Process creation and executing another process

 

🎯 Aim:

1. To write a program that adds two integers provided via the command line and compile it to an executable named myadder.

2. To write another program that uses fork() to create a child process and make the child process execute the myadder program using execvp().

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🧩 Theory:

• The fork() system call creates a new process (a child) by duplicating the calling process.

• The exec() family of functions replaces the current process image with a new program.

• execvp() searches for the program in the PATH and executes it with given arguments.

• When a child process calls execvp(), its memory image is replaced with that of the new program — meaning the child becomes the new program (myadder in this case).

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🧪 Step 1: Program 1 — myadder.c

This program reads two integers from the command line, adds them, and prints the result.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s <num1> <num2>\n", argv[0]);
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[2]);
    int sum = a + b;

    printf("Sum of %d and %d is %d\n", a, b, sum);

    return 0;
}

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🧩 Compile and Test

gcc myadder.c -o myadder
./myadder 10 20

Output:

Sum of 10 and 20 is 30

✅ myadder is ready.

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🧪 Step 2: Program 2 — fork_exec.c

This program creates a child process using fork().
The child process executes the myadder program using execvp().

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main() {
    pid_t pid;
    pid = fork();   // Create a child process

    if (pid < 0) {
        perror("Fork failed");
        exit(1);
    } 
    else if (pid == 0) {
        // Child process
        printf("Child process (PID: %d) executing myadder...\n", getpid());

        // Arguments for myadder
        char *args[] = {"./myadder", "10", "20", NULL};

        // Replace current process with myadder
        execvp(args[0], args);

        // execvp returns only if it fails
        perror("execvp failed");
        exit(1);
    } 
    else {
        // Parent process
        printf("Parent process (PID: %d) created child (PID: %d)\n", getpid(), pid);
        wait(NULL);  // Wait for child to finish
        printf("Child completed execution.\n");
    }

    return 0;
}

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🧩 Compile and Run

gcc fork_exec.c -o fork_exec
./fork_exec

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🧾 Sample Output:

Parent process (PID: 4100) created child (PID: 4101)
Child process (PID: 4101) executing myadder...
Sum of 10 and 20 is 30
Child completed execution.

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🧩 Step 3: Explanation

Concept	Description
fork()	Creates a child process. Both parent and child continue execution.
pid == 0	Indicates the child process.
execvp()	Replaces the child’s process image with another executable (myadder).
wait()	Makes the parent wait until the child finishes.

After execvp():

• The child process stops executing fork_exec.c code.

• It becomes the myadder process.

• Once myadder completes, the child terminates, and the parent resumes.

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🧩 Step 4: Verify Process Replacement

While running fork_exec, observe the process list using another terminal:

ps -ef | grep myadder

You’ll see something like:

user  4101  4100  0 11:45 pts/0  00:00:00 ./myadder

➡️ This confirms that the child process has been replaced by the myadder image.

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🧩 Step 5: Observe Using pstree

Run pstree to visualize the process hierarchy:

pstree -p

Example Output:

systemd(1)─┬─bash(4000)───fork_exec(4100)───myadder(4101)

This shows:

• fork_exec created a child process

• The child was replaced by the myadder program

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🧾 Summary Table

Step	Operation	Function/Command	Observation
1	Create process	fork()	Child and parent created
2	Replace process image	execvp()	Child replaced with myadder
3	Synchronization	wait()	Parent waits for child
4	Verify tree	pstree	Visual process hierarchy

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✅ Result:

• A child process was successfully created using fork().

• The child process was replaced with the myadder executable using execvp().

• The parent process waited until the child finished execution.

• Verified using pstree that the child process became myadder.

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🧠 Extension Exercise (Optional for Students):

Modify fork_exec.c so that the user enters two integers, and the child passes them to myadder dynamically using execvp() arguments.