GDB in Action

Posted 2021-02-05 冯煜博

GDB in Action

入门

编译

gcc -g -O0 -o word2vec.c word2vec

-g 选项：要求 gcc 编译器保留调试符号信息。

-O0 选项表示不优化，从 O1 ~ O4 优化级别越来越高，O4 最高。

strip 命令 = 不加 -g 选项。

启动

第一种

gdb word2vec

第二种

gdb attach (pid)

第三种

gdb word2vec core.21985

(gdb) 命令助记

run (r)

运行程序。也可以设置被调试程序的运行参数，如下例。

(gdb) run -train text8 -output vectors.bin -cbow 1 -size 200 -window 8 -negative 25 -hs 0 -sample 1e-4 -threads 20 -binary 1 -iter 15

show args

Show argument list to give program being debugged when it is started.
Follow this command with any number of args, to be passed to the program.

continue (c)

当 GDB 触发断点或者使用 Ctrl + C 命令中断下来后，想让程序继续运行，只要输入 continue 命令即可。

break (b)

添加断点。

(gdb) break function_name  # 在函数入口加断点
(gdb) break line_No  # 在当前文件行号为 line_no 加断点
(gdb) break file_name:line_no  # 在文件名为 file_name 文件，第 line_no 行加断点

### backtrace (bt)

查看当前调用堆栈。

frame (f)

切换堆栈。

(gdb) f 2  #切换到编号为 2 的堆栈（第三层）

disble

Disable some breakpoints.
Arguments are breakpoint numbers with spaces in between.
To disable all breakpoints, give no argument.
A disabled breakpoint is not forgotten, but has no effect until re-enabled.
This command may be abbreviated "disable".

enable

Enable some breakpoints.
Give breakpoint numbers (separated by spaces) as arguments.
This is used to cancel the effect of the "disable" command.
May be abbreviated to simply "enable".

delete

Delete some breakpoints or auto-display expressions.
Arguments are breakpoint numbers with spaces in between.
To delete all breakpoints, give no argument.
This command may be abbreviated "delete".

list (l)

查看当前断点处的代码。往前和往后显示代码，命令分别是“list + （加号）”和“list - （减号）”。

print (p)

可以查看、计算和修改变量的值

(gdb) print a  # 查看变量 a 的值
10

(gdb) print &a  # 查看变量 a 的地址
(int *) 0x7ffffffde52c

(gdb) print a=12  # 打印并设置变量 a 的值为 12
12

(gdb) print a+a+a  # 打印并计算 a+a+a 的值
36

(gdb) print a=a+a  # 打印并设置变量 a 的值为 a+a
22

(gdb) print *p  # 打印指针 p 指向对象的值

(gdb) print node->name  # 打印结构体变量 node 的 name 字段的值

ptype

打印变量的类型

info

info break (info b)

查看断点们的状态，可以显示断点被触发的次数。

info thread

Display currently known threads.

info threads [-gid] [ID]...  # -gid: Show global thread IDs.

If ID is given, it is a space-separated list of IDs of threads to display.
Otherwise, all threads are displayed.

info args

查看当前函数被执行的，实参的值。

next

next over，运行下一行，遇到函数调用不进入。

step

step into，运行下一行，遇到函数调用时会进入。

finsh

结束当前函数，返回上层。

return

结束当前函数，返回上层。同时可以指定该函数的返回值。

(gdb) return 99  # 指定当前函数返回值 99

until (u)

指定程序运行到某一行停下来。

jump

让程序执行流跳转到指定位置执行。

可以执行一些我们想要执行的代码，而这些代码在正常的逻辑下可能并不会执行

disassemble

反汇编

tbreak

Set a temporary breakpoint.
Like "break" except the breakpoint is only temporary, so it will be deleted when hit. Equivalent to "break" followed by using "enable delete" on the breakpoint number.

watch

监视一个变量或者一段内存，当这个变量或者该内存处的值发生变化时，GDB 就会中断下来。被监视的某个变量或者某个内存地址会产生一个 watch point（观察点）。

当设置的观察点是一个局部变量时，局部变量无效后，观察点也会失效。

display

监视变量或者内存地址，每次程序中断下来都会自动输出这些变量或内存的值。

signal

Continue program with the specified signal.

signal [SIGNAL]

The SIGNAL argument is processed the same as the handle command.

SIGNAL: SIGCHLD, SIGPIPE, SIGINT, SIGTERM......

handle

Specify how to handle signals.

handle [SIGNAL]

for example

handle SIGINT nostop print

告诉 GDB 在接收到 SIGINT 时不要停止，并把该信号传递给调试目标程序 。

break [lineNo] if [condition]

条件断点。

break 11 if i==5000  # 意为当 i = 5000 时，程序中断在第 11 行

添加条件断点还有一个方法就是先添加一个普通断点，然后使用“condition 断点编号断点触发条件”这样的方式来添加。

condition

Specify breakpoint number N to break only if COND is true.

condition N COND

where N is an integer and COND is an expression to be evaluated whenever breakpoint N is reached.

set

scheduler-locking

Set mode for locking scheduler during execution.

• off - no locking (threads may preempt at any time)
• on - full locking (no thread except the current thread may run). This applies to both normal execution and replay mode.
• step - scheduler locked during stepping commands (step, next, stepi, nexti). In this mode, other threads may run during other commands. This applies to both normal execution and replay mode.
• replay - scheduler locked in replay mode and unlocked during normal execution.

follow-fork-mode

Set debugger response to a program call of fork or vfork.
A fork or vfork creates a new process. follow-fork-mode can be:

• parent - the original process is debugged after a fork
• child - the new process is debugged after a fork

The unfollowed process will continue to run.
By default, the debugger will follow the parent process.

print element

Set limit on string chars or array elements to print.
"set print elements unlimited" causes there to be no limit.

"set print element 0" 完整地显示变量的整个字符串。

args

Set argument list to give program being debugged when it is started.
Follow this command with any number of args, to be passed to the program.

(gdb) set args -train text8 -output vectors.bin -cbow 1 -size 200 -window 8 -negative 25 -hs 0 -sample 1e-4 -threads 20 -binary 1 -iter 15

Reference

[1] 《Linux GDB 调试指南》. 范蠡. https://gitbook.cn/gitchat/column/5c0e149eedba1b683458fd5f