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。

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。