#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

//定义进程的终止函数
void term_fun1(void)
{
    printf("first term function\n");
}

void term_fun2(void)
{
    printf("second term funciton\n");
}

void term_fun3(void)
{
    printf("third term function\n");
}

int main(int argc, char* argv[])
{
    if(argc < 3){
        fprintf(stderr, "usage: %s file [exit|_exit|return]\n");
        exit(1);
    }    

    //调用atexit函数向内核登记终止函数
    //注意：注册终止函数的顺序为func1->fun2->fun3，函数被调用顺序fun3->fun2->fun1
    atexit(term_fun1);
    atexit(term_fun2);
    atexit(term_fun3);

    FILE* fp = fopen(argv[1], "w");
    fprintf(fp, "hello world!\n"); //fprintf带缓存，调用exit或return时会被
                                   //流会被关闭，即会被写入指定的文件中。


    if(!strcmp(argv[2], "exit")){
        exit(0);   //标准C的库函数
    }else if(!strcmp(argv[2], "_exit")){
        _exit(0);  //系统调用
    }else if(!strcmp(argv[2], "return")){
        return 0;
    }else{
        fprintf(stderr, "usage: %s file [exit|_exit|return]\n", argv[0]);
    }

    return 0;
}
/*输出结果：
[root@localhost 5.process]# bin/process_term term.txt return  //正常退出（调用终止函数和刷新缓存）
third term function
second term funciton
first term function
[root@localhost 5.process]# cat term.txt
hello world!
[root@localhost 5.process]# bin/process_term term.txt exit   //正常退出（调用终止函数和刷新缓存）
third term function
second term funciton
first term function
[root@localhost 5.process]# cat term.txt
hello world!
[root@localhost 5.process]# bin/process_term term.txt _exit //异常退出（不调用终止函数和刷新缓存）
[root@localhost 5.process]# cat term.txt
[root@localhost 5.process]# bin/process_term term.txt quit
usage: bin/process_term file [exit|_exit|return]
third term function
second term funciton
first term function
[root@localhost 5.process]# cat term.txt
hello world!
[root@localhost 5.process]# 
 */