Processes 2: Primary process system calls

Overview

In this lecture, we finish our discussion of the primary process control system calls, then discuss inter-process communication, including termination notification and stream communication.

Full lecture notes — Textbook readings

Primary process control system calls

• What sub-tasks are required for process coordination?
• Create a process: fork
• Run a program: exec family
• Exit a process: _exit
• Stop a process: kill
• Await completion: waitpid

fork vs. execvp

• fork creates a new process
  • Cloned program image
  • New identity
  • Cloned environment view
  • Shared underlying environment
• execvp starts a new program in the current process
  • New program image
  • Unchanged identity
  • Unchanged environment view
  • Shared underlying environment

Putting them together: processex

fork: Which runs first?

• forkorder.cc

The uniq utility

• uniq searches for consecutive duplicate lines

  Example 1
  Example 2
  Example 2
  Example 3
  Example 2

• uniq: Print only one of each set of duplicates
• uniq -c: Precede each line with a count of duplicates
• uniq -u: Only print non-repeated lines
• uniq -d: Only print repeated lines

Again: fork: Which runs first?

• forkorder.cc

Voluntary termination: Exit

• exit (= return from main)
  • Process transcends the earthly sphere, leaving with its last breath a message for its parent
  • exit(STATUS) library function performs cleanup actions, such as flushing stdio files
  • _exit(STATUS) system call exits without cleanup
• The STATUS is the process’s ‘last words’
  • Remembered by kernel
  • Can be collected by the process’s parent
• Status convention
  • 0 (EXIT_SUCCESS) means success, non-zero (1-255) means failure

Speed of exit notification

• Exiting allows a process to communicate a single byte’s worth of data to its parent!
• How fast is this inter-process communication channel?
• storage1/r-exitbyte 🤡🤪

Involuntary termination

• A process can transcend involuntarily (segmentation fault; being killed)
  • This is called termination by signal
• The exit status indicates the reason for transcendence (exit vs. signal)

Collecting a termination notification

• waitpid(pid, &status, options)
  • Complex system call! Read the manual page! Check the return values!
  • options == 0 means blocking, options == WNOHANG means nonblocking
  • pid == -1 means wait for any child process, pid > 0 means wait for that specific process
  • Returns the terminated process’s ID (or 0 or -1)
  • Can only wait for children
• On success, int status filled in with the status
  • WIFEXITED(status) ≠ 0 iff process terminated by exit
  • If WIFEXITED, then WEXITSTATUS(status) is the exit status

Question

• Fill in the ???s with the most complete assertion you can relating the ps! (Assume fork does not fail.)

pid_t p1 = getpid();
pid_t p2 = getppid();
pid_t p3 = fork();
pid_t p4 = getpid();
pid_t p5 = getppid();
assert(???);

Some answers

• assert(p1 > 0 && p2 > 0 && p4 > 0 && p5 > 0): all process PIDs are >0
• assert(p3 >= 0): fork did not fail (it’s >0 in parent, 0 in child)
• assert(p1 != p2 && p4 != p5): a process’s PID ≠ its PPID
• assert(p4 != p2): new PID ≠ original PPID
• assert(p1 != p3 && p2 != p3): child PID ≠ parent or grandparent PID
• assert(p3 != 0 ? p1 == p4 : p1 == p5)
  • In parent (p3 != 0), original PID == new PID
  • In child (p3 == 0), original PID == new PPID
• assert(p3 != 0 ? p2 == p5 : p2 != p5)
  • In parent (p3 != 0), original PPID == new PPID
  • In child (p3 == 0), original PPID ≠ new PPID