c - 使用 __thread 时 gdb 优化了值

Question

我所有的static __thread值显示为 <optimized out>当在调试时我想观察变量的值时；即使使用 -o0 和/或 volatile .

Static没有 __thread 的变量正确显示。

即使我正在使用线程，是否仍然可以显示变量的值？

使用 Win10 ( CreateThread )、eclipse CDT、c11、mingw64-w64 和 gdb 7.11.1

Answer 1

解决方法可能是:在代码中添加一些线程局部变量的打印机，并让 gdb 调用它们。 (或者，如果您熟悉x86汇编，请编写一些hackish插件来修改可执行内存以读取fs:offset/gs:offset(线程局部变量值)并恢复内存和寄存器)

更具体地说，在 C 代码中添加一个函数，该函数除了返回有趣的 __thread 变量之外什么也不做，当您使用 gdb 中断程序时，您始终可以让 gdb 调用该函数您(假设该函数未优化)不会破坏原始程序的堆栈帧。它应该很简单:

(gdb) p rand()
$1 = 1804289383
(gdb) p rand()
$2 = 846930886
(gdb) p rand()
$3 = 1681692777

虽然rand不是一个很好的例子，因为它有副作用。 TLS 变量读取没有副作用。

示例:(在 Ubuntu 16.04 下，但由于功能非常基本，因此应该不会有太大差异)

tls.cpp:

#include <stdio.h>

__thread long thread_cand;

long what_is_thread_cand()
{
    return thread_cand;
}

int main()
{
    while ( !feof ( stdin ) )
    {
        scanf ( "%ld", &thread_cand );
        printf ( "%p : %ld\n", &thread_cand, thread_cand );
    }
    return 0;
}

终端:

$ g++ -O2 -g3 tls.cpp -o tls
tls.cpp: In function ‘int main()’:
tls.cpp:14:38: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
         scanf ( "%ld", &thread_cand );
                                      ^
$ gdb tls --nh
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from tls...done.
(gdb) r
Starting program: /home/ubuntu/tls 
123
0x7ffff7fcb6f8 : 123
432
0x7ffff7fcb6f8 : 432
^C
Program received signal SIGINT, Interrupt.
0x00007ffff7b04230 in __read_nocancel ()
    at ../sysdeps/unix/syscall-template.S:84
84  ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) p thread_cand
Cannot find thread-local storage for process 6472, executable file /home/ubuntu/tls:
Cannot find thread-local variables on this target
(gdb) p what_is_thread_cand() 
$1 = 432
(gdb)