Lecture 4: Process / Shell (Part 2)

Question 1

When a fork() occurs, what is copied?

Answer 1

Everything about the original process including:

• current line of code program is at
• copy of all variables and their values
• the process’s file descriptor table where whatever its entries are pointing at will be pointing to the same entries

Question 2

When is it considered to be better to open files: before a fork() or after a fork()?

Answer 2

After a fork() because then the child’s file will then get its own entry in the System File Descriptor table and behaviour with this file will not affect the parent’s file

Question 3

When a pipe is created via “int pipe(int fields[2])”, what is used for reading and writing?

Answer 3

filedes[0] is used for reading

filedes[1] is used for writing

Question 4

Be default: if a writing process attempts to write to a full pipe, what happens?

Answer 4

System will automatically block the write until there is space. The OS has a limit on the buffer space used by the pipe. If you hit the limit, the write will block.

Question 5

By default: if a read process attempts to read from an empty pipe, what happens?

Answer 5

System will automatically block the read until there is data to read.

Question 6

What does the dup() function do?

Answer 6

It re-routes input and/or output to a user-specified destination. This function is critical to utilizing pipes properly.