What are Unix signals?

一个信号，是一个信息，可以发送到运行的过程。

Signals can be initiated by programs, users, or administrators.信号可启动程式，使用者或管理员。

For example, to the proper method of telling the Internet Daemon (inetd) to re-read its configuration file is to send it a SIGHUP signal.举例来说，以适当的方法告诉互联网守护进程 （ inetd的） ，以重看其配置文件，是发送一个sighup信号。

For example, if the current process ID (PID) of inetd is 4140, we would type:举例来说，如果当前的进程ID （ PID控制）的inetd的是4140年，我们会类型：

 kill -SIGHUP 4140 

Another common use of signals is to stop a running process.另一种普遍使用的信号是停止一个运行的过程。 To stop the inetd process completely, we would use this command:停止inetd的进程完全中，我们会用这个命令：

 kill 4140 

By default, the kill command sends the SIGTERM signal.默认情况下，杀死指挥移送sigterm信号。 If SIGTERM fails, we can escalate to using the SIGKILL signal to stop the process:如果sigterm失败的话，我们可以升级使用sigkill信号，以阻止过程：

 kill -9 4140 

Because SIGKILL cannot be handled, stopping a process with SIGKILL is generally considered a bad idea.因为sigkill不能处理，制止过程中与sigkill被普遍认为是一个坏主意。 Using SIGKILL prevents a process from cleaning up after itself and exiting gracefully.用sigkill阻止一个进程，从清理后，本身并退出缓慢下降。

Handling Signals信号处理

Each Unix signal has a default set of effects on a Unix program.每个Unix信号具有默认设置的效果上的Unix程序。 Programmers can code their applications to respond in customized ways to most signals.程序员可以码及其应用响应定制的方式，以最信号。 These custom pieces of code are called signal handlers .这些习俗件代码是所谓的信号处理 。

Two signals are unable to be redefined by a signal handler.两路信号无法重新定义，由一个信号处理器。 SIGKILL always stops a process and SIGSTOP always moves a process from the foreground to the background. sigkill总是站有一个过程， sigstop总是一个过程，从前台到后台。 These two signals cannot be "caught" by a signal handler.这两个信号不能被"捕获"的一个信号处理器。

FreeBSD Signals

Signal Name	Signal Number	Signal Description
SIGHUP	1	Terminal line hangup
SIGINT	2	Interrupt program
SIGQUIT	3	Quit program
SIGILL	4	Illegal instruction
SIGTRAP	5	Trace trap
SIGABRT	6	Abort
SIGEMT	7	Emulate instruction executed
SIGFPE	8	Floating-point exception
SIGKILL	9	Kill program
SIGBUS	10	Bus error
SIGSEGV	11	Segmentation violation
SIGSYS	12	Bad argument to system call
SIGPIPE	13	Write on a pipe with no one to read it
SIGALRM	14	Real-time timer expired
SIGTERM	15	Software termination signal
SIGURG	16	Urgent condition on I/O channel
SIGSTOP	17	Stop signal not from terminal
SIGTSTP	18	Stop signal from terminal
SIGCONT	19	A stopped process is being continued
SIGCHLD	20	Notification to parent on child stop or exit
SIGTTIN	21	Read on terminal by background process
SIGTTOU	22	Write to terminal by background process
SIGIO	23	I/O possible on a descriptor
SIGXCPU	24	CPU time limit exceeded
SIGXFSZ	25	File-size limit exceeded
SIGVTALRM	26	Virtual timer expired
SIGPROF	27	Profiling timer expired
SIGWINCH	28	Window size changed
SIGINFO	29	Information request
SIGUSR1	30	User-defined signal 1
SIGUSR2	31	User-defined signal 2
SIGTHR	32	Thread interrupt

Solaris Signals

Signal Name	Signal Number	Signal Description
SIGHUP	1	Hangs up
SIGINT	2	Interrupts
SIGQUIT	3	Quits
SIGILL	4	Illegal instruction
SIGTRAP	5	Trace trap
SIGABRT	6	Used by abort
SIGEMT	7	EMT instruction
SIGFPE	8	Floating-point exception
SIGKILL	9	Kill (cannot be caught or ignored)
SIGBUS	10	Bus error
SIGSEGV	11	Segmentation violation
SIGSYS	12	Bad argument to system call
SIGPIPE	13	Writes on a pipe with no one to read it
SIGALRM	14	Alarm clock
SIGTERM	15	Software termination
SIGUSR1	16	User-defined signal 1
SIGUSR2	17	User-defined signal 2
SIGCHLD	18	Child status change alias (POSIX)
SIGPWR	19	Power-fail restart
SIGWINCH	20	Window size change
SIGURG	21	Urgent socket condition
SIGPOLL /SIGIO	22	Pollable event occurred or Socket I/O possible
SIGSTOP	23	Stop (cannot be caught or ignored)
SIGTSTP	24	User stop requested from TTY
SIGCONT	25	Stopped process has been continued
SIGTTIN	26	Background TTY read attempted
SIGTTOU	27	Background TTY write attempted
SIGVTALRM	28	Virtual timer expired
SIGPROF	29	Profiling timer expired
SIGXCPU	30	Exceeded CPU limit
SIGXFSZ	31	Exceeded file size limit
SIGWAITING	32	Process' LWPs are blocked
SIGLWP	33	Special signal used by thread library
SIGFREEZE	34	Special signal used by CPR
SIGTHAW	35	Special signal used by CPR
SIGCANCEL	36	Thread cancellation signal used by libthread
SIGLOST	37	Resource lost
SIGRTMIN	38	Highest priority real-time signal
SIGRTMAX	45	Lowest priority real-time signal

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