Redis-0x15-库函数signal

1 手册查询

直接输出man手册内容到文件，里面会有很多的控制符，通过col -b来过滤控制符号

cd Desktop
man 3 signal | col -b signal.txt

2 signal函数文档

SIGNAL(3)		   Library Functions Manual		     SIGNAL(3)

NAME
     signal – simplified software signal facilities

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <signal.h>

     void (*
     signal(int sig, void (*func)(int));
     )(int);

     or in the equivalent but easier to read typedef'd version:
     typedef void (*sig_t) (int);

     sig_t
     signal(int sig, sig_t func);

DESCRIPTION
     This signal() facility is a simplified interface to the more general
     sigaction(2) facility.

     Signals allow the manipulation of a process from outside its domain, as
     well as allowing the process to manipulate itself or copies of itself
     (children).  There are two general types of signals: those that cause
     termination of a process and those that do not.  Signals which cause
     termination of a program might result from an irrecoverable error or
     might be the result of a user at a terminal typing the `interrupt'
     character.  Signals are used when a process is stopped because it wishes
     to access its control terminal while in the background (see tty(4)).
     Signals are optionally generated when a process resumes after being
     stopped, when the status of child processes changes, or when input is
     ready at the control terminal.  Most signals result in the termination of
     the process receiving them, if no action is taken; some signals instead
     cause the process receiving them to be stopped, or are simply discarded
     if the process has not requested otherwise.  Except for the SIGKILL and
     SIGSTOP signals, the signal() function allows for a signal to be caught,
     to be ignored, or to generate an interrupt.  These signals are defined in
     the file <signal.h>:

	   Name 	    Default Action	 Description
     1	   SIGHUP	    terminate process	 terminal line hangup
     2	   SIGINT	    terminate process	 interrupt program
     3	   SIGQUIT	    create core image	 quit program
     4	   SIGILL	    create core image	 illegal instruction
     5	   SIGTRAP	    create core image	 trace trap
     6	   SIGABRT	    create core image	 abort program (formerly
						 SIGIOT)
     7	   SIGEMT	    create core image	 emulate instruction executed
     8	   SIGFPE	    create core image	 floating-point exception
     9	   SIGKILL	    terminate process	 kill program
     10    SIGBUS	    create core image	 bus error
     11    SIGSEGV	    create core image	 segmentation violation
     12    SIGSYS	    create core image	 non-existent system call
						 invoked
     13    SIGPIPE	    terminate process	 write on a pipe with no
						 reader
     14    SIGALRM	    terminate process	 real-time timer expired
     15    SIGTERM	    terminate process	 software termination signal
     16    SIGURG	    discard signal	 urgent condition present on
						 socket
     17    SIGSTOP	    stop process	 stop (cannot be caught or
						 ignored)
     18    SIGTSTP	    stop process	 stop signal generated from
						 keyboard
     19    SIGCONT	    discard signal	 continue after stop
     20    SIGCHLD	    discard signal	 child status has changed
     21    SIGTTIN	    stop process	 background read attempted
						 from control terminal
     22    SIGTTOU	    stop process	 background write attempted to
						 control terminal
     23    SIGIO	    discard signal	 I/O is possible on a
						 descriptor (see fcntl(2))
     24    SIGXCPU	    terminate process	 cpu time limit exceeded (see
						 setrlimit(2))
     25    SIGXFSZ	    terminate process	 file size limit exceeded (see
						 setrlimit(2))
     26    SIGVTALRM	    terminate process	 virtual time alarm (see
						 setitimer(2))
     27    SIGPROF	    terminate process	 profiling timer alarm (see
						 setitimer(2))
     28    SIGWINCH	    discard signal	 Window size change
     29    SIGINFO	    discard signal	 status request from keyboard
     30    SIGUSR1	    terminate process	 User defined signal 1
     31    SIGUSR2	    terminate process	 User defined signal 2

     The sig argument specifies which signal was received.  The func procedure
     allows a user to choose the action upon receipt of a signal.  To set the
     default action of the signal to occur as listed above, func should be
     SIG_DFL.  A SIG_DFL resets the default action.  To ignore the signal,
     func should be SIG_IGN.  This will cause subsequent instances of the
     signal to be ignored and pending instances to be discarded.  If SIG_IGN
     is not used, further occurrences of the signal are automatically blocked
     and func is called.

     The handled signal is unblocked when the function returns and the process
     continues from where it left off when the signal occurred.  Unlike
     previous signal facilities, the handler func() remains installed after a
     signal has been delivered.

     For some system calls, if a signal is caught while the call is executing
     and the call is prematurely terminated, the call is automatically
     restarted.  Any handler installed with signal(3) will have the SA_RESTART
     flag set, meaning that any restartable system call will not return on
     receipt of a signal.  The affected system calls include read(2),
     write(2), sendto(2), recvfrom(2), sendmsg(2), and recvmsg(2) on a
     communications channel or a low speed device and during a ioctl(2) or
     wait(2).  However, calls that have already committed are not restarted,
     but instead return a partial success (for example, a short read count).
     These semantics could be changed with siginterrupt(3).

     When a process which has installed signal handlers forks, the child
     process inherits the signals.  All caught signals may be reset to their
     default action by a call to the execve(2) function; ignored signals
     remain ignored.

     If a process explicitly specifies SIG_IGN as the action for the signal
     SIGCHLD, the system will not create zombie processes when children of the
     calling process exit.  As a consequence, the system will discard the exit
     status from the child processes.  If the calling process subsequently
     issues a call to wait(2) or equivalent, it will block until all of the
     calling process's children terminate, and then return a value of -1 with
     errno set to ECHILD.

     See sigaction(2) for a list of functions that are considered safe for use
     in signal handlers.

RETURN VALUES
     The previous action is returned on a successful call.  Otherwise, SIG_ERR
     is returned and the global variable errno is set to indicate the error.

ERRORS
     The signal() function will fail and no action will take place if one of
     the following occur:

     [EINVAL]		The sig argument is not a valid signal number.

     [EINVAL]		An attempt is made to ignore or supply a handler for
			SIGKILL or SIGSTOP.

SEE ALSO
     kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3),
     siginterrupt(3), tty(4)

HISTORY
     The signal facility appeared in 4.0BSD.  The option to avoid the creation
     of child zombies through ignoring SIGCHLD appeared in FreeBSD 5.0.

macOS 13.5			 June 7, 2004			    macOS 13.5

3 翻译

Signal分为两种

• 一种可以中断进程，可以中断进程的信号来源于何处

  • 异常引起的

  • 用户输入了中断符号

• 一种不可以中断进程

signal()这个函数的作用包括如下

• 捕获一个signal
• 忽略一个signal
• 根据signal生成一个中断

那么这个signal函数怎么使用呢

• signal这个函数返回值是一个函数指针，这个函数指针指向的函数的入参是int，返回值是void

• signal这个函数有2个入参

  • 第一个参数是int，就是Signal，上述手册罗列了多达31种Signal

  • 第二个参数是一个函数指针，这个函数指针指向的函数的语义是让系统如何处理上面的Signal，该函数有1个入参

    • 入参是int

    • 返回值为void

3.1 Signal

上述手册罗列了31种Signal

3.2 处理Signal的处理器

通过一个函数告知系统如何处理Signal

• SIG_DFL 让系统执行Signal的默认行为

• SIG_IGN 让系统忽略Signal