什么是__gmon_start__象征？ [英] What is __gmon_start__ symbol?

问题描述

我编译这个code用gcc -o hello.c的-O3你好

I'm compiling this code with gcc hello.c -o hello -O3

#include <stdio.h>

int main(void) {
    printf("Hello world\n");
    return 0;
}

当我列出搬迁，我得到：

when I list the relocations I get:

test@southpark$ readelf -r hello | grep gmon
080495a4  00000106 R_386_GLOB_DAT    00000000   __gmon_start__
080495b4  00000107 R_386_JUMP_SLOT   00000000   __gmon_start__

在我列出的符号在这个文件中，我得到：

when I list the symbols in this file I get:

test@southpark$ readelf -s hello | grep gmon
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
    48: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

是否gmon_start有什么gprof的待办事项？为什么有这个符号，甚至我没有编译一个搬迁/使用-pg或-g链接？什么库就解决了这个符号？

Does gmon_start have anything todo with gprof? Why does have an relocation for that symbol even I didn't compiling/linking with -pg or -g ? What library would resolve this symbol?

推荐答案

做了一些谷歌上搜索，发现这个从这里：

Did a little googling and found this from here:

功能call_gmon_start初始化gmon分析系统。
  当二进制文件与-pg标志编译该系统已启用，
  和用于使用gprof（1）使用创建输出。在该方案的情况下，
  二进制call_gmon_start坐落直接驶往该_start
  功能。该call_gmon_start功能中发现的最后一个条目
  全局偏移表（也称为__gmon_start__），如果不为空，
  将控制传递到指定的地址。该__gmon_start__元素
  指向gmon初始化函数，它开始记录
  分析的信息，并注册了一个清理函数
  的atexit（）。在我们的情况下，然而gmon在不使用，因此
  __gmon_start__为NULL。

The function call_gmon_start initializes the gmon profiling system. This system is enabled when binaries are compiled with the -pg flag, and creates output for use with gprof(1). In the case of the scenario binary call_gmon_start is situated directly proceeding that _start function. The call_gmon_start function finds the last entry in the Global Offset Table (also known as __gmon_start__) and, if not NULL, will pass control to the specified address. The __gmon_start__ element points to the gmon initialization function, which starts the recording of profiling information and registers a cleanup function with atexit(). In our case however gmon is not in use, and as such __gmon_start__ is NULL.

所以...

1. 是，它确实有事可做使用gprof


2. 我不知道为什么符号越来越留在了那里。也许只是当它编译为gprof的吗？一个占位符

更新：的

Update:

好吧，所以我编你的code带和不带 -pg 。它看起来像 __ __ gmon_start 被编译的程序中映射到一个地址。所以，他这样说，我不认为有哪些解决了符号库，但是程序本身。

Okay, so I compiled your code with and without -pg. It looks like __gmon_start__ gets mapped to an address within the compiled program. So with that being said, I don't think there's a library which resolves that symbol, but the program itself.

与 -pg ：

with -pg:

akyserr@orion:~$ readelf -r hello

Relocation section '.rel.dyn' at offset 0x32c contains 1 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049fec  00000806 R_386_GLOB_DAT    08048460   __gmon_start__

Relocation section '.rel.plt' at offset 0x334 contains 6 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
0804a000  00000607 R_386_JUMP_SLOT   080483b0   _mcleanup
0804a004  00000107 R_386_JUMP_SLOT   00000000   __monstartup
0804a008  00000207 R_386_JUMP_SLOT   00000000   mcount
0804a00c  00000307 R_386_JUMP_SLOT   00000000   __cxa_atexit
0804a010  00000407 R_386_JUMP_SLOT   00000000   puts
0804a014  00000507 R_386_JUMP_SLOT   00000000   __libc_start_main

__gmon_start__ code的objdump的：

akyserr@orion:~$ objdump -S hello  | grep "460 <__gmon_start__>:" -A 20

08048460 <__gmon_start__>:
 8048460:       83 ec 1c                sub    $0x1c,%esp
 8048463:       a1 20 a0 04 08          mov    0x804a020,%eax
 8048468:       85 c0                   test   %eax,%eax
 804846a:       75 2a                   jne    8048496 <__gmon_start__+0x36>
 804846c:       c7 05 20 a0 04 08 01    movl   $0x1,0x804a020
 8048473:       00 00 00 
 8048476:       c7 44 24 04 36 86 04    movl   $0x8048636,0x4(%esp)
 804847d:       08 
 804847e:       c7 04 24 30 84 04 08    movl   $0x8048430,(%esp)
 8048485:       e8 36 ff ff ff          call   80483c0 <__monstartup@plt>
 804848a:       c7 04 24 b0 83 04 08    movl   $0x80483b0,(%esp)
 8048491:       e8 1a 01 00 00          call   80485b0 <atexit>
 8048496:       83 c4 1c                add    $0x1c,%esp
 8048499:       c3                      ret    
 804849a:       90                      nop
 804849b:       90                      nop
 804849c:       90                      nop
 804849d:       90                      nop

通过在 __ gmon_start __ present编译你好程序，你可以看到， __ monstartup 被称为成。 （ monstartup手册页）

With the __gmon_start__ present in the compiled hello program, you can see that that __monstartup is called into. (monstartup man page)

没有 -pg ：

without -pg:

akyserr@orion:~$ readelf -r hello 

Relocation section '.rel.dyn' at offset 0x290 contains 1 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049ff0  00000206 R_386_GLOB_DAT    00000000   __gmon_start__

Relocation section '.rel.plt' at offset 0x298 contains 3 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
0804a000  00000107 R_386_JUMP_SLOT   00000000   puts
0804a004  00000207 R_386_JUMP_SLOT   00000000   __gmon_start__
0804a008  00000307 R_386_JUMP_SLOT   00000000   __libc_start_main

您可以在这里看到，该 __ gmon_start __ 的标志值设置为 00000000 。

You can see here, that the symbol value of __gmon_start__ is set to 00000000.

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