06 - Pipes

Question 1

Explain the pipe operation in the shell.

Answer 1

• Connects the standard output of one program to the standard input of another
• A chain of programs connected in this way is called a pipeline
• All the stages in a pipeline run concurrently
  • Each stage waits for input on the output of the previous one
  • No stage has to exit before the next can run
• It is unidirectional
  • p1 | p2 p1 -> p2
  • Impossible to pass information back
• Protocol for passing data is simply the receiver’s input format

Question 2

Give two examples of pipe types.

Answer 2

• File operations (read/write)
• Processes - (related -father/son or brothers)

Question 3

Describe pipe creation with relevant code.

Answer 3

• Pipe creation

  •   int pipe(int pipefd[2]);
      int pipe2(int pipefd[2], int flags); /* O_NONBLOCK */

• Opens two files
  • fd[0] descriptor open for reading
  • fd[1] descriptor open for writing
• Returns
  • 0 successful
  • 1 unsuccessful (errno variable set)
• Pipes can only connect processes with a common antecessor
• Pipe information is managed as an open file

Question 4

Describe the communication process in pipes with relevant code.

Answer 4

• Communication (data read/write)

  • ssize_t read(int fd, void *buf, size_t count);
    ssize_t write(int fd, void *buf, size_t count);

• 1st argument is the file descriptor (fd[0] or fd[1])
• 2nd argument is the data buffer address (data destination/source)
• 3rd argument number of bytes to read/write
• Return the number of bytes read/write
• Blocks or not (O_NONBLOCK)

Question 5

What happens if a process attempts to read from an empty pipe?

Answer 5

read(2) will block until data is available

Question 6

What happens if a process attempts to write to a full pipe?

Answer 6

write(2) blocks until sufficient data has been read from the pipe to allow the write to complete

Question 7

How can nonblocking I/O be achieved?

Answer 7

Using O_NONBLOCK status flag

Question 8

What is the communication channel provided by a pipe?

Answer 8

It is a byte stream, so there is no concept of message boundaries.

Question 9

What is the major limitation of pipes?

Answer 9

Processes should be related.

Question 10

How to implement pipes that are accessible by other processes?

Answer 10

• Give a name to the pipes
• Register them in the file system

Question 11

Explain the process of closing pipes.

Answer 11

• Closing all write ends makes the read return 0
• Closing all read ends makes the write produce SIGPIPE
• After, fork processes should close not needed ends

Question 12

List the characteristics of pipes.

Answer 12

• Implementation – Kernel/syscall
• Scope - local
• No Duplex
• Time-coupling
• Space-coupling +-
• Explicit
• Synchronization – Yes, by default
• Process relation - related
• Identification - NA
• API – file operations

Question 13

Name an alternative to pipes to solve its limitations, what problem they fix and what characteristics change.

Answer 13

FIFOs, also called named pipes.

• Can be used by unrelated processes (Process relation - unrelated)
• Referred and identified by a file in the file system (Identification - file name)
• Created in a different way. Instead of an anonymous communications channel, FIFO is entered into the file system by calling mkfifo()

Question 14

Explain process creation.

Answer 14

To start a new process call:

\#include \<unistd.h\>
pid_t fork();

• A new process is created
• The new instruction to be executed is the one following the fork (on the father and son)
• All variables are duplicated with the same value. After the fork, all changes in values are local to each process
• Fork is a system call that returns two different values
  • In the son, returns 0
  • In the father, return the son’s PID

Slide 19, 20, chapter 6

Question 15

Explain UNIX process creation and management.

Answer 15

Unix process creation - two steps

• Copy of the current process
• Optional - execution of a different program (slide 17, chapter 6)

Management

• UNIX fork - system call to create a copy of the current process and start it running with no arguments
• UNIX wait - system call to wait for a process to finish

Question 16

How to execute different programs with a fork?

Answer 16

Replace the code after the fork.