L7: Processes

Question 1

Process Creation:

Key Process Routines

Answer 1

• fork
• exec
• wait

Question 2

Process Communication:

Key Routines

Answer 2

These facilitate read/write between processes:

• pipe
• dup
• sockets

Question 3

Process Controlling:

Key Routines

Answer 3

Cause and Handle exeptions and interrupts:

• signal
• alarm

Question 4

3 Major Categories of

Process Operations

Answer 4

• Process Creation
• Process Communication
• Process Control

Question 5

fork() function:

Basic Idea

Answer 5

Clone an existing process,

then have it run an alternate set of code from

the parent.

The child “forks” from the parent.

Question 6

Which library contains the “fork()” function?

Answer 6

unistd.h

Question 7

Forking:

Sequence of steps

Answer 7

1. execute “fork()”
2. The complete parent state is cloned to create the child process.
   
   • Child is created.
3. The “fork()” call returns
   
   • pid of child, in parent process
   • 0, in child process
4. Return value is used to determine next step:
   
   • parent executes parent code
   • child executes child code

Question 8

Code:

Have Parent Process

wait for

Child Process to finish

Answer 8

int status;

if( fork() ) {

//parent process stuff

wait(&status); //wait for child to update status

exit(0); //parent exits

}

Question 9

exec() function:

Basic Use

Answer 9

1. Use fork() to get a child process
2. In the child process, use “execlp()” or “execvp()”
   
   • This will “overlay” the image of the new program onto the child image
   • The existing context(such as the file descriptors) is preserved
3. The Child process now runs as the second program, and exits with a status code
4. The parent process waits for the child to complete

Question 10

exec:

Two Formats

Answer 10

• execlp( filename, argv0, argv1, …, (char *)0);
  
  • The last parameter must be null
• execvp(filename, argv);
• filename can be a program or a script
• image of the current process is replaced, can only return if that fails
• Most Context is preserved
• stdio buffers are NOT FLUSHED
  
  • must close or flush them manually using fflush or fclose before exec

Question 11

What is a

Process?

Answer 11

• An instance of an executing program
• A fundamental concept in Computer Science and a powerful coding technique
• Performs independent computations
• Processes interact via messages
• Identified using a process id (pid)

Question 12

Possible Process States

Answer 12

• running
  
  • actually executing on machine
• sleeping
  
  • ready to run, waiting for its turn
• idle
  
  • sleeping for more than 20 seconds
• waiting
  
  • waiting for an event, such as disk I/O
• zombie
  
  • processes ended, but the parent has not yet recieved notification from the system