Gdb Introduction

[AngeloYin]

Gdb Introduction

What is gdb？

• "GNU Debugger"
• A debugger for several laguages, include C and C++

Basic Usage

If you want to use gdb, you need to add -g option when you use gcc to compile your program.
Gdb has an interactive shell, so it also has some features like the ordinary shell such as recall history with arrow keys and auto-complete words wit TAB.

Start gdb

you can start it directly

gdb ./xxx //program name

or you can start gdb first

gdb
(gdb) file xxx

Basic Commands

Here is some basic commands in gdb, and you can use the whole name or the abbreviation in the parenthese.

//run the program
(gdb) run(r)
//set breakpoints
(gdb) break(b) file.c:6//set with line number
(gdb) break(b) my_func//set with function name
//continue the program, run will resatrt from the beginning
(gdb) continue(c)
//run step by step, will step into a func if next line is a func
(gdb) step(s)
//run step by step, won't step into a func if next line is a func
(gdb) next(n)
//print value
(gdb) print(p) my_var
(gdb) print(p)/x my_var
//watch a value, it will stop when  is modified
(gdb) watch(w) my_var
//produce a stack trace of function calls
(gdb) backtrace(bt)

Deal with segmentation fault

As we all know, it is quite normal for you to meet a seg fault problem. With gdb, you can detect the problem more eaisly.

//let the core dump
ulimit -c unlimited
//start gdb with the core
gdb ./xxx core
//produce the stack trace
(gdb)bt

Tips & Skills

Play With Pointers

Multi Thread

Change to another thread

Sometimes we may find that our program stuck in an infinite loop. And if you have a moniter thread, you can use gdb to find the loop easily. You can set a breakpoint in your monitor thread, and when it stops, you can check other thread.

(gdb) info thread //check all thread
(gdb) thread id //change thread into another one with id

Also, it is useful anytime you want to break and check the status of other thread.

Lock other threads

On the other hand, you just want to continue one thread, and you can also lock other thread using this:

(gdb)set scheduler-locking on //only this thread will run

(gdb)set scheduler-locking step //optimize only when run step by step

You can unlock

(gdb)set scheduler-locking off

Useful Breakpoints

[liukeshu]

正好需要，收藏了！

[zhaoyonghui]

想起了拆炸弹

[geniusrabbit]

太强了，收藏了～

[MengyingYu]

昨天用gdb解决了一个bug，多谢阴队

[ZhangXin]

多线程多进程调试好厉害

[luck]

gdb 的coredump 定位段错误简直不要太好用

[BC]

收藏了~好好学习一下

[Kacedas]

这个必须好好学学