为什么GCC编译的应用程序总是包含_mcount符号？

问题描述

库并不总是包含_mcount符号，但应用程序可以（您可以使用gobjdump或nm工具来验证）。我读过_mcount用于实现分析，但即使禁用了分析并启用了优化（-O2），符号仍然存在。它是否有其他用途？

更新：我在Solaris上，所以这是Solaris链接器与GCC的组合，我不确定这是否有所作为或不。 GCC版本是4.2.2。即使我编译的文件只包含代码 int main（）{return 0;

Update2：I type：

  $ g ++ -O2 mytest.cpp 
 $ nm a.out | grep _mcount 
 [65] | 134547444 | 1 | FUNC | GLOB | 0 | 11 | _mcount 
  

g ++不是别名。另外，我尝试使用Sun CC编译器进行编译，但没有这个问题。我也试过更新GCC，符号依然存在于4.4.1中。

奇怪的是，在我的机器上（ubuntu 9.10）这没有发生。

对于一个测试，我编写了一个小小的hello-word：

  #include  
 
 int main（int argc，char ** args）
 {
 printf（hello world\\\
）; 
 
 $ / code> 

使用

编译

  gcc test.c 
  

它没有_mcount符号。我查了一下：

  nm a.out | grep -i count 
  

尝试使用一些编译器开关（-g，-pg等）只有在使用-pg编译应用程序时才会出现该符号，在这种情况下，您可以在启用分析功能的情况下进行编译，因此_mcount符号存在理由。

Libraries don't always contain the _mcount symbol, but applications do (you can verify this with gobjdump or the nm utility). I've read that _mcount is used to implement profiling, but the symbol is present even when profiling is disabled and optimization is enabled (-O2). Does it serve some other additional purpose?

Update: I am on Solaris, so this is the Solaris linker combined with GCC, I'm not sure if that makes a difference or not. The GCC version is 4.2.2. This happens even if I compile a file that only contains the code int main() { return 0; } with no libraries linked.

Update2: I type:

$ g++ -O2 mytest.cpp
$ nm a.out | grep _mcount
[65]    | 134547444|       1|FUNC |GLOB |0    |11     |_mcount

And g++ is not aliased to anything. Also, I tried compiling with the sun CC compiler, and it doesn't have this issue. I also tried updating GCC, symbol still exists in 4.4.1.

解决方案

Hm. strange, on my machine (ubuntu 9.10) this does not happen.

For a test I just compiled a small hello-word:

#include <stdio.h>

int main (int argc, char **args)
{
  printf ("hello world\n");
}

compiled with

gcc test.c

It didn't has the _mcount symbol. I checked with:

nm a.out | grep -i count

Trying some compiler switches (-g, -pg ect.) it turns out that the symbol only appears if you compile your application with -pg, In this case you compile with profiling enabled, so the _mcount symbol has a reason to exist.

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