Redirection and Pipes

Last time: Processes and Inter-Process Communication

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Reminder: Project 9 is due Wednesday.
• Homework 8 is online, due Friday.
• Today:
  • Redirection and Pipes

Redirection Principles

• Every program gets three open file descriptors at startup
  • 0: stdin, 1: stdout, 2: sterr
• These are often attached to the tty
• Most UNIX tools send output to stdout and provide no way to output to a file
• When you want output to a file, we use shell redirection
  • Tool is unaware of the redirection

Redirection Principles

• For every process, the kernel maintains an array of open files.
• A file descriptor is an index into that array.
• Useful: When you open a file, you ALWAYS get the lowest available spot in that array.

Redirection Techniques

• How to attach stdin to a file
• Method 1: close ... open
  • Standard Plumbing: File descriptors 0,1,2 attached to /dev/tty
    • 0 for reading, 1 for writing, 2 for writing
  • close(0): if the process closes file descriptor 0, that entry in its array of i/o channels is free
  • fd = open("file",0): if the process opens another file, that connection is attached to the FIRST FREE entry in the array of i/o channels
  
   
  
• See example stdinredir1.c

Redirection Techniques

• How to attach stdin to a file
• Method 2: open, close, dup, close

  fd = open("data", 0);
  close(0);
  dup(fd);
  close(fd);
  

Redirection Techniques

• How to attach stdin to a file
• Method 3: uses dup2( origfd, dupfd )

  fd = open("data", 0);
  dup2(fd, 0);
  close(fd);
  

• dup2 closes 0, and dups fd to 0
   
  
• See example stdinredir2.c

Redirection I/O for Another Program

• Q: How does the shell allow "who > userlist.txt" without changing who?

Redirection I/O for Another Program

• Q: How does the shell allow "who > userlist.txt" without changing who?
• A: It forks() [as we discussed before]
• Before running who, the child close()s fd 1, creat()s userlist.txt which means it is open for writing (at fd 1)
• The exec()d program (who) inherits the file descriptor array and so who's stdout is attached to userlist.txt
   
  
• See example whotofile.c

Understand redirection?

• Q: How to implement:
  1. who >> userlog
  2. sort < data

Pipes

• Now let's consider implementing "who | sort"
   
  
• Q: What really is a pipe?

Pipes

• Now let's consider implementing "who | sort"
   
  
• Q: What really is a pipe?
• A: A one-way data channel in the kernel with a reading end and a writing end
• The system call pipe(2) creates a pipe in the OS and connects it to two file descriptors.
• Usage: pipe(int a[2]);
• a[0] is the file descriptor of the reading end, and a[1] is the file descriptor of the writing end.

Pipes Example

• We could create a pipe and have the same program write into it and read out of it, but that is not particularly useful.
• Typically a pipe is used to send data from one process to another.
• Let's use fork() to create a new process that can share the pipe.
• See pipedemo2.c
  • If the child writes into the pipe, the parent can read those bytes.
• We are close to being able to re-implement "who | sort". Shall we try?

Final Technical Details About Pipes

• 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