首先,在談這個(gè)問(wèn)題時(shí),先說(shuō)說(shuō)unix中僵尸進(jìn)程的含義,APUE2中如下定義:
In UNIX System terminology, a process that has terminated, but whose parent has not yet waited for it, is called a zombie.
也就是說(shuō),但凡是父進(jìn)程沒(méi)有調(diào)用wait函數(shù)獲得子進(jìn)程終止?fàn)顟B(tài)的子進(jìn)程在終止之后都是僵尸進(jìn)程,這個(gè)概念的關(guān)鍵一點(diǎn)就是父進(jìn)程是否調(diào)用了wait函數(shù).
而關(guān)于SIGCHLD信號(hào),APUE2中又如是說(shuō):
Whenever a process terminates or stops, the SIGCHLD signal is sent to the parent. By default, this signal is ignored, so the parent must catch this signal if it wants to be notified whenever a child's status changes. The normal action in the signal-catching function is to call one of the wait functions to fetch the child's process ID and termination status.
簡(jiǎn)單的說(shuō),子進(jìn)程退出時(shí)父進(jìn)程會(huì)收到一個(gè)SIGCHLD信號(hào),默認(rèn)的處理是忽略這個(gè)信號(hào),而常規(guī)的做法是在這個(gè)信號(hào)處理函數(shù)中調(diào)用wait函數(shù)獲取子進(jìn)程的退出狀態(tài).
這里存在一個(gè)疑問(wèn),既然在SIGCHLD信號(hào)的處理函數(shù)中要調(diào)用wait函數(shù)族,為什么有了wait函數(shù)族還需要使用SIGCHLD信號(hào)?
我們知道,unix中信號(hào)是異步處理某事的機(jī)制,好比說(shuō)你準(zhǔn)備去做某事,去之前跟鄰居張三說(shuō)如果李四來(lái)找你的話就通知他一聲,這讓你可以抽身出來(lái)去做這件事,而李四真正來(lái)訪時(shí)會(huì)有人通知你,這個(gè)就是異步信號(hào)一個(gè)較為形象的比喻.
一般的,父進(jìn)程在生成子進(jìn)程之后會(huì)有兩種情況,一種是父進(jìn)程繼續(xù)去做別的事情,類似上面舉的例子,另一種是父進(jìn)程啥都不做,一直在wait子進(jìn)程退出.SIGCHLD信號(hào)就是為這第一種情況準(zhǔn)備的,它讓父進(jìn)程去做別的事情,而只要父進(jìn)程注冊(cè)了處理該信號(hào)的函數(shù),在子進(jìn)程退出時(shí)就會(huì)調(diào)用該函數(shù),在函數(shù)中wait子進(jìn)程得到終止?fàn)顟B(tài)之后再繼續(xù)做父進(jìn)程的事情.
也就是說(shuō),明確以下幾點(diǎn):
1)凡父進(jìn)程不調(diào)用wait函數(shù)族獲得子進(jìn)程終止?fàn)顟B(tài)的子進(jìn)程在退出時(shí)都會(huì)變成僵尸進(jìn)程.
2)SIGCHLD信號(hào)可以異步的通知父進(jìn)程有子進(jìn)程退出.