firstly. test cpp file show below.

cpp
// t.cpp
#include <iostream>
#include <stdio.h>

int main()
{
        char* c = NULL;
        printf("%d", *c);
        return 0;
}

to check if the coredump enabled in your system by.

shell
ulimit -c // return 0 if disable.
ulimit -c unlimited // this command will enable the coredump feature.

use g++ to compile the program with -g option.

shell
g++ -g t.cpp -o t.o // complile
./t.o // this command will run into fail 
// message is  "Segmentation fault (core dumped)"
// and a file named "core" is generated.

now you can use gdb to debug the code.

shell
gdb t.o core

done !

代码：

#include<iostream>

using namespace std

![20200119145417764.png](https://santi.pub/static

cpp
//回调函数的简单理解
//例子温度计显示温度。

//有两种方式
// 1. 睡眠5