Threads

1. Thread and process overview

① Similar to the process , Threads are a mechanism that allows applications to perform multiple tasks concurrently . A process can contain multiple threads . All threads in the same program will execute the same program independently , And Share the same global memory area , This includes initializing data segments 、 Uninitialized data segment （BSS paragraph ）, And heap memory segments .

② The process is CPU The smallest unit of allocation of resources , Threads are the smallest unit of scheduling execution in the operating system .

③ Threads are lightweight processes , stay LInux In this environment, the essence of thread is still process .

④ View the... Of the specified process LWP（ Threads ） Number ： ps -Lf pid

Such as ： stay unbuntu Open Firefox in .

perform ps aux The process of checking Firefox browser is 42569

Input ：ps -Lf 42569 Get the thread opened in the Firefox browser process .

2. Thread is different from process

There are several differences ：

• Information between processes is difficult to share . Because except for the read-only code segment , The parent-child process does not share memory , Therefore must Adopt some process communication methods , Exchange information between processes .
• call fork() The cost of creating a process is relatively high , Even with Share while reading , Copy on write technology , You still need to copy a variety of process attributes such as memory page tables and file descriptors , It means fork() The time cost of calling is still expensive .
• Threads can be easily 、 Share information quickly . Just copy the data to the share （ Global or heap ） Variable .
• Creating threads is faster than creating processes 10 Times or more . Inter thread time Sharing virtual space , No write time assignment is needed to copy memory , There is no need to copy the page table .