深入理解计算机系统chapter7 学习笔记

Local symbols

It is important to realize that local linker symbols are not the same as local
program variables. The symbol table in .symtab does not contain any symbols
that correspond to local nonstatic program variables. These are managed at run

time on the stack and are not of interest to the linker.

符号表.symtab中不包含对应于本地非静态程序变量的符号。本地非静态变量运行时在栈上管理，

链接对这些变量不感兴趣。（不属于linker的管理范畴)

本地定义成static的变量不在栈上管理

Interestingly, local procedure variables that are defined with the C static
attribute are not managed on the stack. Instead, the compiler allocates space in
.data or .bss for each definition and creates a local linker symbol in the symbol
table with a unique name.

1 int f()
2 {
3 static int x = 0;
4 return x;
5 }
6
7 int g()
8 {
9 static int x = 1;
10 return x;
11 }
In this case, the compiler allocates space for two integers in .data and exports a
pair of unique local linker symbols to the assembler. For example, it might use x.1
for the definition in function f and x.2 for the definition in function g.

C programmers use the static attribute to hide variable and function declarations inside modules

Any global variable or function declared with the static attribute is private to that module

任何声明为static的全局变量或函数是那个模块私有的。

对

gcc -o2 -g -o p main.c swap.c

生成的目标文件

readelf指令

readelf - Displays information about ELF files.

readelf -a p

得到部分消息如下

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:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x400370
  Start of program headers:          64 (bytes into file)
  Start of section headers:          4064 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         8
  Size of section headers:           64 (bytes)
  Number of section headers:         36
  Section header string table index: 33

Symbol table '.symtab' contains 78 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name

66: 0000000000600870     8 OBJECT  GLOBAL DEFAULT   23 bufp0
    67: 0000000000400590     0 OBJECT  GLOBAL HIDDEN   14 __dso_handle
    68: 0000000000600698     0 OBJECT  GLOBAL HIDDEN   18 __DTOR_END__
    69: 00000000004004b0   139 FUNC    GLOBAL DEFAULT   12 __libc_csu_init
    70: 0000000000400460    60 FUNC    GLOBAL DEFAULT   12 swap
    71: 0000000000600878     0 NOTYPE  GLOBAL DEFAULT  ABS __bss_start
    72: 0000000000600888     8 OBJECT  GLOBAL DEFAULT   24 bufp1
    73: 0000000000600890     0 NOTYPE  GLOBAL DEFAULT  ABS _end
    74: 0000000000600864     8 OBJECT  GLOBAL DEFAULT   23 buf
    75: 0000000000600878     0 NOTYPE  GLOBAL DEFAULT  ABS _edata
    76: 0000000000400448    21 FUNC    GLOBAL DEFAULT   12 main
    77: 0000000000400338     0 FUNC    GLOBAL DEFAULT   10 _init

Ndx = The section number the symbol is in.

ABS is for symbols that should not be relocated. UNDEF is for undefined symbols,
that is, symbols that are referenced in this object module but defined elsewhere.
COMMON is for uninitialized data objects that are not yet allocated

UNDEF即为readelf显示的UND

重载函数在符号表中的项

比如这两个重载函数在符号表中的信息

int func(int a,int b){
  return a+b;
 }
 double func(double a){
return a;
 }

  71: 0000000000400648    18 FUNC    GLOBAL DEFAULT   12 _Z4funcii

 69: 000000000040065a    24 FUNC    GLOBAL DEFAULT   12 _Z4funcd