1 What are signals and how are they used

1.当进程接收到信号,进程会暂停,来处理信号

1.1 How we use signals

1.killall cat 杀死所有的cat程序
2.信号源与信号

信号源	信号
kill	`SIGTERM`
Ctrl-c	`SIGINT`
Ctrl-z	`SIGTSTP`
fg	`SIGCONT`

2 The Wide World of Signals

Signal	Value	Action	Comment
SIGHUP	1	Term	Hangup detected on controlling terminal or death of controlling process
SIGINT	2	Term	Interrupt from keyboard
SIGQUIT	3	Core	Quit from keyboard
SIGILL	4	Core	Illegal Instruction
SIGABRT	6	Core	Abort signal from abort(3)
SIGFPE	8	Core	Floating point exception
SIGKILL	9	Term	Kill signal
SIGSEGV	11	Core	Invalid memory reference
SIGPIPE	13	Term	Broken pipe: write to pipe with no readers
SIGALRM	14	Term	Timer signal from alarm(2)
SIGTERM	15	Term	Termination signal
SIGUSR1	30,10,16	Term	User-defined signal 1
SIGUSR2	31,12,17	Term	User-defined signal 2
SIGCHLD	20,17,18	Ign	Child stopped or terminated
SIGCONT	19,18,25	Cont	Continue if stopped
SIGSTOP	17,19,23	Stop	Stop process
SIGTSTP	18,20,24	Stop	Stop typed at tty
SIGTTIN	21,21,26	Stop	tty input for background process
SIGTTOU	22,22,27	Stop	tty output for background process

2.1 Signal Names and Values

each signal has a name, value, and default action.
sys/signal.h

#define SIGHUP  1       /* hangup */
#define SIGINT  2       /* interrupt */
#define SIGQUIT 3       /* quit */
#define SIGILL  4       /* illegal instruction (not reset when caught) */
#define SIGTRAP 5       /* trace trap (not reset when caught) */
#define SIGABRT 6       /* abort() */
#define SIGPOLL 7       /* pollable event ([XSR] generated, not supported) */
#define SIGFPE  8       /* floating point exception */
#define SIGKILL 9       /* kill (cannot be caught or ignored) */

2.2 Default Actions of Signals

动作	描述
Term	The process will terminate
Core	The process will terminate and produce a core dump file that traces the process state at the time of termination.
Ign	The process will ignore the signal
Stop	The process will stop, like with a Ctrl-Z
Cont	The process will continue from being stopped

3 Signals from the Command Line

3.1 Preparing for the kill

3.2 Sending Terminal Signals with Kill

4 Handling and Generating Signals

4.1 Hello world of Signal Handling

1.int signal(int signum, void (*handler)(int))
2.参数1,接收到的信号参数2:接收到指定信号后调用的函数

#include <stdlib.h>
#include <stdio.h>#include <signal.h> /*for signal() and raise()*/void hello(int signum){printf("Hello World!\n");
}int main(){//execute hello() when receiving signal SIGUSR1  signal(SIGUSR1, hello); //再执行//send SIGUSR1 to the calling process  raise(SIGUSR1); //先执行
}

4.2 Asynchronous Execution

1.asynchronous,当程序接收到信号,会暂停,去处理信号,然后再接着刚才的暂停继续执行
2.下面的程序无法用ctrl+c来退出,因为程序接收到ctrl+c的信号后,会执行hello,然后继续循环.所以可以发送ctrl + \来停止发送SIGSTOP信号,此信号不能被处理和忽略

/* hello_loop.c*/
void hello(int signum){printf("Hello World!\n");
}int main(){//Handle SIGINT with hellosignal(SIGINT, hello);//loop forever!while(1);}

4.3 Inter Process Communication

1.int kill(pid_t pid, int signum);
2. `kill函数

/*ipc_signal.c*/
void hello(){printf("Hello World!\n");
}int main(){pid_t cpid;pid_t ppid;//set handler for SIGUSR1 to hello()signal(SIGUSR1, hello);if ( (cpid = fork()) == 0){/*CHILD*///get parent's pidppid = getppid();//send SIGUSR1 signal to parrentkill(ppid, SIGUSR1);exit(0);}else{/*PARENT*///just wait for child to terminatewait(NULL);}}

4.4 Ignoring Signals

忽略信号有两种方法
1. SIG_IGN : Ignore the signal
2. SIG_DFL : Replace the current signal handler with the default handler

int main(){// using SIG_IGNsignal(SIGINT, SIG_IGN);while(1);
}

4.5 Changing and Reverting to the default handler

1.在handler中可以再次接收信号

/*you_shot_me.c*/
void handler_3(int signum){printf("Don't you dare shoot me one more time!\n");//Revert to default handler, will exit on next SIGINTsignal(SIGINT, SIG_DFL);
}void handler_2(int signum){printf("Hey, you shot me again!\n");//switch handler to handler_3signal(SIGINT, handler_3);
}void handler_1(int signum){printf("You shot me!\n");//switch handler to handler_2signal(SIGINT, handler_2);
}int main(){//Handle SIGINT with handler_1signal(SIGINT, handler_1);//loop forever!while(1);}

4.6 Some signals are more equal than others

1.SIGSTOP ctrl+z不可处理

/* ignore_stop.c */
int main(){//ignore SIGSTOP ?signal(SIGSTOP, SIG_IGN);//infinite loopwhile(1);}

2.SIGKILL kill -SIGKILL 不可处理

int main(){//ignore SIGSTOP ?signal(SIGKILL, SIG_IGN);//infinite loopwhile(1);}

4.7 Checking Errors of signal()

/*signal_errorcheck.c*/
int main(){//ignore SIGSTOP ?if( signal(SIGKILL, SIG_IGN) == SIG_ERR){perror("signal");;exit(1);}//infinite loopwhile(1);}

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