Understanding Processes: Basics, Forking, and Lifecycle in Operating Systems

Process

A process is an execution instance of a program on a data set, serving as the basic unit for resource allocation and scheduling in an operating system. It only runs after the system creates the necessary resources.

Difference Between Process and Program

A program is a static file containing ordered code stored on disk, while a process is the running instance of that program with allocated memory, variables, and resources such as heap and stack. When the program finishes, the process ends, but the program file remains.

Process Characteristics

Concurrency: multiple processes can progress simultaneously at a macro level.

Dynamic: processes are created, evolve, and terminate dynamically.

Independence: each process is the basic unit for system resource allocation.

Interaction: processes can communicate with each other.

Asynchrony: processes do not advance in lockstep.

Structure: each process has a control block (PCB).

Parallelism: true parallel execution requires multiple CPUs or processors.

Concurrency vs. Parallelism

Concurrency is the macro‑level perception that processes advance together, achieved by the CPU time‑slice scheduling mechanism; the eye cannot see the rapid switches. Parallelism means multiple processes truly execute at the same instant, which only occurs on multi‑CPU systems.

Creating Multiple Processes

Processes can be created by the operating system or by a parent process. A child process inherits almost all resources from its parent, and child processes can further create their own children, forming a process family.

Orphan Process

An orphan process is one whose parent has terminated before it; the init process (PID 1) adopts such a process.

Zombie Process

A zombie process occurs when a child terminates but its parent has not yet performed a wait, leaving the kernel retain minimal information about the child. The zombie occupies resources until the parent reaps it.

Process Termination Methods

Returning from the main function.

Calling exit().

Calling _exit() or _Exit().

The last thread returning from its start routine.

The last thread calling pthread_exit().

Calling abort() (generates SIGABRT).

Receiving a terminating signal.

The last thread responding to a cancellation request.

Process Group

Every process belongs to a process group, which is a collection of one or more related processes that can receive signals from the same terminal. Each group has a unique group ID, and the process whose PID equals the group ID is the group leader.

Session

A session is a collection of one or more process groups.

Race Condition

A race condition arises when multiple processes access shared data and the final result depends on the order of execution, which is determined by the kernel scheduler and system load. Using wait / waitpid can serialize execution but defeats the purpose of concurrency. Instead, synchronization mechanisms such as signals or inter‑process communication (IPC) should be employed to avoid race conditions.