c – 什么是__gmon_start__符号？

发布时间：2020-12-16 05:22:30 所属栏目：百科来源：网络整理

导读：我用 gcc hello.c -o hello -O3编译这个代码 #include stdio.hint main(void) { printf("Hello worldn"); return 0;} 当我列出我得到的搬迁时： test@southpark$readelf -r hello | grep gmon080495a4 00000106 R_386_GLOB_DAT 00000000 __gmon_start__0804

我用 gcc hello.c -o hello -O3编译这个代码

#include <stdio.h>

int main(void) {
    printf("Hello worldn");
    return 0;
}

当我列出我得到的搬迁时：

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__

当我列出这个文件中的符号我得到：

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__

gmon_start有什么需要使用gprof吗？即使我没有编译/链接-pg还是-g,为什么要使用该符号进行重定位？什么图书馆会解决这个符号？

解决方法

有一点搜索,从 here发现：

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…

是的,它确实与gprof有关
>我不知道为什么这个符号被放在那里.在gprof编译时,可能只是一个占位符？

更新：

好的,所以我编译你的代码有和没有-pg.看起来__gmon_start__映射到编译程序中的一个地址.所以就这样说,我不认为有一个解决这个符号的图书馆,而是程序本身.

与-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

objdump __gmon_start__代码：

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

在编译好的hello程序中,__gmon_start__存在,您可以看到__monstartup被调用. (monstartup man page)

没有-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__gmon_start__的符号值设置为00000000.

（编辑：李大同）

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