LIB_H



void foobar(int i);



#endif



$ gcc -fPIC -shared -o Lib.so Lib.c

$ gcc -o a a.c ./Lib.so







Libc -> [Compiler] -> Lib.o -> [Linker] <- [C Runtime Library]

                            |

                            \/

                            Lib.so

                            |

                            \/

    a.c -> [Compiler] -> a.o -> [Linker] -> a





7.3 地址无关代码



静态库 使用编译时重定位

动态库 使用装载时重定位



如果上面只使用 -shared 参数， 则输出的共享对象 就是使用的 装载时重定位的方法

但是 这样虽然可以解决动态模块中有绝对地址引用，但是缺点是：指令部分无法在多个进程之间共享

这样就失去了动态链接节省内存的一大优势！！！



那么 -fPIC 参数有什么作用呢？



解决 共享对象指令中绝对地址重定位问题基本思路：

把指令中那些需要修改的部分分离出来，和数据部分放在一起，这样指令部分可以保持不变，而数据部分可以个在每个进程中拥有一个副本



这种方案目前被称为：地址无关代码（PIC， Position-independent Code）



共享对象模块中的地址引用按照是否为跨模块分为

（1）模块内部引用

（2）模块外部引用

按照引用方式：

（1）指令引用

（2）数据访问



于是我们得到4中情况：

（1）模块内部 的 函数调用、跳转。

（2）模块内部 的 数据访问，比如模块中定义的全局变量、静态变量。

（3）模块外部 的 函数调用、跳转。

（4）模块外部 的 数据访问，比如其他模块中定义的全局变量。



p217