More Pipes

Last time: Pipes

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Reminder: Project 9 is due today.
• Reminder: Homework 8 is due Friday.
• HW5: High 10, Low 6, Mean 9.1
• HW7: High 10, Low 9, Mean 9.7
• Project 6: High 100, Low 50, Mean 86
• Today:
  • Revisit fork/wait question from last time
  • More Pipes

The wait(3c) system call

• There are a few variations, including waitpid(3c) and waitid(2) that permit the calling process to specify a particular process ID or process group ID (and only wait for them, not all children).
• However, in all cases, the wait() family will only reap zombies that are the immediate children of the current process.
• When a child loses a parent, the special init process becomes its new parent. Init will periodically call wait() to reap any zombie children.

System Programming

• UNIX has three types of data channels, all using the same type of i/o interface
  • disk/device files -- use open() to connect, use read() and write() to transfer data
  • pipes -- use pipe() to create, fork() to share, and use read() and write() to transfer data
  • sockets -- use socket(), listen(), accept(), connect() and use read() and write() to transfer data
    • To learn about sockets and how other aspects of networking work, consider taking CSE342.

bc: a UNIX calculator

• bc is an arbitrary precision calculator. It has variables, loops, functions, etc.
• Try it -- it supports normal order of operations.
• But use ps (in the same terminal after suspending bc) to see what's going on...
   
  

bc: a UNIX calculator

• bc is an arbitrary precision calculator. It has variables, loops, functions, etc.
• Try it -- it supports normal order of operations.
• But use ps (in the same terminal after suspending bc) to see what's going on...
   
  
• bc does not do calculations -- dc does the calculations

Client-server model

• dc provides a service (calculation)
  • it has a well-defined language (RPN)
  • it communicates via stdin and stdout
• bc provides a user interface and uses dc's services
  • bc is called a client of dc

Coding bc

• More pipe(), fork(), dup(), exec()

  int todc[2], fromdc[2];
  pipe(todc); pipe(fromdc);
  fork();
  if ({parent}) {
    close(todc[0]);
    close(fromdc[1]);
  
    repeat
      readline from user
      convert
      write(todc[1], cmd, length)
      read(fromdc[0], reply)
      printf to user
  

  Client would similarly exec dc.

popen() -- making processes look like files

• recall fopen() -- create buffered stream to a file

  FILE *fp = fopen("stuff", "r");
  c = getc(fp); fgets(buf, len, fp); fscanf(fp, ...)
  fclose(fp);
  

• meet popen(cmds, how) -- creates a buffered stream to a program so that you can read and write to this process

  FILE *fp = popen("ls", "r");
  fgets(buf, len, fp);
  pclose(fp);
  

• Args to popen() -- any shell command, "r" or "w", not "a"

more popen() examples

• A simple popen() demo
• A program that sends mail when certain errors occur:

  fp = popen("mail admin", "w");
  fprintf(fp, "Major error!\n");
  pclose(fp);
  

• Note that you need to use pclose()
  • Reason: fclose() just closes the connection.
  • However, we need to wait() for a process, or it becomes a zombie.
  • pclose() wait()s for the child to exit.

Writing popen() featuring fdopen()

• Idea: create a pipe to a program and convert that pipe into a FILE *
• Fact: a pipe is a pair of file descriptors. To convert a file descriptor into a FILE *, use fdopen(fd, "r");
• Approach:

  pipe(p);
  rv = fork();
  if (!rv) { // child
    close(p[0]);
    dup2(p[1],1);
    close(p[1]);
    execl("/bin/sh", "sh", "-c", cmd, NULL);
  } else { // parent
    close(p[1]);
    fp = fdopen(p[0], "r");
    return fp;
  }
  

• Write your own version of popen() using this approach.

That's it

• If you have just been reading along, consider going back and trying out some of the ideas
• Now you can see an implementation of popen() -- it's in the notes for this page, which you can see by pressing the +- button at the bottom of this page.