GDB远程调试中trace点收集变量显示<unavailable>的原因排查

在GDB会话中使用trace定义跟踪点收集变量时，所有数据均显示<unavailable>，详细信息如下：

C代码 #

#include <stdio.h>

int main() {
    // Define variables for square, cube, and the sums
    int square, cube;
    int sum_of_squares = 0, sum_of_cubes = 0;
    int i;

    // Loop from 1 to 10
    for (i = 1; i <= 10; i++) {
        // Calculate square and cube of the current number
        square = i * i;
        cube = i * i * i;

        // Accumulate the sums
        sum_of_squares += square;
        sum_of_cubes += cube;
    }

    // After the loop, print the accumulated sums
    printf("Sum of squares: %d\n", sum_of_squares);
    printf("Sum of cubes: %d\n", sum_of_cubes);

    // Wait for the user to press any key before exiting
    printf("Press any key to exit...\n");
    getchar();  // Wait for user input

    return 0;
}

编译命令 #

gcc -g -O0 main.c -o test

调试操作步骤 #

1. 启动gdbserver：

gdbserver :0 ./test
# Process ./test created; pid = 70878
# Listening on port 43457

2. 启动GDB并连接：

gdb -q ./test

3. GDB会话指令：

(gdb) target remote :43457
(gdb) break main
(gdb) continue
(gdb) trace main.c:17
(gdb) actions
# Enter actions for tracepoint 2, one per line.
# End with a line saying just "end".
# >collect square,cube
# >end
(gdb) tstart
(gdb) continue

^C

(gdb) tstop
(gdb) tfind 0
(gdb) tdump
# Data collected at tracepoint 2, trace frame 0:
# square = <unavailable>
# cube = <unavailable>
(gdb) tfind
# Found trace frame 1, tracepoint 2
# 17              sum_of_cubes += cube;
(gdb) tdump
# Data collected at tracepoint 2, trace frame 1:
# square = <unavailable>
# cube = <unavailable>

系统环境 #

mohammad@ubuntu:~$ uname -a
Linux ubuntu 6.8.0-35-generic #35-Ubuntu SMP PREEMPT_DYNAMIC Tue May 21 07:52:29 UTC 2024 aarch64 aarch64 aarch64 GNU/Linux
mohammad@ubuntu:~$ cat /etc/os-release 
PRETTY_NAME="Ubuntu 24.04.1 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04.1 LTS (Noble Numbat)"
VERSION_CODENAME=noble
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=noble
LOGO=ubuntu-logo
mohammad@ubuntu:~$ gdb --version
GNU gdb (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git
Copyright (C) 2024 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
mohammad@ubuntu:~$ gcc --version
gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

mohammad@ubuntu:~$ gdbserver --version
GNU gdbserver (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git
Copyright (C) 2024 Free Software Foundation, Inc.
gdbserver is free software, covered by the GNU General Public License.
This gdbserver was configured as "aarch64-linux-gnu"
mohammad@ubuntu:~$ cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-6.8.0-35-generic root=UUID=974aad07-eefd-4d23-a2c7-3c132d00c155 ro
mohammad@ubuntu:~$

核心原因 #

跟踪点设置在main.c:17（sum_of_cubes += cube;）时，square变量在该行之后不再被使用，即使-O0编译级别，编译器也可能将其从内存中释放或复用寄存器；cube虽仍被读取，但也可能仅存于寄存器而非栈内存。GDB远程跟踪时无法直接访问寄存器中的临时数据，导致显示<unavailable>。

解决办法 #

1. 调整跟踪点位置
   将跟踪点移到square和cube仍处于活跃状态的行，比如main.c:15（sum_of_squares += square;）：

   (gdb) trace main.c:15
   

   此时两个变量刚完成赋值且尚未被优化，GDB可以正常收集数据。

2. 阻止编译器优化变量
   修改C代码，给square和cube添加volatile关键字，强制变量存储在内存中：

   volatile int square, cube;
   

   重新编译后，跟踪点即使在原位置也能收集到变量值。

3. 收集表达式值替代变量
   不修改代码的前提下，直接收集表达式计算结果，绕开变量存储问题：

   (gdb) actions
   >collect i*i, i*i*i
   >end
   

   GDB会在跟踪点执行表达式计算并保存结果。

内容的提问来源于stack exchange，提问作者Mohammad Rahimi