Re: [PATCH] Linux Kernel Markers 0.11 for 2.6.17

From: Frank Ch. Eigler
Date: Tue Sep 26 2006 - 12:42:51 EST

Mathieu Desnoyers <compudj@xxxxxxxxxxxxxxxxxx> writes:

> [...]
> > I believe [printf formatting directives] are not
> > quite general enough either e.g. to describe a raw binary blob.
> If you want to dump a raw binary blob, what about :
> MARK(mysubsys_myevent, "char %p %u", blobptr, blobsize); where %p is
> a pointer to an array of char and %u the length ?

That involves new conventions beyond printf. Why not "%p %p %u %u"
for two blobs ... or why implicitly dereference the given pointers. A
probe handler unaware of a specific marker's semantics would not know
whether or not this is implied.

> My idea is to use the string to identify what is referred by a
> pointer, so it can be casted into this type with some kind of
> coherency between the marker and the probe.

I understand what you're using them for. To me, they just don't look
like a good fit.

> > I realize they serve a useful purpose in abbreviating what otherwise
> > one might have to do (like that multiplicity of STAP_MARK_* type/arity
> > permutations). [...]
> I think that duplicating the number of marker macros could easily make
> them unflexible and ugly. [...]

Inflexible and ugly in what way? Remember, the macro definitions can
be automatically generated. At the macro call site, there needs to be
little difference.

> [...] Good point, I will setup a va_args in the probe. When
> correctly used, however, there is no need to use the format string :
> we can directly get the variables from the var arg list if we know
> in advance what the string will be.

Do I understand you correctly that the probe handlers would be given
va_list values, and would have to call va_arg to yank out individual
actual arguments? So again type safety is a matter of explicit coding
(equivalent to correctly casting each type)?

- FChE
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