Beginning Linux Programming 学习--chapter 11 Processes and Signals

 What's process--什么是进程？

The UNIX standards, specifically IEEE Std 1003.1, 2004 Edition, defines a process as “an address space with one or more threads (线程) executing within that address space, and the required system resources for those threads.”

Linux允许多个用户同时登陆系统，同时运行相同的程序：

As a multiuser system, Linux allows many users to access the system at the same time. Each user can run many programs, or even many instances of the same program, at the same time. The system itself runs other programs to manage system resources and control user access.

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Process Structures:

If two users, neil and rick, both run the grep program at the same time to look for different strings in different files :

如果在搜索结束前，运行ps -ef 命令来查看进程，输出的结果中可能会包含下两项：

操作系统为每一个进程分配一个process identifier(PID). PID的值通常会在2--32768之间，1通常分配给init 进程，该进程的作用是维护其他进程。

进程的共享东东：

grep这个软件对应的可执行code，被加载到内存中的只读区域，然后被两个进程共享！

另外，系统的libraries也可以被共享！例如printf函数，它的可执行代码应该只需要被加载到内存中的一个地方，然后其他用到该函数的所有进程共享这块read-only内存！--这种机制跟windows系统中的dll类似，但是更加复杂！

另一个好处是，可执行程序中如果有printf等库函数，这些库函数不被保存在可执行程序中，可以是可执行程序的占用的磁盘控件变小！---- 意思是最后的可执行文件对应的那些二进制代码中 不包括prinf库函数的二进制代码？

进程的独自的东东：

• 每一个进程都有自己的stack space ,used for local variables in functions and for controlling function call and returns.

• 每一个进程都有自己的environment space, containing environment variables that may be established solely for this process to use, as you saw with putenv and getenv in Chapter 4.-----???

• 每一个进程都有自己的 program counter, a record of where it has gotten to in its execution, which is the execution thread. In the next chapter you will see that when you use threads, processes canhave more than one thread of execution.

/proc 文件的作用：On many Linux systems, and some UNIX systems, there is a special set of “files” in a directory called /proc. These are special in that rather than being true files they allow you to “look inside” processes while they are running as if they were files in directories. We took a brief look at the /proc file system back in Chapter 3. -----------？？？？？？？

虚拟内存的好处：Finally, because Linux, like UNIX, has a virtual memory system that pages(映射) code and data out to an area of the hard disk, many more processes can be managed than would fit into the physical memory.

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现在，一个系统可以同时运行的线程数量取决于内存空间的大小。

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View Processes:

PS命令是基本的进程查看命令：https://www.cnblogs.com/wxgblogs/p/6591980.html

PS命令的一些参数： 

-A ：所有的进程均显示出来，与 -e 具有同样的效用；

-f ：做一个更为完整的输出。--？？

By default, the ps program shows only processes that maintain a connection with a terminal, a console,
a serial line, or a pseudo terminal. Other processes run without needing to communicate with a user on a terminal. These are typically system processes that Linux uses to manage shared resources. You can use ps to see all such processes using the -e option and to get “full” information with -f

PS命令的输出：

TTY 列：shows which terminal the process was started from. tty1-tty6 是本机上面的登入者程序，若为 pts/0 等等的，则表示为由网络连接到主机的程序。

TIME ：gives the CPU time used so far--？？

CMD： shows the command used to start the process.

用PS命令查看进程的状态：

STAT：进程目前的状态，主要的状态有：

R ：Running，on the run queue either executing or about to run.
S ：Usually waiting for an event to occur, such as a signal or input to become available.
T ：Usually stopped by shell job control (?) or the process is under the control of a debugger.
Z ：该程序应该已经终止，但是其父程序却无法正常的终止他，造成 zombie (疆尸) 程序的状态.--??
D:   Uninterruptible Sleep (Waiting). Usually waiting for input or output to complete.
N:  Low priority task, “nice"
s （小写） : Process is a session leader.---------??
+:  Process is in the foreground process group.---------??
l :  Process is multi-threaded.

<:  High priority task

父子进程：

In general, each process is started by another process known as its parent process. A process so started
is known as a child process. When Linux starts, it runs a single program, the prime ancestor and process number 1, init. This is, if you like, the operating system process manager and the grandparent of all processes. Other system processes you’ll meet soon are started by init or by other processes started by init.

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Process Scheduling （进程调度）：