pthread_self() in C

The pthread_self() is a method in C programming language that is found in a library called pthreads (POSIX threads). This function is used to fetch the calling thread's unique identifier-Thread ID (TID). These TIDs are highly useful as different multithreaded applications can recognize itself using these TIDs.

Introduction:

Multithreading: In this programming technique, many threads are enabled within a process to work independently while using the same resources, such as memory space.
POSIX Threads (pthreads): These standard threading libraries define a set of C programming language types, functions, and constants.
Thread ID (TID): A unique thread ID identifies every thread in a multi-threaded program. The TID is an identifier assigned by the system that distinguishes between different threads within the same process.

History:

POSIX threads originated during the 1980s as a part of the POSIX work to standardize threading interfaces on UNIX-like operating systems. The pthreads API was designed to be completely compatible with different UNIX platforms so that programmers can write portable multi-threaded applications that run on multiple OS platforms.

Another function in pthreads library is pthread_self(), which helps a thread obtain its own thread ID easily.

Key Features of POSIX Threads:

There are several features of POSIX threads in C. Some main features of the POSIX threads are as follows:

1. Thread Creation:

The library provides functions for creating threads. The primary function for thread creation is pthread_create. It allows developers to create a new thread within a program, specifying a function that the thread will execute.

#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void     *(*start_routine)(void *), void *arg);

2. Thread Termination:

Threads can be terminated using pthread_exit. This function allows a thread to exit, returning a value to the calling thread.

#include <pthread.h>
void pthread_exit(void *value_ptr);

3. Thread Synchronization:

POSIX threads provide various mechanisms for synchronizing threads, including mutexes (pthread_mutex_t), condition variables (pthread_cond_t), and semaphores (sem_t). These synchronization primitives help coordinate the execution of multiple threads and prevent race conditions.

#include <pthread.h>
int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t *attr);
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);
int pthread_cond_init(pthread_cond_t *cond, const pthread_condattr_t *attr);
int pthread_cond_signal(pthread_cond_t *cond);
int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);

4. Thread Joining:

The pthread_join function is used to wait for a thread to complete its execution. It allows one thread to wait for the termination of another thread.

#include <pthread.h>
int pthread_join(pthread_t thread, void **value_ptr);

5. Thread Attributes:

Thread attributes can be specified when creating a thread. It includes attributes related to scheduling policy, stack size, and other characteristics.

#include <pthread.h>
int pthread_attr_init(pthread_attr_t *attr);
int pthread_attr_setdetachstate(pthread_attr_t *attr, int detachstate);
int pthread_attr_setstacksize(pthread_attr_t *attr, size_t stacksize);

6. Thread-Safe Functions:

The POSIX threads library includes many thread-safe versions of standard C library functions. These thread-safe functions ensure that they can be safely called from multiple threads without interference.

#include <stdio.h>
#include <pthread.h>
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);
FILE *fopen(const char *filename, const char *mode);

Example:

Let us take an example to illustrate the pthread_self() function in C.

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

void *thread_function(void *arg) {
    pthread_t tid = pthread_self();
    printf("Inside the thread. Thread ID: %lu\n", tid);
    return NULL;
}

int main() {
    pthread_t thread;

    // Create a new thread
    if (pthread_create(&thread, NULL, thread_function, NULL) != 0) {
        fprintf(stderr, "Error creating thread\n");
        return 1;
    }

    // Wait for the thread to finish
    if (pthread_join(thread, NULL) != 0) {
        fprintf(stderr, "Error joining thread\n");
        return 1;
    }

    // In the main thread
    pthread_t main_tid = pthread_self();
    printf("Back in the main thread. Main Thread ID: %lu\n", main_tid);

    return 0;
}

Output:

Inside the thread. Thread ID: 139836372760128
Back in the main thread. Main Thread ID: 139836372764480

Explanation:

1. Include Headers:

#include <stdio.h>: It includes the standard input/output library for functions like printf.
#include <pthread.h>: It includes the POSIX threads library for multithreading functionalities.

2. Thread Function (thread_function):

This function is executed by the newly created thread.
pthread_t tid = pthread_self();: It retrieves the thread ID of the current thread using pthread_self() and stores it in the variable tid.
printf("Inside the thread. Thread ID: %lu\n", tid);: It prints the thread ID to the console.

3. Main Function:

pthread_t thread;: It declares a variable thread of type pthread_t to store the thread ID of the newly created thread.
pthread_create(&thread, NULL, thread_function, NULL): It creates a new thread using pthread_create. The new thread will execute the thread_function. The thread ID is stored in the variable thread.
pthread_join(thread, NULL): It waits for the newly created thread to finish its execution. It ensures that the main thread doesn't proceed until the new thread has been completed.
pthread_t main_tid = pthread_self();: It retrieves the thread ID of the main thread using pthread_self() and stores it in the variable main_tid.
printf("Back in the main thread. Main Thread ID: %lu\n", main_tid);: It prints the thread ID of the main thread to the console.
return 0;: It indicates successful program execution.

4. Compilation:

The program should be compiled with the appropriate thread library flags, such as -lpthread for GCC, to link the pthread library.

5. Execution:

When the program is executed, it will create a new thread, print its thread ID, wait for it to finish, and then print the thread ID of the main thread.

Time and Space Complexities:

Time Complexity:

The operation inside the thread_funtion and the main function dominate the time complexity.
The primary operations involve creating a thread ('pthread_create') and waiting for the thread to finish ('pthread_join').
These two operations generally have a time complexity of constant complexity O(1) because neither operation time increases with the size of the input.
Therefore, the overall time complexity of the provided code is O(1).

Space Complexity:

The primary space usage is related to the thread creation and the storage of thread IDs.
The 'pthread_t' variables ('thread' and 'main_tid') and the 'int' variable ('thread_ids') are used to store thread IDs.
The space required for these variables is constant and does not depend on the size of the input.
Therefore, the overall space complexity of the provided code is also O(1).

Example 2:

Let us take another example to illustrate the pthread_self() function in C.

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

#define NUM_THREADS 3

// Function to be executed by the threads
void *thread_function(void *arg) {
    int thread_id = *(int *)arg;
    
    // Print the thread ID and perform some task
    printf("Thread %d: Hello from the thread!\n", thread_id);

    // Return from the thread
    return NULL;
}

int main() {
    pthread_t threads[NUM_THREADS];  // Array to store thread IDs
    int thread_ids[NUM_THREADS];     // Array to store unique IDs for each thread

    // Create multiple threads
    for (int i = 0; i < NUM_THREADS; ++i) {
        thread_ids[i] = i;
        if (pthread_create(&threads[i], NULL, thread_function, &thread_ids[i]) != 0) {
            fprintf(stderr, "Error creating thread %d\n", i);
            return 1;
        }
    }

    // Wait for all threads to finish
    for (int i = 0; i < NUM_THREADS; ++i) {
        if (pthread_join(threads[i], NULL) != 0) {
            fprintf(stderr, "Error joining thread %d\n", i);
            return 1;
        }
    }

    // In the main thread
    printf("Back in the main thread.\n");

    return 0;
}

Output:

Thread 0: Hello from the thread!
Thread 1: Hello from the thread!
Thread 2: Hello from the thread!
Back in the main thread.

Explanation:

1. Thread Function (thread_function):

The thread_function is the function that will be executed by each thread. It takes a single argument void *arg, which is expected to be a pointer to an integer representing the unique ID of the thread.
Inside the function, the argument is cast to an integer (int thread_id = *(int *)arg;), representing the thread's unique identifier.
The thread prints a message to the console, including its unique ID, indicating that it has started.
After that, the thread returns (return NULL;), indicating the completion of its task.

2. Main Function:

#define NUM_THREADS 3: A preprocessor directive to define the number of threads as 3.
pthread_t threads[NUM_THREADS];: An array to store the thread IDs of the created threads.
int thread_ids[NUM_THREADS];: An array to store unique IDs for each thread.

3. Creating Threads:

A loop (for (int i = 0; i < NUM_THREADS; ++i)) is used to create multiple threads.
thread_ids[i] = i;: It assigns a unique ID to each thread.
pthread_create(&threads[i], NULL, thread_function, &thread_ids[i]): It creates a new thread using pthread_create. The thread will execute thread_function with the unique ID passed as an argument.
If an error occurs during thread creation, an error message is printed to the standard error stream (stderr), and the program exits with an error code.

4. Waiting for Threads to Finish:

Another loop is used to wait for all threads to finish.
pthread_join(threads[i], NULL): It waits for the thread with ID threads[i] to finish its execution. If an error occurs during thread joining, an error message is printed to stderr, and the program exits with an error code.

5. Main Thread:

After all threads have finished, the main thread prints a message indicating that it is back in the main thread.

6. Compilation and Execution:

The code needs to be compiled with the appropriate thread library flags, such as -lpthread for GCC:
gcc -o multithread_example multithread_example.c -lpthread
After compilation, run the executable:
./multithread_example

Time and Space Complexity:

1. Time Complexity:

Thread Creation (pthread_create):
The loop that creates multiple threads runs NUM_THREADS times, so the overall time complexity for thread creation is O(NUM_THREADS).
The overall time complexity is O(NUM_THREADS).

2. Space Complexity:

The space required for storing thread IDs is proportional to the number of threads (NUM_THREADS). Therefore, the space complexity of these arrays is O(NUM_THREADS).
The overall space complexity is O(NUM_THREADS).

Advantages of pthread_self():

There are several advantages of the pthread_self() function in C. Some main advantages are as follows:

Portability: Using a standardized API, users can develop on different Unix-like systems multi-threaded services that are shared by all compliant systems. In the new scheme, every piece of code may involve multiple threads that can cooperate even if they are deployed on different platforms.
Parallelism and Concurrency: Pthreads allow developers to create multiple tasks within one process. As the hardware executes these tasks simultaneously, this concurrent execution mode boosts performance on multicore systems.
Scalability: Pthreads multi-threading allows applications to scale as hardware resources change. Therefore, tasks in programs can be divided into multiple threads and then run on different processors or cores. Certain types of programs run faster when it is done, giving them better performance in general.
Flexible Thread Creation: With Pthreads, developers can design their own thread creation routine to meet their needs. For example, when a developer creates a thread, he can set the stack size, scheduling policy, and thread priority. It helps fine-tune specific application requirements.
Thread-Safe Functions: Pthreads also provides thread-safe versions of many standard C library functions. A function is said to be "thread-safe" in these circumstances since it protects against data corruption or deadlock, among others. Multiple threads share common resources and use these types of functions without any problems.

Applications of pthread_self():

There are several applications of the pthread_self() function in C. Some main applications are as follows:

Servers: Server applications such as web, database servers and the like often use threads (pthreads) to enable multiple client requests to be handled concurrently. Therefore, each incoming connection can be treated as a separate thread, allowing a single server to service more than one client at any given moment.
Multiprocess File Systems: Pthreads making it possible for multiple users or processes to access their files at once can significantly improve the efficiency of file I/O operations. In contrast with distributed file systems and file servers, there are many file systems supporting this basic function.
Network Programming: Threads can provide a solution for network applications like chat servers, messaging systems, and peer-to-peer applications to manage multiple network connections simultaneously.
Simulation and Gaming: Pthreads are often used in simulation software and video games. In these programs, physics simulations, AI processing and rendering, and interpolation, among others, run in parallel, which enhances the realism and responsiveness of the simulations or games for human beings.
Data Analysis and Machine Learning: Pthreads should be found helpful for some kinds of problems, where large amounts of data can be processed cooperatively. Examples could be machine-learning algorithms that use numerical optimization techniques and statistical processing or data-analysis programs working on big databases. It is especially useful for tasks to be executed as independent sub-problems.

Disadvantages of pthread_self():

There are several disadvantages of the pthread_self() function in C. Some main disadvantages are as follows:

Platform Dependency: This is a result of the way systems support these features with different levels of help and how various POSIX standards are complied with on different Operating Systems, meaning all the subtleties are hard to spot and platform specific.
Complexity and error risk: Concurrent programming introduces complexity into the logical flow of the program, in particular when dealing with shared resources. Deadlocks and race conditions should be avoided by using such a synchronization mechanism correctly as mutual exclusion (mutexes) and conditional variables. Finally, misuse of these constructs accelerates difficult-to-find bugs.
Lack of native windows support: Pthreads is mainly developed for UNIX like systems. Thus, there is no support for it on Windows. While there are other third-party implementations in Windows of pthreads but they don't have portability.
Some abstraction limitations: The thread model provided by Pthreads is relatively low-level. On the other hand, developers must handle many of these aspects, like creation and termination of threads. This results in extra code (also known as 'boilerplate code instance') unlike higher-level threading models found in some other languages.

Conclusion:

In conclusion, Transporting programs written with POSIX Threads (pthreads) is currently a long established and widely used method to develop multithreaded applications for Unix-like operating systems. This thread library has many advantages, such as parallelism, portability, and flexibility, among many other features that are not present so easily in RPC-based systems. With the help of pthreads it is possible to execute several tasks concurrently within one process, which enables better utilization of multicore platforms.

If we speak in general, POSIX Threads are still an essential asset for concurrent programming in C language through maintaining some level of control while abstracting developers who work with POSIX-supported systems. As long as users use such devices only as necessary, they will greatly enhance how software and systems perform, thus their effective employment spans everything from server-side applications to scientific computing.

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