Why is the same log printed twice when using probe alias?

Why is the same log printed twice when using probe alias?

[Nan Xiao]

Hi all,

I am writing a simple script to test probe alias:

# cat test.stp
#!/usr/bin/stap

probe timer_alias = timer.s(3) {printf("Entering timer\n")}
probe timer_alias += timer.s(3) {printf("Leaving timer\n")}
probe timer_alias {printf("In timer\n")}

But executing it will outputs "In timer" twice:

# ./test.stp
Entering timer
In timer
In timer
Leaving timer
Entering timer
In timer
In timer
Leaving timer
.......

Is it the expected behavior? How to understand it?

Thanks very much in advance!

Best Regards
Nan Xiao

Re: Why is the same log printed twice when using probe alias?

[Frank Ch. Eigler]

Nan Xiao <xiaonan830818@gmail.com> writes:

> [...]
> probe timer_alias = timer.s(3) {printf("Entering timer\n")}
> probe timer_alias += timer.s(3) {printf("Leaving timer\n")}
> probe timer_alias {printf("In timer\n")}

> # ./test.stp
> Entering timer
> In timer
> In timer
> Leaving timer
> [...]


This is correct.  Each probe alias definition is independent of the
others, so in this case, timer_alias expands to two separate
timer.s(3) probes.  The += just means that the {} block associated
with the latter is supposed to be executed -after- the user's handler
rather than before.  (It is not an addition to the other alias's
probe handler or something like that.)


- FChE