[Linux Multithreading] Thread Concept

Table of Contents

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introduction

What is a thread?

Threading model in Linux

Some calls to the thread library

pthread_create

pthread_join

pthread_exit

Introduction

Process is the basic unit of OS scheduling resources

Thread is the basic unit of OS scheduling

Threads are a key concept in modern computer programming, and they play an important role in multitasking and concurrent programming. This article will explore the concept of threads and their application in the Linux operating system. We’ll learn how threads are defined and why they are so important in operating systems. In addition, we will emphasize the historical background of Linux as an operating system that supports multi-threading.

What is a thread?

Threads, simply put, are execution units within a program. Unlike processes, they are not independent execution entities, but are attached to subtasks within the process. A thread can be viewed as a smaller granularity of a process, and a process can contain one or more threads that share the same address space and resources. The main purpose of threads is to allow programs to execute concurrently more efficiently to improve performance and resource utilization.

Threads serve a variety of purposes, from parallel processing of tasks to achieving faster response times, all without the support of threads. Because threads can perform multiple tasks simultaneously, they are highly utilized on multi-core processors and can better utilize the performance of modern computers. In addition, threads can be used to solve problems in concurrent programming, such as resource contention and synchronization.

Threading Model in Linux

The Linux operating system provides multiple thread models, each with its own characteristics and applicable scenarios. These models include Many-to-One, One-to-One and Many-to-Many threading models.

Many-to-One model: In this model, multiple user-level threads are mapped to one kernel thread. Although they can execute concurrently at the user level, if one thread blocks in the kernel, all threads will be affected. This model is suitable for lightweight threads, but not for maximum utilization of multi-core systems.
One-to-One model: In this model, each user-level thread is mapped to a kernel thread. This provides better parallelism as each thread can execute in parallel on a multi-core system. This is the default threading model for many modern operating systems, including Linux.
Many-to-Many model: This model is a hybrid of many-to-one and one-to-one, allowing multiple user-level threads to be mapped to multiple kernel threads. This provides a degree of parallelism and flexibility and is adaptable to the needs of different applications.

Some calls of the thread library

In Linux, you can use the POSIX thread library (Pthreads) to create and manage threads. The following are basic concepts about thread creation and management:

`pthread_create`

Function is a function used to create a new thread, which allows you to start a new thread in Linux to execute a specified function. The following is a detailed explanation of the pthread_create function:

Function prototype:

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg);

Parameter explanation:

pthread_t *thread: This is a pointer to type pthread_t used to store the ID of the new thread. After the thread is successfully created, it will contain the ID of the new thread. You can use this ID to reference or wait for the thread to complete.
const pthread_attr_t *attr: This is a pointer to the pthread_attr_t type, used to specify the attributes of the thread. Typically, you can set this to NULL to use the default attribute. If you need to customize thread attributes, you can create a pthread_attr_t structure and pass it to this parameter.
void *(*start_routine)(void *): This is a pointer to a function that will become the entry point of the new thread. A new thread will start executing from this function. This function should accept a parameter of type void * and return a pointer of type void *. The execution of the new thread will start from the start_routine function, which is usually a thread function written by you.
void *arg: This is the argument passed to the start_routine function. It is a void * pointer that can be used to pass any needed data to the new thread. You can pass data to the new thread’s start_routine function to make the data available to the new thread.

Return value:

If pthread_create successfully creates the thread, it will return 0.
If an error occurs, a non-zero error code is returned to indicate failure. You can use errno to get detailed error information.

`pthread_join`

A function is used to wait for a thread to complete execution. It allows one thread to wait for the completion of another thread in order to coordinate the execution order and share data of multiple threads. The following is a detailed explanation of the pthread_join function:

Function prototype:

int pthread_join(pthread_t thread, void **retval);

Parameter explanation:

pthread_t thread: This is the ID of the thread to wait for. You need to provide the ID of the thread to wait on so that pthread_join knows which thread to wait for to complete.
void **retval: This is a pointer to a void * pointer used to store the return value of the thread. When the waiting thread completes, its return value will be stored at the location pointed to by the retval pointer. This allows you to get the return value of the waiting thread.

Return value:

If pthread_join successfully waits for the thread to complete, it will return 0.
If an error occurs, a non-zero error code is returned to indicate failure. You can use errno to get detailed error information.

`pthread_exit`

The function is used for the exit of the thread. It allows a thread to terminate execution gracefully and optionally return a value that other threads can obtain if needed. The following is a detailed explanation of the pthread_exit function:

Function prototype:

void pthread_exit(void *retval);

Parameter explanation:

void *retval: This is the return value of the thread, usually a void * pointer. When a thread calls pthread_exit, it exits and returns the value of retval. This return value can be obtained by other threads through pthread_join to understand the execution results of the thread.

Notes on using pthread_exit:

pthread_exit is a method for threads to exit normally. A thread can call pthread_exit anywhere to end its execution.
The thread can optionally return a value when exiting (via the retval parameter). This return value can be any type of data, usually a pointer, so that the execution results of the thread can be obtained in other threads.
Calling pthread_exit will terminate the execution of the current thread but will not affect the running of other threads.
Once a thread calls pthread_exit, it will no longer perform pthread_join or other thread wait operations because it has terminated.
A thread should ensure that its allocated resources, such as memory or open file descriptors, are released when it exits.