From: fche@redhat.com (Frank Ch. Eigler)
To: Nan Xiao <xiaonan830818@gmail.com>
Cc: systemtap@sourceware.org
Subject: Re: How to understand "?" in probe alias?
Date: Fri, 13 Nov 2015 14:00:00 -0000 [thread overview]
Message-ID: <y0moaeyxan4.fsf@fche.csb> (raw)
In-Reply-To: <CA+MhoaMpa3iG5KJA6eL03eez=24qwwwyMjymozXD7K4MbWf52A@mail.gmail.com> (Nan Xiao's message of "Fri, 13 Nov 2015 15:14:02 +0800")
Nan Xiao <xiaonan830818@gmail.com> writes:
> [...]
> probe vm.pagefault = kernel.function("__handle_mm_fault@mm/memory.c") ?,
> kernel.function("handle_mm_fault@mm/memory.c") ?
> [...] How to understand "?" in probe alias? After checking Language
> reference [...] I can't find the explanations of it.
You're right; that particular book doesn't seem to mention it.
But [man stap] refers to [man stapprobes], which includes:
However, a probe point may be followed by a "?" character, to
indicate that it is optional, and that no error should result if
it fails to resolve. Optionalness passes down through all levels
of alias/wildcard expansion. Alternately, a probe point may be
followed by a "!" character, to indicate that it is both optional
and sufficient. (Think vaguely of the Prolog cut operator.) If it
does resolve, then no further probe points in the same
comma-separated list will be resolved. Therefore, the "!"
sufficiency mark only makes sense in a list of probe point
alternatives.
- FChE
prev parent reply other threads:[~2015-11-13 14:00 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-13 7:14 Nan Xiao
2015-11-13 14:00 ` Frank Ch. Eigler [this message]
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