More Pipes

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Reminder Project 8 (last one!) is due Thursday.
• Reminder: Final exam is scheduled for Wednesday May 4, noon-3pm in Packard 416
  • Contact me if you already have two finals scheduled for that day to arrange to take the exam earlier.
• Today
  • Return p7: range 28-100, mean 89
  • Short Quiz 10
  • More Pipes
  • Course Evaluation

System Programming

• UNIX has three types of data channels, but one 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 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 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
  

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

  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);
  fork();
  if (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;
  }
  

• Now we can write popen().

Course Evaluation

• Additional questions:
  1. How prepared are you for future work requiring programming in C?
  2. How much have you learned about using and programming in UNIX?
  3. What did you like best about the course?
  4. What should be changed for the next time the course is taught?