Xv6

Lab: Xv6 and Unix utilities

pingpong

pingpong (easy) Write a program that uses UNIX system calls to ‘‘ping-pong’’ a byte between two processes over a pair of pipes, one for each direction. The parent should send a byte to the child; the child should print “: received ping”, where is its process ID, write the byte on the pipe to the parent, and exit; the parent should read the byte from the child, print “: received pong”, and exit. Your solution should be in the file user/pingpong.c.

Some hints:

Use pipe to create a pipe.
Use fork to create a child.
Use read to read from the pipe, and write to write to the pipe.
Use getpid to find the process ID of the calling process.
Add the program to UPROGS in Makefile.
User programs on xv6 have a limited set of library functions available to them. You can see the list in user/user.h; the source (other than for system calls) is in user/ulib.c, user/printf.c, and user/umalloc.c. Run the program from the xv6 shell and it should produce the following output:

    $ make qemu
    ...
    init: starting sh
    $ pingpong
    4: received ping
    3: received pong
    $

Your solution is correct if your program exchanges a byte between two processes and produces output as shown above. pingpong诚然是比较简单的一个程序,但是需要了解的system function如fork和pipe却都并不简单.

fork

使用chatgpt可以大概了解fork的阅读xv6-book 也可以大概了解fork.

dirent stat

#define DIRSIZ 14

struct dirent {
  ushort inum;
  char name[DIRSIZ];
  char isvalid;
};
``` c
struct stat {
  short type;  // 文件类型，如 T_DIR, T_FILE 等
  int dev;     // 磁盘设备号
  uint ino;    // inode 号
  short nlink; // 硬链接数
  uint size;   // 文件大小（字节）
  uint mtime;  // 最近的修改时间
};

暂时就了解这么多吧,真正底层的东西在后面文件系统中会详细学到.

int fd;
  // dirent 即目录项,
  struct dirent de;
  struct stat st;

  if((fd = open(path, 0)) < 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:
    printf("%s %d %d %l\n", fmtname(path), st.type, st.ino, st.size);
    break;

ls和find会利用到stat结构体中的type来判断文件类型

buf缓冲区

使用buf缓冲区来作为绝对路径,使用char *p来作为文件名,通过加’/‘前缀拼接在buf后面 p = buf + strlen(buf);

void find(char *path,char *target){
    int fd;
    struct dirent dt;
    struct stat st;
    char buf[512],*p;
    if ((fd = open(path,0)) < 0)
    {
      printf("cann't not open %s\n",path);
      return;
    }
    if(fstat(fd,&st)<0){
      printf("can't stat %s\n",path);
      close(fd);
      return;
    }
    if(st.type!=T_DIR){
      fprintf(2,"Usage: find: %s is not a directory",path);
      close(fd);
      return;
    }
    strcpy(buf,path);
    //buf是绝对路径,p是一个文件名,通过加'/'前缀拼接在buf后面
    p=buf+strlen(buf);
    *p++='/';
    //读取fd,如果read返回字节数和de长度相等则循环
    while(read(fd,&dt,sizeof(dt))==sizeof(dt)){
      if(dt.inum==0){
        continue;
      }
      if(!(strcmp(dt.name,"."))||!(strcmp(dt.name,".."))){
        continue;
      }
      memmove(p,dt.name,DIRSIZ);
      //设置文件名结束符
      p[DIRSIZ]=0;
      //int stat(char*,DIRSIZ);
      //stat 系统调用,可以获得一个已存在文件的模式,并将此模式赋值给它的副本
      if(stat(buf,&st)<0){
        fprintf(2,"find: can't stat %s\n",buf);
        continue;
      }
      if(st.type==T_DIR){
        find(buf,target);
      }
      else if(st.type==T_FILE&&!strcmp(dt.name,target)){
        printf("%s\n",buf);
      }
    }   
}

system call

在你开始写代码之前，请阅读xv6手册《book-riscv-rev1》的第2章、第4章的第4.3节和第4.4节以及相关源代码文件：

系统调用的用户空间代码在user/user.h和user/usys.pl中
内核空间代码是kernel/syscall.h、kernel/syscall.c
与进程相关的代码是kernel/proc.h和kernel/proc.c

Xv6#

Lab: Xv6 and Unix utilities#

pingpong#

fork#

dirent stat#

buf缓冲区#

system call#

Xv6