6.1810-utils

算是寒假稍微干的一点正事吧，趁着旅游美赛拜年的空隙试着尝试了一下lab，和jyy的网课配套使用。

课程官网在 https://pdos.csail.mit.edu/6.1810/2023/ ， hint和评测都十分用心。

在lab1犯的很蠢的错误就是自己折腾了好久gcc ， 试着

gcc -I ./ -MMD -c your.c -o your.o

去编译，后来发现大部分时候都不需要这样，直接make就好。

甚至因为在primes里开了无数的线程然后给我的wsl宕机了（逃

boot xv6

照着做就可以喽qwq

sleep

有现成的系统调用可以用，照着提示配好存根，开睡！

sleep(atoi(argv[1]));

pingpong

int
main(int argc, char *argv[])
{
  if(argc != 1){
    fprintf(2, "Usage: pingpong");
    exit(1);
  }
  int p1[2];
  int p2[2];
  pipe(p1);
  pipe(p2);
  char buf1[4];
  char buf2[4];
  memset(buf1,0,sizeof(buf1));
  memset(buf2,0,sizeof(buf2));
  if (fork() == 0) {
    write(p1[1],"ping",4);
    read(p2[0],buf1,4);
    fprintf(2,"%d: received %s\n",getpid(),buf1);
  } else {
    read(p1[0],buf2,4);
    fprintf(2,"%d: received %s\n",getpid(),buf2);
    write(p2[1],"pong",4);
  }
  sleep(5);
  exit(0);
}

回头写blog的时候发现自己写的好烂啊，比如内存空间会被fork复制所以不需要这么多buf，比如sleep不如wait，总之过了。

如果既不sleep也不wait的话主线程提前结束会先输出下一个命令提示符$.

primes

是一个很符合直觉的筛，但是写起来有点坑（

void cal(int p[])
{
  int write_p[2];
  pipe(write_p);
  int i = 0;
  int num;
  int status = read(p[0], &num, sizeof(num));
  if (status == 0)
  { 
    close(p[1]);
    close(write_p[1]);
    close(write_p[0]);
    exit(0);
  }
  int pid = fork();
  if (pid < 0)
  {
    exit(1);
  }
  if (pid == 0)
  {
    close(write_p[1]);
    cal(write_p);
  }
  else
  {
    for (;;)
    {
      int firstnum;
      if (status == 0){
        close(p[1]);
        close(p[0]);
        close(write_p[1]);
        exit(0);
      }
      else if (i == 0)
      {
        firstnum = num;
        printf("prime %d\n", firstnum);
      }
      else
      { 
        if ((num % firstnum) != 0)
        { 
          write(write_p[1], &num, sizeof(num));
        }
      }
      i++;
      status = read(p[0], &num, sizeof(num));
    }
  }
  wait(0);
  exit(0);
}

int main(int argc, char *argv[])
{
  if (argc != 1)
  {
    fprintf(2, "Usage: primes");
    exit(1);
  }
  int p[2];
  pipe(p);
  int pid = fork();
  if (pid < 0)
  {
    exit(1);
  }
  if (pid == 0)
  { 
    close(p[1]);
    cal(p);
  }
  else
  {
    for (int i = 2; i < 35; i++)
    {
      write(p[1], &i, sizeof(i));
    }
    close(p[1]);
  }
  wait(0);
  exit(0);
}

重要的hint就是read returns zero when the write-side of a pipe is closed.
所以需要判断是否读出0。

另一点是fork了以后fd同样被复制了一份，所以在pipe用完以后要在父子进程中都关闭fd。

find

int my_strncmp(const char *s1, const char *s2, int n)
{
  for (int i = 0; i < n; i++)
  {

    if (s1[i] != s2[i] || s1[i] == '\0' || s2[i] == '\0')
    {
      return (unsigned char)s1[i] - (unsigned char)s2[i];
    }
  }
  return 0;
}

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

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

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

  switch (st.type)
  {
  case T_FILE:
    if (mode == 1)
    {
      uint l1 = strlen(name);
      uint l2 = strlen(path);
      if (l2 >= l1)
      {
        if (my_strncmp(path + l2 - l1, name, l1) == 0)
        {
          printf("%s\n", path);
        }
      }
    } else {
      printf("%s\n", path);
    }
    break;
  case T_DIR:
    if (strlen(path) + 1 + DIRSIZ + 1 > sizeof buf)
    {
      printf("ls: 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;
      if (!strcmp(de.name, ".") || !strcmp(de.name, ".."))
        continue;
      memmove(p, de.name, DIRSIZ);
      p[DIRSIZ] = 0;
      if (stat(buf, &st) < 0)
      {
        printf("find: cannot stat %s\n", buf);
        continue;
      }
      find_name(buf, name, mode);
      *p = 0;
    }
    break;
  }
  close(fd);
}

int main(int argc, char *argv[])
{
  if (argc == 1)
  {
    find_name(".", "", 0);
  }
  if (argc == 2) // only path no name
  {
    find_name(argv[1], "", 0);
  }
  if (argc == 3) // path and name
  {
    find_name(argv[1], argv[2], 1);
  }
  exit(0);
}

通过维护一个buffer和p来记录当前的路径，对DIR里读出的特殊的.和..特判一下就好

xargs

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

#define MAXARG 10

int main(int argc, char *argv[])
{
  char buf[512];
  char *cmd[MAXARG];

  if (argc < 2)
  {
    fprintf(2, "Usage: xargs command\n");
    exit(1);
  }

  for (int i = 1; i < argc; i++)
  {
    cmd[i - 1] = argv[i];
  }
  int fd = 0;
  char *p;
  while (1)
  {
    p = buf;
    int n = 0;
    while (1)
    {
      if (read(fd, p, 1) != 1)
      {
        *p = 0;
        break;
      }
      if (*p == '\n' || *p == '\r')
      {
        *p = 0;
        break;
      }
      if (++n >= sizeof(buf))
      {
        fprintf(2, "xargs: line too long\n");
        exit(1);
      }
      p++;
    }

    if (n == 0)
    {
      break;
    }

    if (fork() == 0)
    {
      cmd[argc - 1] = buf;
      cmd[argc] = 0;
      exec(cmd[0], cmd);
      fprintf(2, "exec %s failed\n", cmd[0]);
      exit(1);
    }
    else
    {
      wait(0);
    }
  }
  exit(0);
}

一个exec教学（？

感觉jyy老师讲的很好，exec就是替换状态机（