Multithreading in C

Hello there, fellow coder! Are you ready to dive into the advanced world of multithreading in C? If you’re nodding your head (or even if you’re not, we’ll win you over soon enough), you’re in the right place. Let’s unravel the mysteries of multithreading together, shall we?

What is a Thread?

In the realm of programming, a thread is a single sequence stream within a process. Think of it as a path that execution can follow. Threads are sometimes called lightweight processes because they have some properties of processes, but they’re not entirely independent. They share their code section, data section, and OS resources like open files and signals with other threads. But, like processes, a thread has its own program counter (PC), a register set, and a stack space.

Why Multithreading?

Multithreading is a popular way to improve application performance through parallelism. For example, in a browser, multiple tabs can be different threads. MS Word uses multiple threads, one thread to format the text, another to process inputs, and so on. Threads operate faster than processes for several reasons:

  1. Thread creation is much faster.
  2. Context switching between threads is much faster.
  3. Threads can be terminated easily.
  4. Communication between threads is faster.

Multithreading in C

Unlike Java, multithreading is not supported by the C language standard. However, POSIX Threads (or Pthreads) is a POSIX standard for threads. Implementation of pthread is available with the gcc compiler.

Code Examples

Let’s take a look at some code examples to better understand multithreading in C.

Basic Pthread Functions

Here’s a simple C program to demonstrate the use of pthread basic functions:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>  //Header file for sleep(). man 3 sleep for details.
#include <pthread.h>

void *myThreadFun(void *vargp){
    printf("Printing GeeksQuiz from Thread \n");
    return NULL;

int main(){
    pthread_t thread_id;
    printf("Before Thread\n");
    pthread_create(&thread_id, NULL, myThreadFun, NULL);
    pthread_join(thread_id, NULL);
    printf("After Thread\n");

In main(), we declare a variable called thread_id, which is of type pthread_t, an integer used to identify the thread in the system. After declaring thread_id, we call pthread_create() function to create a thread.

Multithreading with Global and Static Variables

As mentioned earlier, all threads share data segment. Global and static variables are stored in data segment. Therefore, they are shared by all threads. The following example program demonstrates the same.

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

int g = 0;

void *myThreadFun(void *vargp){
    int *myid = (int *)vargp;
    static int s = 0;
    ++s; ++g;
    printf("Thread ID: %d, Static: %d, Global: %d\n", *myid, ++s, ++g);

int main(){
    int i;
    pthread_t tid;
    for (i = 0; i < 3; i++)
        pthread_create(&tid, NULL, myThreadFun, (void *)&tid);
    return 0;

Wrapping Up

Multithreading in C, while not directly supported by the language standard, can be implemented using POSIX Threads (or Pthreads). It’s a powerful tool that can significantly improve the performance of your applications by allowing multiple operations to be executed concurrently. However, it’s important to manage threads carefully to avoid issues like race conditions and deadlocks.

Frequently Asked Questions (FAQ)

  • What is multithreading in C?

    Multithreading in C is a technique where a process, the smallest unit of execution, is divided into smaller parts known as threads. Thread is a flow of execution through the process code, with its own program counter, register set, and stack.

  • Is C good for multithreading?

    Yes, C is a great language for multithreading. Although the C language standard does not directly support multithreading, it can be implemented using POSIX Threads (or Pthreads).

  • Is C single or multi-threaded?

    By default, a C program is single-threaded, meaning it has only one thread of execution. However, you can create multi-threaded programs in C using libraries like Pthreads.

  • What is the difference between multithreading and multiprocessing in C?

    Multiprocessing involves multiple processes, each running in its own memory space. On the other hand, multithreading involves multiple threads within the same process, sharing the same memory space.

  • How does multithreading improve performance?

    Multithreading can improve performance by allowing multiple operations to be performed concurrently. This is particularly beneficial in applications that involve a lot of waiting, such as server applications.

  • How does multithreading improve performance?

    Multithreading can improve performance by allowing multiple operations to be performed concurrently. This is particularly beneficial in applications that involve a lot of waiting, such as server applications.

  • What are the challenges with multithreading?

    Multithreading can introduce challenges such as race conditions, where two threads try to access and modify the same data concurrently, and deadlocks, where two or more threads are unable to proceed because each is waiting for the other to release a resource.

  • What is a Pthread?

    Pthread, or POSIX Threads, is a POSIX standard for threads. It provides a way to create and manage threads in C.

  • How do you create a thread in C?

    You can create a thread in C using the pthread_create() function. This function takes four arguments: a pointer to a pthread_t variable, a pointer to a pthread_attr_t structure, the function to be executed by the thread, and a pointer to the arguments of the function.

  • How do you wait for a thread to finish in C?

    You can wait for a thread to finish in C using the pthread_join() function. This function blocks the calling thread until the specified thread terminates.

  • What are global and static variables in the context of multithreading?

    Global and static variables are shared by all threads in a process. This means that any changes made to these variables by one thread will be visible to all other threads.

If you enjoyed this tutorial and want to learn more about C programming, check out these related tutorials:

  1. Command Line Arguments in C
  2. Function Pointers in C
  3. Efficient Memory Management in C
  4. Hashing in C
  5. Network Programming in C
  6. Building a Web Server in C
  7. Introduction to Algorithms in C
  8. Advanced Algorithms in C
  9. Building a Chat Application in C
  10. Searching Algorithms in C
  11. Sorting Algorithms in C

Remember, practice makes perfect. So, keep coding and exploring new concepts. Happy coding!

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