当重复使用pid时,选择哪种pid,例如是最低的pid？
解决方法
Process IDs are usually allocated on a sequential basis,beginning at
0 and rising to a maximum value which varies from system to system.
Once this limit is reached,allocation restarts at 300 and again
increases. In Mac OS X and HP-UX,allocation restarts at 100. However,
for this and subsequent passes any PIDs still assigned to processes
are skipped. Some consider this to be a potential security
vulnerability in that it allows information about the system to be
extracted,or messages to be covertly passed between processes. As
such,implementations that are particularly concerned about security
may choose a different method of PID assignment.[1] On some systems,
like MPE/iX,the lowest available PID is used,sometimes in an effort
to minimize the number of process information kernel pages in memory.
原始答案：https://superuser.com/questions/135007/how-are-pids-generated