MIT 6.828 - 1. Lab 01: Xv6 and Unix utilities

实验指导

上来直接：

$ cd xv6-riscv-fall19
$ git checkout util

实验指导简要介绍了如何把 xv6 跑起来（make then make qemu），如何交作业（make handin），如何测试成绩（make grade）。

下面介绍各个子任务如何写。

1. sleep

顾名思义写一个 sleep 例程，休眠一定的 tick 数，tick 的定义是时间中断。

Hints:
Look at some of the other programs in user/ to see how you can obtain the command-line arguments passed to a program. If the user forgets to pass an argument, sleep should print an error message.
The command-line argument is passed as a string; you can convert it to an integer using atoi (see user/ulib.c).
Use the system call sleep (see user/usys.S and kernel/sysproc.c).
Make sure main calls exit() in order to exit your program.
Add the program to UPROGS in Makefile and compile user programs by typing make fs.img.
Look at Kernighan and Ritchie's book The C programming language (second edition) (K&R) to learn about C.

首先 make clean，然后照猫画虎写一下 user/sleep.c：

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char *argv[])
{
  if(argc != 2)
    write(2, "Error message", strlen("Error message"));
  
  int x = atoi(argv[1]);

  sleep(x);

  exit(0);
}

按照说明修改 Makefile 运行即可。

2. pingpong

如法炮制。

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char *argv[])
{
  int parent_fd[2], child_fd[2];
  pipe(parent_fd); pipe(child_fd);
  char buf[64];

  if(fork()) {
    write(parent_fd[1], "ping", strlen("ping"));
    read(child_fd[0], buf, 4);
    printf("%d: received %s\n", getpid(), buf);
  } else {
    read(parent_fd[0], buf, 4);
    printf("%d: received %s\n", getpid(), buf);
    write(child_fd[1], "pong", strlen("pong"));
  }

  exit(0);
}

3. primes

照猫画虎。

#include "kernel/types.h"
#include "user/user.h"

void source() {
  int i;
  for (i = 2; i < 36; i++) {
    write(1, &i, sizeof(i));
  }
}

void cull(int p) {
  int n;
  while (read(0, &n, sizeof(n))) {
    if (n % p != 0) {
      write(1, &n, sizeof(n));
    }
  }
}

void redirect(int k, int pd[]) {
  close(k);
  dup(pd[k]);
  close(pd[0]);
  close(pd[1]);
}

void sink() {
  int pd[2];
  int p;

  if (read(0, &p, sizeof(p))) {
    printf("prime %d\n", p);
    pipe(pd);
    if (fork()) {
      redirect(0, pd);
      sink();
    } else {
      redirect(1, pd);
      cull(p);
    }
  }
}

int main(int argc, char *argv[]) {

  int pd[2];
  pipe(pd);

  if (fork()) {
    redirect(0, pd);
    sink();
  } else {
    redirect(1, pd);
    source();
  }

  exit();
}

4. find

Bonus 是加 regex 支持，好，抄 user/grep.c 即可。

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"
#include "kernel/fs.h"

char*
fmtname(char *path)
{
  static char buf[DIRSIZ+1];
  char *p;

  // Find first character after last slash.
  for(p=path+strlen(path); p >= path && *p != '/'; p--)
    ;
  p++;

  // Return blank-padded name.
  if(strlen(p) >= DIRSIZ)
    return p;
  memmove(buf, p, strlen(p));
  memset(buf+strlen(p), ' ', DIRSIZ-strlen(p));
  return buf;
}

// Regexp matcher from Kernighan & Pike,
// The Practice of Programming, Chapter 9.

int matchhere(char*, char*);
int matchstar(int, char*, char*);

int
match(char *re, char *text)
{
  if(re[0] == '^')
    return matchhere(re+1, text);
  do{  // must look at empty string
    if(matchhere(re, text))
      return 1;
  }while(*text++ != '\0');
  return 0;
}

// matchhere: search for re at beginning of text
int matchhere(char *re, char *text)
{
  if(re[0] == '\0')
    return 1;
  if(re[1] == '*')
    return matchstar(re[0], re+2, text);
  if(re[0] == '$' && re[1] == '\0')
    return *text == '\0';
  if(*text!='\0' && (re[0]=='.' || re[0]==*text))
    return matchhere(re+1, text+1);
  return 0;
}

// matchstar: search for c*re at beginning of text
int matchstar(int c, char *re, char *text)
{
  do{  // a * matches zero or more instances
    if(matchhere(re, text))
      return 1;
  }while(*text!='\0' && (*text++==c || c=='.'));
  return 0;
}

void
find(char *path, char *re)
{
  char buf[512], *p;
  int fd;
  struct dirent de;
  struct stat st;

  if((fd = open(path, 0)) < 0){
    fprintf(2, "find: cannot open %s\n", path);
    return;
  }

  if(fstat(fd, &st) < 0){
    fprintf(2, "find: cannot stat %s\n", path);
    close(fd);
    return;
  }

  switch(st.type){
  case T_FILE:
    if(match(re, fmtname(path)))
      printf("%s\n", path);
    break;

  case T_DIR:
    if(strlen(path) + 1 + DIRSIZ + 1 > sizeof buf){
      printf("find: path too long\n");
      break;
    }
    strcpy(buf, path);
    p = buf+strlen(buf);
    *p++ = '/';
    while(read(fd, &de, sizeof(de)) == sizeof(de)){
      if(de.inum == 0)
        continue;
      memmove(p, de.name, DIRSIZ);
      p[DIRSIZ] = 0;
      if(stat(buf, &st) < 0){
        printf("find: cannot stat %s\n", buf);
        continue;
      }

      if(strlen(de.name) == 1 && de.name[0] == '.')
        continue;
      if(strlen(de.name) == 2 && de.name[0] == '.' && de.name[1] == '.')
        continue;

      find(buf, re);
    }
    break;
  }
  close(fd);
}


int main(int argc, char *argv[]) {
  if(argc <= 2)
    fprintf(2, "find: not enough params provided");
  find(argv[1], argv[2]);
  
  exit();
}

5. xargs

未完成，TODO。

实验总结

无。