Detailed explanation of how Python uses the signal module to implement scheduled execution methods

In the liunx system, if you want to execute a command every one minute, the most common method is crontab. If you don't want to use crontab, you can use timer to achieve this function in the program as instructed by a colleague. , so I started to explore and found that I need some knowledge of signals...

Check which signals yourlinux supports: kill -l

root@server:~# kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
 6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX
root@server:~#

Signal: The method of communication between processes is a software interrupt. Once a process receives a signal, it interrupts the original program execution flow to process the signal. The operating system stipulates the default behavior of the process after receiving the signal. However, we can modify the behavior of the process after receiving the signal by binding the signal processing function. There are two signals that cannot be changed, SIGTOP and SIGKILL.

There are generally two reasons for sending signals:

1 (Passive) The kernel detects a system event. For example, when the child process exits, it will send SIGCHLD to the parent process. Signal. Pressing control+c on the keyboard will send the SIGINT signal

2 (active). Send a signal to the specified process through the system call kill

In C language There is a setitimer function. The function setitimer can provide three timers, which are independent of each other. When any timer is completed, a timing signal will be sent to the process and the timer will be automatically retimed. The parameter which determines the type of timer:

ITIMER_REAL Timing real time, the same as the alarm type. SIGALRM

ITIMER_VIRT The actual execution time of the scheduled process in user mode. SIGVTALRM

ITIMER_PROF The actual execution time of the scheduled process in user mode and core mode. SIGPROF

The three types of timers send different signals to the process when the timing is completed. Among them, the ITIMER_REAL class timer sends the SIGALRM signal, the ITIMER_VIRT class timer sends the SIGVTALRM signal, and the ITIMER_REAL class timer sends the SIGPROF signal.

The function alarm essentially sets a low-precision, non-overloaded ITIMER_REAL timer. It can only be accurate to seconds, and each setting can only generate timing once. The timers set by the function setitimer are different. Not only can they time to microseconds (theoretically), but they can also automatically cycle the timer. In a Unix process, alarm and ITIMER_REAL timers cannot be used at the same time.

SIGINT Terminate process Interrupt process (control+c)

SIGTERM Terminate process Software termination signal

SIGKILL Terminate process Kill process

SIGALRM Alarm clock signal

The preliminary knowledge is almost ready, and it’s time to move towards python’s signal.

Define signal names

The signal package defines each signal name and its corresponding integer, such as

import signal
print signal.SIGALRM
print signal.SIGCONT

The signal name used by Python is the same as Linux. You can query the preset signal processing function through

$man 7 signal

The core of the signal package is to use the signal.signal() function to preset (register) the signal processing function. As shown below:

singnal.signal(signalnum, handler)

signalnum is a signal, and handler is the processing function of the signal. We mentioned in the signal basics that a process can ignore signals, take default actions, or customize actions. When handler is signal.SIG_IGN, the signal is ignored. When the handler is singal.SIG_DFL, the process takes the default action (default). When handler is a function name, the process takes the action defined in the function.

import signal
# Define signal handler function
def myHandler(signum, frame):
  print('I received: ', signum)
 
# register signal.SIGTSTP's handler 
signal.signal(signal.SIGTSTP, myHandler)
signal.pause()
print('End of Signal Demo')

In the main program, we first use the signal.signal() function to preset the signal processing function. Then we execute signal.pause() to pause the process waiting for the signal to wait for the signal. When the signal SIGUSR1 is passed to the process, the process resumes from the pause and executes the SIGTSTP signal processing function myHandler() according to the default. One of the two parameters of myHandler is used to identify the signal (signum), and the other is used to obtain the status of the process stack (stack frame) when the signal occurs. Both parameters are passed by the signal.singnal() function.

上面的程序可以保存在一个文件中(比如test.py)。我们使用如下方法运行:

$python test.py

以便让进程运行。当程序运行到signal.pause()的时候，进程暂停并等待信号。此时，通过按下CTRL+Z向该进程发送SIGTSTP信号。我们可以看到，进程执行了myHandle()函数, 随后返回主程序，继续执行。(当然，也可以用$ps查询process ID, 再使用$kill来发出信号。)

(进程并不一定要使用signal.pause()暂停以等待信号，它也可以在进行工作中接受信号，比如将上面的signal.pause()改为一个需要长时间工作的循环。)

我们可以根据自己的需要更改myHandler()中的操作，以针对不同的信号实现个性化的处理。

定时发出SIGALRM信号

一个有用的函数是signal.alarm()，它被用于在一定时间之后，向进程自身发送SIGALRM信号:

import signal
# Define signal handler function
def myHandler(signum, frame):
  print("Now, it's the time")
  exit()
 
# register signal.SIGALRM's handler 
signal.signal(signal.SIGALRM, myHandler)
signal.alarm(5)
while True:
  print('not yet')

我们这里用了一个无限循环以便让进程持续运行。在signal.alarm()执行5秒之后，进程将向自己发出SIGALRM信号，随后，信号处理函数myHandler开始执行。 

发送信号

signal包的核心是设置信号处理函数。除了signal.alarm()向自身发送信号之外，并没有其他发送信号的功能。但在os包中，有类似于linux的kill命令的函数，分别为

os.kill(pid, sid)
os.killpg(pgid, sid)

分别向进程和进程组(见Linux进程关系)发送信号。sid为信号所对应的整数或者singal.SIG*。

实际上signal, pause，kill和alarm都是Linux应用编程中常见的C库函数，在这里，我们只不过是用Python语言来实现了一下。实际上，Python 的解释器是使用C语言来编写的，所以有此相似性也并不意外。此外，在Python 3.4中，signal包被增强，信号阻塞等功能被加入到该包中。我们暂时不深入到该包中。

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