More Pipes

Last time: Redirection and Pipes

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Project 8 is due Friday.
• Final exam is scheduled for Friday May 4, 8-11am in Maginnes 290
• Wednesday we will discuss character sets and internationalization
• Friday will only be review -- bring your questions!
• Today:
  • Return Short Quiz 10
  • More on Redirection and Pipes
  • Course Evaluation

Final Technical Details About Pipes

We didn't quite get to this slide on Friday, but we did figure out some of these facts on our own.

• read() on a pipe blocks until data appear
• write() on a pipe blocks until space is available in the pipe
• When all writers close the writing end, read() returns 0 (i.e., EOF)
• When all readers close the reading end, write() causes SIGPIPE

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 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()
  • fclose() just closes the connection.
  • 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?