Thread Creation and Synchronization

 

Creation and Synchronization of Threads using pthread_create() and pthread_join()

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1. Aim

To study and implement multithreading using POSIX threads and understand thread creation and synchronization using pthread_create() and pthread_join().

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2. Objectives

• To create multiple threads in a process

• To pass arguments to threads

• To synchronize threads using pthread_join()

• To observe concurrent execution of threads

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3. Theory

Thread

A thread is the smallest unit of execution within a process.
All threads of a process:

• Share the same address space

• Share code and global data

• Execute independently

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pthread_create()

pthread_create() is used to create a new thread.

When called:

• A new thread is created

• It starts executing a specified function

• Both threads run concurrently

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pthread_join()

pthread_join() is used for thread synchronization.

• The calling thread waits

• Execution resumes only after the specified thread finishes

This prevents premature termination of the main thread.

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4. Problem Description

Create two threads:

• Thread 1 adds two numbers

• Thread 2 adds another two numbers
  The main thread waits for both threads and prints the largest sum.

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5. Algorithm

Main Thread

1. Declare thread variables

2. Initialize input values

3. Create two threads using pthread_create()

4. Wait for both threads using pthread_join()

5. Compare results and display the largest sum

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Thread Function

1. Receive input data

2. Add two numbers

3. Store result

4. Exit thread

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6. Program (C Implementation)

#include <stdio.h>
#include <pthread.h>

/* Structure to pass data to thread */
typedef struct {
    int a;
    int b;
    int sum;
} data_t;

/* Thread function */
void *add(void *arg) {
    data_t *d = (data_t *)arg;
    d->sum = d->a + d->b;
    return NULL;
}

int main() {
    pthread_t t1, t2;

    data_t d1 = {10, 20, 0};
    data_t d2 = {15, 25, 0};

    pthread_create(&t1, NULL, add, &d1);
    pthread_create(&t2, NULL, add, &d2);

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    printf("Sum calculated by Thread 1 = %d\n", d1.sum);
    printf("Sum calculated by Thread 2 = %d\n", d2.sum);

    if (d1.sum > d2.sum)
        printf("Largest Sum = %d (Thread 1)\n", d1.sum);
    else
        printf("Largest Sum = %d (Thread 2)\n", d2.sum);

    return 0;
}

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7. Explanation of Important Functions

pthread_create()

pthread_create(&t1, NULL, add, &d1);

• Creates a new thread

• Executes the function add()

• Passes input data using structure

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pthread_join()

pthread_join(t1, NULL);

• Makes the main thread wait

• Ensures the thread finishes before proceeding

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8. Compilation and Execution

gcc thread_example.c -lpthread
./a.out

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9. Sample Output

Sum calculated by Thread 1 = 30
Sum calculated by Thread 2 = 40
Largest Sum = 40 (Thread 2)

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10. Observations

• Threads execute concurrently

• Each thread performs independent computation

• Main thread waits for completion

• No race condition occurs

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11. Result

The program successfully demonstrated thread creation and thread synchronization using pthread_create() and pthread_join().

Important Functions to Learn

1️⃣ pthread_create()

What does it do?

👉 pthread_create() is used to create a new thread in a process.

When it is called:

• A new thread is created

• The new thread starts executing a specified function

• The parent (main) thread continues execution in parallel

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Syntax

pthread_create(thread_id, attributes, function, argument);

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Simple Meaning of Each Parameter

Parameter	Meaning
thread_id	Stores the ID of the newly created thread
attributes	Thread properties (NULL = default)
function	Function executed by the new thread
argument	Data passed to the function

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Simple Example

pthread_create(&t1, NULL, add, &data);

What happens here?

• A new thread is created

• The function add() starts executing in that thread

• &data is passed to the function

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Analogy

Think of pthread_create() as:

“Start a new worker to do this job.”

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2️⃣ pthread_join()

What does it do?

👉 pthread_join() makes the calling thread wait until another thread finishes execution.

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Syntax

pthread_join(thread_id, return_value);

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Simple Meaning of Each Parameter

Parameter	Meaning
thread_id	Thread to wait for
return_value	Value returned by the thread (usually NULL)

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Simple Example

pthread_join(t1, NULL);

What happens here?

• The main thread waits

• It resumes only after t1 finishes

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Analogy

Think of pthread_join() as:

“Wait until this worker finishes before continuing.”

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3️⃣ Why are both needed together?

Without pthread_join():

• Main thread may finish early

• Program may exit before child threads complete

With pthread_join():

• Main thread waits

• Ensures correct and complete execution