Linux 进程有哪些状态
通过ps的帮助手册，能看到进程有几种状态
man ps
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进程_exit退出后，进程占用的内存和其他资源会被回收，同时在操作系统的process table中依旧保留一条记录(存储PID, termination status, resource usage information)，此时进程的状态是zombie / defunct的 。
父进程会使用waitpid系统调用，回收处于zombie状态的子进程，回收后进程的信息才会从process table去除。
wiki里的例子
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
int main(void)
{
pid_t pids[10];
int i;
for (i = 9; i >= 0; --i) {
pids[i] = fork();
if (pids[i] == 0) {
sleep(i+1);
_exit(0);
}
}
for (i = 9; i >= 0; --i)
waitpid(pids[i], NULL, 0);
return 0;
}man 2 wait 里的解释
A child that terminates, but has not been waited for becomes a "zom-
bie". The kernel maintains a minimal set of information about the zom-
bie process (PID, termination status, resource usage information) in
order to allow the parent to later perform a wait to obtain information
about the child. As long as a zombie is not removed from the system
via a wait, it will consume a slot in the kernel process table, and if
this table fills, it will not be possible to create further processes.
If a parent process terminates, then its "zombie" children (if any) are
adopted by init(8), which automatically performs a wait to remove the
zombies.如果zombie进程的父进程被terminate了，那么zombie进程将会被init进程接管，init进程也会异步的调用wait回收处于zombie的进程。
什么时候会进入Zombie状态
前面已经解释了，当进程exit后，会进入zombie状态。
然后它的父进程会通过waitpid调用，回收处于zombie状态的进程。
如果处于zombie状态进程的父进程被terminate了，没有被回收的zombie进程就会被init接管，init也会调用waitpid来回收它的zombie子进程。
另外需要注意，POSIX.1-2001标准的系统，允许设置SIGCHLD to SIG_IGN，这样不需要通过wait来回收zombie子进程。 因为它fork的子进程进程不会进入zombie状态，exit后不会在process table中保留任何信息。
On modern UNIX-like systems (that comply with SUSv3 specification in this respect), the following special case applies: if the parent explicitly ignores SIGCHLD by setting its handler to SIG_IGN (rather than simply ignoring the signal by default) or has the SA_NOCLDWAIT flag set, all child exit status information will be discarded and no zombie processes will be left
POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN.
POSIX.1-2001 allows this possibility, so that ignoring SIGCHLD can be
used to prevent the creation of zombies (see wait(2)). Nevertheless,
the historical BSD and System V behaviors for ignoring SIGCHLD differ,
so that the only completely portable method of ensuring that terminated
children do not become zombies is to catch the SIGCHLD signal and per-
form a wait(2) or similar.如何清理Zombie状态的进程
通过zombie进程的父进程，发起waitpid调用，清理process table中对应子进程的信息。
如果父进程没有处理，可以手工向他发起 SIGCHLD 信号，让他处理。
kill -SIGCHLD $(ps -A -ostat,ppid | awk '/[zZ]/{print $2}')如果发这个信号还不行，那可能是parent进程没有处理这个信号，把父进程也干掉，然后让init去回收zombie进程。
如果没有清理掉会有什么危险
zombie进程会占用process table的slot，如果有非常多的zombie，可能最终会导致process table slot满，导致系统不能创建新的进程。
参考
http://stackoverflow.com/questions/16944886/how-to-kill-zombie-process
https://manpages.debian.org/cgi-bin/man.cgi?query=wait&sektion=2
https://en.wikipedia.org/wiki/Zombie_process
http://unix.stackexchange.com/questions/11172/how-can-i-kill-a-defunct-process-whose-parent-is-init
http://stackoverflow.com/questions/20535438/cant-cleanup-a-zombie-process-whose-parent-is-init
man 2 exit
man 2 wait
man 7 signal
PostgreSQL 如何处理退出的子进程
也能看到waitpid的影子
注册信号处理函数
src/backend/postmaster/postmaster.c: pqsignal(SIGCHLD, reaper); /* handle child termination */
信号处理函数内容
/*
* Reaper -- signal handler to cleanup after a child process dies.
*/
static void
reaper(SIGNAL_ARGS)
{
int save_errno = errno;
int pid; /* process id of dead child process */
int exitstatus; /* its exit status */
PG_SETMASK(&BlockSig);
ereport(DEBUG4,
(errmsg_internal("reaping dead processes")));
while ((pid = waitpid(-1, &exitstatus, WNOHANG)) > 0)
{
...