参考材料
https://pdos.csail.mit.edu/6.828/2020/labs/util.html
Look at some of the other programs in user/ (e.g., user/echo.c, user/grep.c, and user/rm.c) to see how you can obtain the command-line arguments passed to a program.
grep.c echo.c rm.c int main(){} 括号里的参数是int argc, char* argv[]。argc表示参数个数，argv表示内容。
If the user forgets to pass an argument, sleep should print an error message.
要写一个提示错误的语句。printf(“exec errorn”);
exit(1)括号里是1表示出现异常。
The command-line argument is passed as a string; you can convert it to an integer using atoi (see user/ulib.c).
如果传入的数字是10，存储到argv[1]里，调用atoi()把字符类型转换成整型。
Use the system call sleep.
See kernel/sysproc.c for the xv6 kernel code that implements the sleep system call (look for sys_sleep), user/user.h for the C definition of sleep callable from a user program, and user/usys.S for the assembler code that jumps from user code into the kernel for sleep.
调用sleep（）传入时间参数就好了。他让看sleep的具体实现和汇编代码。
Make sure main calls exit() in order to exit your program.
调用exit(0)正常退出就可以了。
完整代码：
#include "kernel/types.h"
#include "user/user.h"
int main(int argc, char* argv[]){
if(argc != 2){
printf("exec errorn");
exit(1);
}
int num=atoi(argv[1]);
sleep(num);
exit(0);
}`
