常见系统Signals解析

介绍

系统实现 中断机制 依赖于Signals，而其存在是为了当一个程序（Process）运行时，响应“用户请求” or 系统强制介入。

Signals are software interrupts sent to a program to indicate that an important event has occurred. The events can vary from user requests to illegal memory access errors. Some signals, such as the interrupt signal, indicate that a user has asked the program to do something that is not in the usual flow of control.

以下为常见的Signals，更多的可以尝试kill -l查看。总共64个(0 预留，所以0 - 64 占据 7 bits)
Note: 以下 Examples 实现使用 Perl5

Signal Name	Signal Number	Description	Examples
SIGHUP	1	Hang up detected on controlling terminal or death of controlling process	$SIG{HUP}
SIGINT	2	Issued if the user sends an interrupt signal (Ctrl + C).	$SIG{INT}
SIGQUIT	3	Issued if the user sends a quit signal (Ctrl + D).	TBD
SIGFPE	8	Issued if an illegal mathematical operation is attempted	TBD
SIGKILL	9	If a process gets this signal it must quit immediately and will not perform any clean-up operations. This signal cannot be handled (caught), ignored or blocked -- rough termination	TBD
SIGALRM	14	Alarm Clock signal (used for timers)	$SIG{ALRM}
SIGTERM	15	Software termination signal immediately (sent by kill by default). This is used for graceful termination of a process, unlike SIGKILL, it's a graceful termination	TBD
SIGCHLD	17	Child process exit signal, sent to parent process	$SIG{CHLD}

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赋值%SIG

大多数的Signals是用来被捕获并作出回应（无论是由用户or系统）, 赋值可以为　IGNORE, DEFAULT and \&SOMESUB，依次列举如下：

$SIG{XXX} = 'IGNORE';

字面意思：忽略它，即使catch a signal也当没发生。例如$SIG{CHLD}，parent process 在catch a SIGCHLD后通常会回收该Child process资源。好吧，现在它不管了。还好OS提供backup。这是最省事的回收方式，常见REAPER

$SIG{XXX} = 'DEFAULT';

每种signal 都有 default action，即程序(Process)　catch后应有的动作，可能如下：

• Terminate the process.
• Ignore the signal.
• Dump core. This creates a file called core containing the memory image of the process when it received the signal.
• Stop the process.
• Continue a stopped process.

$SIG{XXX} = '&SOMESUB';

此处\&SOMESUB就是用户对该SIGXXX作出的回应，定义在sub SOMESUB{# what shold do}, 实例常见上文任意Examples

PS: %SIG 为Perl预定义的supervar，存储着所有Signals，遵循POSIX，常见perlvars

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总结

每种Signals都有自身的特殊含义，虽然只用一个数字精简的表示, 就像汽车鸣笛, 当行人听见, 意味着等待或避让; 但前车听见了, 意思可就多了, 如催促其别龟速了; 也可能好心提醒车胎爆了. 所以，只有日常使用中实践使用，才能认识其应用场景，后续将继续补充Examples