只有在ret probe函数中，在这个函数中才会使用＠entry函数去提取变量
是因为ret probe 有什么特殊的吗？在中间这个变量会变化吗？
A new operator, @entry, is available for automatically saving an expression at entry time for use in a .return probe.
但是
A SystemTap script can also accept simple command-line arguments using a
$
or
@
immediately followed
by the number of the argument on the command line. Use
$
if you are expecting the user to enter an
integer as a command-line argument, and
@
if you are expecting a string.
stap中的entry函数
原文：https://www.cnblogs.com/honpey/p/9011805.html
