Process Tree

 

🧠 Experiment: Process Creation Using fork() and Viewing the Process Tree

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🎯 Aim:

To create a new process using the fork() system call, print the parent and child process IDs, and use the pstree command to observe the process tree starting from the init (systemd) process.

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🧩 Theory:

• The fork() system call is used to create a new process.

• The newly created process is called the child process, and the original is the parent process.

• After a successful fork(), two processes run concurrently:

  • Both execute the same code following the fork()

  • But they receive different return values:

    • 0 in the child process

    • The child’s PID in the parent process

• The init (or systemd) process is the ancestor of all processes in Linux.

The pstree command displays processes in a hierarchical tree format, showing parent–child relationships.

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🧪 Step 1: Write the C Program

Create a file named fork_example.c:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main() {
    pid_t pid;

    pid = fork();   // create a new process

    if (pid < 0) {
        perror("Fork failed");
        return 1;
    } 
    else if (pid == 0) {
        // Child process
        printf("Child Process:\n");
        printf("   Child PID: %d\n", getpid());
        printf("   Parent PID: %d\n", getppid());
    } 
    else {
        // Parent process
        printf("Parent Process:\n");
        printf("   Parent PID: %d\n", getpid());
        printf("   Child PID: %d\n", pid);
        sleep(5);  // delay so pstree can show both processes
    }

    return 0;
}

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🧪 Step 2: Compile and Run

gcc fork_example.c -o fork_example
./fork_example

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🧾 Sample Output:

Parent Process:
   Parent PID: 3412
   Child PID: 3413
Child Process:
   Child PID: 3413
   Parent PID: 3412

This shows that:

• The parent has PID 3412

• The child has PID 3413

• The child’s parent PID matches the parent’s PID

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🧪 Step 3: Observe the Process Tree

While the program is running (especially during the 5-second sleep), run in another terminal:

pstree -p 3412

or, to view the entire tree from the init process:

pstree -p 1

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🧾 Sample Output (Simplified):

systemd(1)─┬─bash(3200)─┬─fork_example(3412)─┬─fork_example(3413)
            │            └─pstree(3420)

Explanation:

• systemd(1) is the init process (PID 1)

• bash(3200) is your shell (parent of your program)

• fork_example(3412) is the parent process created by you

• fork_example(3413) is the child process created by fork()

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🧩 Step 4: Understanding Process Hierarchy

• Every process (except init/systemd) is a child of another process.

• The fork() system call duplicates the parent process.

• Using pstree, you can visualize this relationship easily.

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🧩 Step 5: Verify with ps command

You can also verify the parent-child relationship using ps:

ps -ef | grep fork_example

Sample Output:

user    3412 3200  0 11:20 pts/0    00:00:00 ./fork_example
user    3413 3412  0 11:20 pts/0    00:00:00 ./fork_example

This shows:

• The child (3413) has parent 3412

• The parent (3412) has parent 3200 (bash)

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🧩 Step 6: After Process Termination

Once both processes finish, re-running pstree will no longer show them — they were temporary.

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🧾 Summary Table

Concept	Command / Output
Create process	fork() system call
Parent PID	getpid()
Child PID	Returned by fork()
Parent of child	getppid()
Show process tree	pstree -p
Show parent-child relationship	`ps -ef

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✅ Result:

• A new process was successfully created using the fork() system call.

• The parent and child PIDs were displayed.

• The pstree command was used to visualize the process hierarchy, confirming that both parent and child originate from the init (systemd) process.

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🧠 Extension Exercise (for students):

Modify the program to:

1. Have the child execute another command using execlp() (e.g., ls).

2. Observe the change in process tree.

3. Notice that the child replaces its code with the new program.