Pipes

Depends on	Queues • Streams

In Unix (and similar operating systems) a ‘pipe’ is a means of passing a serial stream of data from one process to another. A pipe is a FIFO queue with two interfaces – one for inserting data and one for removing them.

Pipes can be created to link processes, as is illustrated in this exercise.

A typical use for pipes is as a redirection of standard I/O streams, often invoked from a shell. As a random sort of example, find how many filenames in the current directory contain the letter ‘a’: ls | grep a | wc

In an example like this (or, maybe, one which generates more traffic) the three child processes run independently, blocking if and when they are unable to input or output due to a pipe becoming empty or full or, possibly, timeslicing at other times.

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Named pipes

It is sometimes said of Unix “Everything looks like a file”. Whilst not entirely true, lots of things can look like files – including pipes. Like a file, these need a name.

Try this experiment:

• Find a nice, secluded directory somewhere and open two separate shell windows upon it.
• In one window, type mkfifo -m 666 FIFO
  (“FIFO” is the name, “666” specifies the access permission.)
• Try ls -lsa to see the pipe.
• Attach one window to the pipe output with: cat < FIFO
• Attach the other window to the pipe input with: cat > FIFO
• Type things in your input window.
• Marvel!
• When finished, try to remember to rm FIFO

Such a named pipe can also be created and used from within software etc. Note that (unlike a file) the open call can block; opening one end of the pipe will block the opening process until the other end is open too. In Unix see man 3 mkfifo for the C language interface, for example.

Similar mechanisms are available in Windows et alia, although the details of the interface are different.

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Standard Streams

Unix processes have three standard I/O streams, created when the process is started:

• stdin - the default input: typically the keyboard
• stdout - the default output: typically the display (window)
• stderr - the diagnostic output: typically also the display (window)

A process will inherit its streams from a parent, so, for example, a process started by a Shell will run ‘within’ the shell window unless instructed otherwise.

stdin and stdout can conveniently be redirected. One application is to use pipes to connect one process to another.

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Also refer to:	Operating System Concepts, 10th Edition: Chapter 3.7.4, pages 139-145

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