Unix Processes in C: Introduction

Realised by: Rahma Rahali

What is a process?

A process is simply a program in execution. When you open an application or launch a program, Linux creates a dedicated process for that program.

Each process has:

• A unique ID (PID): used by the system to track the process.
• Its own memory space: independent from the other processes.
• A state: in Linux, a process can transition between various states, each representing its current execution status or the specific resource or event it is awaiting. These states are: Running, Waiting (Blocked) and Terminated.
  • Running: the process is actively executing on the CPU.
  • Waiting (Blocked): the process is paused, waiting for a specific resource or event.
  • Terminated: the process has completed its execution or has been stopped by the system or a user.

The diagram below illustrates the transitions between those different states:

stateDiagram-v2
    [*] --> Running : Process launched
    Running --> Waiting : Waiting for a resource / event
    Waiting --> Waiting : Resource / event not available
    Waiting --> Running : Resource / event available
    Running --> Terminated : Process finished / stopped
    Waiting --> Terminated : Timeout reached

How are processes created in Linux?

In Linux, process creation relies on two key system calls:

1. fork(): creates an exact copy of the current process.
2. exec(): replaces the content of a process with a new program.

Inter-Process Communication (IPC)

In Linux, processes often need to communicate to share data or synchronize actions. Inter-Process Communication (IPC) provides several mechanisms for this purpose:

• Pipes: unidirectional channels for communication between related processes (a main process and its child process).
• FIFOs: bidirectional pipes that can be used by unrelated processes.
• Shared Memory: allows multiple processes to access the same memory region.
• Signals: asynchronous notifications sent between processes to signal events or actions.
• Message Queues: enable processes to send and receive messages in a queue.

Why are processes important?

Processes allow Linux to:

• Isolate programs: if one process crashes, it doesn’t affect the others.
• Run multiple programs simultaneously thanks to task scheduling (the scheduler).

Useful commands

Command	Description
ps	Lists running processes.
top or htop	Shows processes in real time.
kill	Terminates a running process.