Notes on Wait System Call Lecture

Introduction

• Lecture on the weight system call and its relation to child process signals.
• Previously discussed the exit system call and its execution when a process terminates.

Exit System Call

• Every process executes the exit system call on termination (explicitly via exit or implicitly on returning from main).
• Involves two key signals:
  • SIGINT: Resets the leader process ID if the process is a leader.
  • SIGCHLD: Notifies the parent process of the death of a child process.

Death of Child Signal (SIGCHLD)

• SIGCHLD is sent by the terminating child process to its parent.
• By default, the parent ignores the SIGCHLD signal, which could lead to the child becoming a zombie process.

Wait System Call

• The wait system call allows the parent to manage child processes and retrieve their exit status.
• Discussion on the necessity of wait and its algorithm, including pseudo code.
• Mentioned in Morris Bach's book, pages 212 to 270.

Example Code Explanation

• Code example discussed where a parent and child process are created.
• The parent sleeps for two seconds while the child displays its ID and exits.
• If the parent ignores the SIGCHLD signal, the child remains a zombie.
• If the parent catches the signal, it handles it but does not automatically reap the child; this requires executing wait.

Importance of Handling SIGCHLD

• If the parent handles the signal, it can perform actions like logging but needs to call wait to properly clean up and avoid zombie processes.

Reaping Zombie Processes

• If the parent process dies, the init process automatically reaps the zombie child process.
• Only the wait system call allows the parent to reap its child manually.

Advantages of Using Wait

• No need to guess how long to wait, as wait blocks until the child terminates.
• wait can return the child process ID and the exit code of the child.
• Exit codes are typically passed back as integer values, with higher bits for status.

Variants of Wait System Call

1. Multiple Children: Parent can wait for specific children or all children to terminate.
2. Waitpid: Allows the parent to specify which child process to wait for.

Summary of Key Points

• The wait system call synchronizes the parent with the child's termination.
• Importance of properly reaping child processes to avoid zombies.
• Various implementations of the wait system call can handle multiple child processes efficiently.

Discussion on Assignments

• Students encouraged to complete assignments related to synchronization using the wait system call.
• Offers to assist with any challenges faced during assignments.