Linux system programming 43 signal-signal(), the signal will interrupt the blocking system call

View signal

mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ kill -l

Insert picture description here 0-31: Standard signal
32-64: Real-time signal

NAME
       signal - ANSI C signal handling

SYNOPSIS
       #include <signal.h>

       typedef void (*sighandler_t)(int);

       sighandler_t signal(int signum, sighandler_t handler);

==> 
void (*signal(int signum, void (*func)(int)))(int)

signal()将信号符号的配置设置为handler, handler可以是SIG_IGN、SIG_DFL，也可以是程序员定义的函数(“信号处理器”)的地址。

SIG_IGN：向内核表示忽略此信号
SIG_DFL：向内核表示接到此信息的动作是系统默认动作
当指定函数地址的时候，则在信号发生时，调用该函数，即调用信号处理函数。

The signal SIGINT refers to the program termination (interrupt) signal, which is sent when the user types the INTR character (usually Ctrl-C) to notify the foreground process group to terminate the process.

Experiment 1: Ignore the SIG_IGN signal

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

int main()
{
	int i;

	signal(SIGINT,SIG_IGN);

	for(i=0 ; i<12 ; i++)
	{
		write(1,"*",1);
		sleep(1);
	}

	exit(0);
}



mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ gcc test.c 
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ./a.out 
***^C*^C^C^C*^C^C***^C^C^C**^C^C*^C^C^C*mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ 
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ 
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$

You can see that the program will not be affected by Ctrl-C while running

Experiment 2: Responding to the SIG_IGN signal

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

void sig_handler(int s)
{
	write(1,"!",1);
}

int main()
{
	int i;

	signal(SIGINT,sig_handler);

	for(i=0 ; i<12 ; i++)
	{
		write(1,"*",1);
		sleep(1);
	}

	exit(0);
}

mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ gcc test.c 
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ./a.out 
*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!**^C!*^C!mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ^C
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$

Signals will interrupt blocked system calls

Experiment 3: Signals will interrupt blocked system calls

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

void sig_handler(int s)
{
	write(1,"!",1);
}

int main()
{
	int i;

	signal(SIGINT,sig_handler);

	for(i=0 ; i<12 ; i++)
	{
		write(1,"*",1);
		sleep(1);
	}

	exit(0);
}

mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ./a.out 
*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!*^C!mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ^C
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ^C
mhr@ubuntu:~/Desktop/xitongbiancheng/parallel/signal$ ^C

If you keep pressing Ctrl-C during execution, the program will end quickly, taking less than 15 seconds.

This is because the signal will interrupt the blocking system call.

Such as: also explained in open()

EINTR While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a signal handler; see signal(7).

When blocking and waiting to open a slower device, this call may be interrupted by a signal.
If a device is being opened, the device may be slow, or the device is busy, and open() will take a while to open it, that is, open() will block. At this time, if there is a signal, the blocking will be interrupted. System call.

As explained in read()

EINTR The call was interrupted by a signal before any data was read; see signal(7)
Before I read anything, if there is a signal, it will be interrupted. That is, the signal will interrupt the blocking system call.

Therefore, the previous programs, such as open(), read(), need to add blocking error judgments:
Insert picture description here

The system call will not be interrupted during execution, but will wait, and will be interrupted during blocking. If waiting for opening, waiting for reading.

Linux system programming 43 signal-signal(), the signal will interrupt the blocking system call

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