场景

在程序中遇到耗时较长的操作（如等待用户响应，连接remote server），通常会想到创建 Child process 去处理。在Perl 中使用fork()

## fork a child process
die "$@" unless defined( my $child_pid = fork());
if ($child_pid) {  # If I have a child PID, then I must be the parent
    push @children_pids, $child_pid;
    print "children's PIDs: @children_pids\n";
} else { # I am the child
    my $wait_time = int(rand(10));
    sleep $wait_time;
    my $localtime = localtime;
    print "Child: Some child exited at $localtime\n";
    exit 0; # Exit the child
}

那么 Child process 的回收问题怎么解决？如果 Parent process 任由它自生自灭，就会导致Zombie process。

回收方法

有两种方式如下，无论哪种都涉及waitpid(),当然也可以使用wait()（不建议）

Blocking wait
Non blocking wait
在此提一下waitpid()下文code有使用

waitpid PID,FLAGS
PID: -1(stands for any child process) | pid ( > 0 )
FLAGS: 0 (for a blocking wait) | WHOHANG (return 0 or -1if no dead children)
i.e,. waitpid($pid, 0) ## in a blocking wait
waitpid(-1, WHOHANG) ## no blocking wait

Blocking wait

见如下code:

my @children_pids;
print "Parent: my pid $$\n";
for my $count (1..10){
    die "$@" unless defined( my $child_pid = fork());
    if ($child_pid) {  # If I have a child PID, then I must be the parent
        push @children_pids, $child_pid;
        print "children's PIDs: @children_pids\n";
    } else { # I am the child
        my $wait_time = int(rand(10));
        sleep $wait_time;
        my $localtime = localtime;
        print "Child: Some child exited at $localtime\n";
        exit 0; # Exit the child
    }
}

foreach my $child (@children_pids) {
        print "Parent: Waiting on $child\n";
        waitpid($child, 0);　## will not go to next step unless $child reaped
        my $localtime = localtime;
        print "Parent: Child $child was reaped - $localtime.\n";
}

Output:

[root@VTB93-PC1 ~]# perl /tmp/test_fork.pl
Parent: my pid 15117
children's PIDs: 15118
children's PIDs: 15118 15119
children's PIDs: 15118 15119 15120
children's PIDs: 15118 15119 15120 15121
children's PIDs: 15118 15119 15120 15121 15122
children's PIDs: 15118 15119 15120 15121 15122 15123
children's PIDs: 15118 15119 15120 15121 15122 15123 15124
children's PIDs: 15118 15119 15120 15121 15122 15123 15124 15125
children's PIDs: 15118 15119 15120 15121 15122 15123 15124 15125 15126
children's PIDs: 15118 15119 15120 15121 15122 15123 15124 15125 15126 15127
Parent: Waiting on 15118
Child: Some child exited at Thu Apr 21 13:28:48 2016
Parent: Child 15118 was reaped - Thu Apr 21 13:28:48 2016.
Parent: Waiting on 15119
Child: Some child exited at Thu Apr 21 13:28:48 2016
Child: Some child exited at Thu Apr 21 13:28:48 2016
Child: Some child exited at Thu Apr 21 13:28:50 2016
Child: Some child exited at Thu Apr 21 13:28:51 2016
Parent: Child 15119 was reaped - Thu Apr 21 13:28:51 2016.
Parent: Waiting on 15120
Child: Some child exited at Thu Apr 21 13:28:51 2016
Child: Some child exited at Thu Apr 21 13:28:52 2016
Child: Some child exited at Thu Apr 21 13:28:53 2016
Child: Some child exited at Thu Apr 21 13:28:54 2016
Parent: Child 15120 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15121
Parent: Child 15121 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15122
Parent: Child 15122 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15123
Child: Some child exited at Thu Apr 21 13:28:54 2016
Parent: Child 15123 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15124
Parent: Child 15124 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15125
Parent: Child 15125 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15126
Parent: Child 15126 was reaped - Thu Apr 21 13:28:54 2016.
Parent: Waiting on 15127
Parent: Child 15127 was reaped - Thu Apr 21 13:28:54 2016.

可以看出规律，回收的顺序是按照｀fork()｀的顺序, 可在实际中 child process 耗时有长有短,后fork()的process很有可能比较早的先结束，所以引入第二种Reap机制，使用$SIG{CHLD}

Non blocking wait

这里先提一下$SIG{CHLD}, 可接受的赋值如下：

$SIG{CHLD} = 'IGNORE'; ## Children reaped by system
$SIG{CHLD} = 'DEFAULT'; ## System defined
$SIG{CHLD} = &REAPER; ## do REAPER if SIGCHLD catched

use POSIX ":sys_wait_h";

$SIG{CHLD}=\&REAPER;
sub REAPER {
    my $child;
    while(( $child = waitpid(-1, &WNOHANG)) > 0){
        my $localtime = localtime;
        print "Parent: Child $child was reaped - $localtime.\n";
    }
    $SIG{CHLD}=\&REAPER;
}
my @children_pids;
print "Parent: my pid $$\n";
for my $count (1..10){
    die "$@" unless defined( my $child_pid = fork());
    if ($child_pid) {  # If I have a child PID, then I must be the parent
        push @children_pids, $child_pid;
        print "children's PIDs: @children_pids\n";
    } else { # I am the child
        my $wait_time = int(rand(10));
        sleep $wait_time;
        my $localtime = localtime;
        print "Child: Some child exited at $localtime\n";
        exit 0; # Exit the child
    }
}
## Keep parent alive to reap all children
while (1) {
    sleep;
}

Outpuit:

Parent: my pid 15189
children's PIDs: 15194
children's PIDs: 15194 15195
children's PIDs: 15194 15195 15196
children's PIDs: 15194 15195 15196 15197
children's PIDs: 15194 15195 15196 15197 15198
children's PIDs: 15194 15195 15196 15197 15198 15199
children's PIDs: 15194 15195 15196 15197 15198 15199 15200
children's PIDs: 15194 15195 15196 15197 15198 15199 15200 15201
children's PIDs: 15194 15195 15196 15197 15198 15199 15200 15201 15202
children's PIDs: 15194 15195 15196 15197 15198 15199 15200 15201 15202 15203
Child: Some child exited at Thu Apr 21 13:42:25 2016
Parent: Child 15202 was reaped - Thu Apr 21 13:42:25 2016.
Child: Some child exited at Thu Apr 21 13:42:27 2016
Parent: Child 15197 was reaped - Thu Apr 21 13:42:27 2016.
Child: Some child exited at Thu Apr 21 13:42:29 2016
Parent: Child 15201 was reaped - Thu Apr 21 13:42:29 2016.
Child: Some child exited at Thu Apr 21 13:42:30 2016
Parent: Child 15194 was reaped - Thu Apr 21 13:42:30 2016.
Child: Some child exited at Thu Apr 21 13:42:31 2016
Parent: Child 15198 was reaped - Thu Apr 21 13:42:31 2016.
Child: Some child exited at Thu Apr 21 13:42:31 2016
Parent: Child 15200 was reaped - Thu Apr 21 13:42:31 2016.
Child: Some child exited at Thu Apr 21 13:42:31 2016
Parent: Child 15203 was reaped - Thu Apr 21 13:42:31 2016.
Child: Some child exited at Thu Apr 21 13:42:32 2016
Parent: Child 15199 was reaped - Thu Apr 21 13:42:32 2016.
Child: Some child exited at Thu Apr 21 13:42:33 2016
Parent: Child 15195 was reaped - Thu Apr 21 13:42:33 2016.
Child: Some child exited at Thu Apr 21 13:42:34 2016
Parent: Child 15196 was reaped - Thu Apr 21 13:42:34 2016.

可以看出但某个Child exit 后会发送SIGCHLD给Parent，顺序依据exit先后。当然，这是Parent跟踪Children（如获取返回信息local $?，以上都返回 0 ）作出相应操作。如果无需如此，可直接$SIG{CHLD}='INGNORE'; 交给OS吧。

总结

使用 SIGCHLD REAPER 更符合现实场景，结合waitpid()以 Non blocking wait 方式
Parent 负责 Children 的生命周期，以免出现 Zombie process
PID 1 init ## 所有Process 的 parent

[Perl]REAPER

场景

回收方法

Blocking wait

Non blocking wait

总结

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