THREADS, LIKE PROCESSES, ARE A MECHANISM TO ALLOW A PROGRAM to do more than one thing at a time. As with processes, threads appear to run concurrently; the Linux kernel schedules them asynchronously, interrupting each thread from time to time to give others a chance to execute. Conceptually, a thread exists within a process.Threads are a finer-grained unit of execution than processes.When you invoke a program, Linux creates a new process and in that process creates a single thread, which runs the program sequentially.That thread can create additional threads; all these threads run the same program in the same process, but each thread may be executing a different part of the program at any given time. We’ve seen how a program can fork a child process.The child process is initially running its parent’s program, with its parent’s virtual memory, file descriptors, and so on copied.The child process can modify its memory, close file descriptors, and the like without affecting its parent, and vice versa.When a program creates another thread, though, nothing is copied.The creating and the created thread share the same memory space, file descriptors, and other system resources as the original. If one thread changes the value of a variable, for instance, the other thread subsequently will see the modi- fied value.
