zombie进程与orphan进程

(Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)
参考：https://www.tutorialspoint.com/zombie-and-orphan-processes-in-linux
参考：http://www.geekride.com/orphan-zombie-process/

zombie进程、orpahn进程是在linux中常被提到的进程。它们有着各自的特点，被系统识别标示出来。

orphan进程

孤儿进程：orphan
父进程终止，子进程还在执行；子进程被init-1号进程接管；
orphan进程是指进程还在执行，进程的父进程已经终止或结束；
有意创建的孤儿进程，类似于我们主动后台起的服务或是后台执行一个长时间的任务，这个是一个比较常见的情况；
无意创建的孤儿进程，类似于父进程异常终止了。
Orphan processes are those processes that are still running even though their parent process has terminated or finished.
A process can be orphaned intentionally or unintentionally.
An intentionally orphaned process runs in the background without any manual support. This is usually done to start an indefinitely running service or to complete a long-running job without user attention.
An unintentionally orphaned process is created when its parent process crashes or terminates. Unintentional orphan processes can be avoided using the process group mechanism.

在linux系统或类linux系统中，当父进程中止，子进程仍在执行这种情况，会立刻被重新指定父进程，把子进程的父进程指向1号进程-init进程；不过进程依然是orphan进程，因为原父进程已经终止。
In Linux/Unix like operating systems, as soon as parents of any process are dead, re-parenting occurs, automatically.
Re-parenting means processes whose parents are dead, means Orphaned processes, are immediately adopted by special process “init”.
Thing to notice here is that even after re-parenting, the process still remains Orphan as the parent which created the process is dead,

zombie进程

僵尸进程：zombie
父进程在持续运行中，子进程终止，父进程未使用wait检查子进程终止状态；此时子进程的终止状态始终保存在process table中。
A zombie process is a process whose execution is completed but it still has an entry in the process table.
Zombie processes usually occur for child processes, as the parent process still needs to read its child’s exit status.
Once this is done using the wait system call, the zombie process is eliminated from the process table. This is known as reaping the zombie process.
A diagram that demonstrates the creation and termination of a zombie process is given as follows

zobmie进程不再使用系统资源，但是会保留它的进程id；如果有许多的zombie进程，进程id就会被占用很多，可能会导致新进程的创建，因为进程id用完了。
Zombie processes don’t use any system resources but they do retain their process ID.
If there are a lot of zombie processes, then all the available process ID’s are monopolized by them. This prevents other processes from running as there are no process ID’s available.

zombie状态查看

显示所有zombie进程
ps -elf|head -1; ps -elf| awk ‘{if ($2 == “Z”) {print $0}}’

执行zombie测试程序：./a.out 12
(下面有测试程序代码样例）

[top]命令可以查到zombie进程数量：
top - 14:31:43 up 2:39, 6 users, load average: 0.15, 0.52, 0.64
Tasks: 597 total, 1 running, 595 sleeping, 0 stopped, 12 zombie

[ps -fu]命令可以看到进程状态
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
testuser 14454 0.0 0.0 4220 496 pts/4 S+ 14:32 0:00 _ ./a.out 12
testuser 14455 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14456 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14457 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14458 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14459 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14460 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14461 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14462 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14463 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14464 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14465 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]
testuser 14466 0.0 0.0 0 0 pts/4 Z+ 14:32 0:00 _ [a.out]

orphan状态查看

显示所有orphan进程：
ps -elf | head -1; ps -elf | awk ‘{if ($5 == 1 && $3 != “root”) {print $0}}’

执行arphan测试程序：./a.out 3
(下面有测试程序代码样例）

[ps -fu]命令可以看到进程状态
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
testuser 19193 0.0 0.0 4220 92 pts/4 S 14:37 0:00 ./a.out 3
testuser 19192 0.0 0.0 4220 92 pts/4 S 14:37 0:00 ./a.out 3
testuser 19191 0.0 0.0 4216 88 pts/4 S 14:37 0:00 ./a.out 3

[ps -ef|grep a.out]命令查看进程父进程变成了init-1
UID PID PPID C STIME TTY TIME CMD
testuser 19191 1 0 14:37 pts/4 00:00:00 ./a.out 3
testuser 19192 1 0 14:37 pts/4 00:00:00 ./a.out 3
testuser 19193 1 0 14:37 pts/4 00:00:00 ./a.out 3

关于processs stat code说明
PROCESS STATE CODES
Here are the different values that the s, stat and state output
specifiers (header “STAT” or “S”) will display to describe the state of
a process.
D Uninterruptible sleep (usually IO)
R Running or runnable (on run queue)
S Interruptible sleep (waiting for an event to complete)
T Stopped, either by a job control signal or because it is being
traced.
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z Defunct (“zombie”) process, terminated but not reaped by its
parent.

   For BSD formats and when the stat keyword is used, additionalcharacters may be displayed:<    high-priority (not nice to other users)N    low-priority (nice to other users)L    has pages locked into memory (for real-time and custom IO)s    is a session leaderl    is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)+    is in the foreground process group

orphan测试代码

#include "stdio.h"
#include "stdlib.h"
#include "unistd.h"
int main(int argc, char** argv){int subproc = 1;if (argc == 2){subproc = atoi(argv[1]);}printf("start of parent: %d\n", getpid());int pid = 0;for (int i=0; i<subproc; i++){pid = fork();if (pid == 0){break;}else{printf("create child proc: %d\n", pid);}}if (pid == 0){getchar();printf("end of child: %d\n", getpid());}else{printf("end of parent: %d\n", getpid());}return 0;
}

zombie测试代码

#include "stdio.h"
#include "stdlib.h"
#include "unistd.h"
int main(int argc, char** argv){int subproc = 1;if (argc == 2){subproc = atoi(argv[1]);}printf("start of parent: %d\n", getpid());int pid = 0;for (int i=0; i<subproc; i++){pid = fork();if (pid == 0){break;}else{printf("create child proc: %d\n", pid);}}if (pid == 0){printf("end of child: %d\n", getpid());}else{getchar();printf("end of parent: %d\n", getpid());}return 0;
}

最后再标注一个问题点：orphan与zombie相同吗？
两者较明显的不同；
orphan进程是一个运行着的进程，alive；
zombie进程是一个终止了的进程但在进程表中仍有记录，not alive；
Orphan process are totally different from Zombie processes.
Zombie processes are the ones which are not alive but still have entry in process table.

(Owed by: 春夜喜雨 http://blog.csdn.net/chunyexiyu)