# Signals






# Data Structures Associated with Signals

Signal descriptor related to signal handling

# The Role of Signals

Linux系統各個processes會有不同的state，這些processes要嘛屬於user applications要嘛就屬於kernel.我們需要一種機制來協調他們的活動，其中一種辦法就是讓process可以在有重要的事情發生時去通知其他process.

signal基本上是一種one way notification.
* Kernel -> process
* process -> other process
* process -> 自己

不只hardware可以interrupt cpu, signals也可以interrupt process execution.(software interrupts)

* hardware interrupt -> interrupt handlers
* signal interrupt -> signal handlers

有些singals直接mapping到一些hot key

* SIGINT -> ctrl+c
* SIGSTOP -> ctrl+z
* SIGQUIT -> ctrl+\

# How does a signal affect the state of a Linux process?

有些signals會直接終結掉被通知到process
* SIGHUP
* SIGINT
* SIGTERM
* SIGKILL

有些signals終結掉process後會附送core dump讓人debug
* SIGABRT -> abort signal
* SIGBUS -> bus error
* SIGILL -> illegal instruction
* SIGSEGV -> invalid memory reference
* SIGSYS -> bad system call

有些signals stop the process
* SIGSTOP
* SIGTSTP

有些signals resumes a stopped process
* SIGCONT

你可以在program裡覆寫掉signal的行為
* SIGKILL and SIGSTOP除外

例子

# Aren't signals similar to exceptions?


# Are Linux signals synchronous or asynchronous?

都可
* Synchronous(伴隨CPU內部指令的順序發生)
* 當CPU執行到錯誤指令例如illegal addresss access的時候，signals會send給發生的thread.這也被稱為 traps,因為會觸發trap進kernel trap handler
* Asynchronous(外部發生的，跟CPU內部指令順序無關)
* 例如被其他process send SIGKILL，這也被稱為software interrupts

# What's the typical lifecycle of a signal?

System calls and functions that can generate signals
* raise
* kill
* killpg
* pthread_kill
* sigqueue

Delivery:
* signal還沒被送達目標會是pending狀態
* 如果一個process目前是block signal狀態, signal會一直pending直到block解除

Processing: signal可以怎樣被處理?
* 每個signal都有一個default動作
* ignore the signal
* terminate the process
* core dump
* stop/continue
* .......
* 對於non default的行為，可以call handler來處理--> sigaction function

# blocking and unblocking signals

Signals可能會interrupt一個正在執行重要任務的code，process這時可以選擇Blocking signals.

* 每個procss都可以指定要block哪種signals
* 如果被指定block的signals被送來了，kernel會先pending住，等unblock在送給你，這個block的動作叫做signal mask

signal mask
* 可以用sigprocmask -> singlethreaded
* pthread_sigmask -> multi-threaded