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
