If I have foo(), where
foo() {
return(bar());
}
then should I expect anything unusual about args[1]
in fbt::foo:return? Should I expect args[1] in such
a probe to give me X where
X = foo()
?
I ask after finding something odd about the following
code in mapin():
145 pfnum = hat_getpfnum(as->a_hat, addr);
146 if (pf_is_memory(pfnum)) {
dtrace alleged that the return value of hat_getpfnum()
differed from the argument to pf_is_memory(); recompiling
with cmn_err() showed that the latter was correct. The value
of args[1] in hat_getpfnum:return looked like the second
argument passed in.
Robert
Hey Robert,> If I have foo(), where > > foo() { > return(bar()); > } > > then should I expect anything unusual about args[1] > in fbt::foo:return? Should I expect args[1] in such > a probe to give me X where > > X = foo()If the code above were compiled into a tail call, the foo:return probe would fire before the bar:entry probe. The value of arg1 or args[1] will not contain the value returned to the caller of foo(). To trace that value, one can write a simple script: foo:entry { self->follow = 1; } bar:return /self->follow/ { trace(arg1); self->follow = 0; } This applies both for user-land and the kernel. Adam -- Adam Leventhal, Fishworks http://blogs.sun.com/ahl
Hi Adam, On 06/11/10 22:44, Adam Leventhal wrote:> If the code above were compiled into a tail call, the > foo:return probe would fire before the bar:entry probe. > The value of arg1 or args[1] will not contain the value > returned to the caller of foo().Many thanks for the clarification. May I suggest that the fbt chapter in the DTrace wiki be updated to reflect this? The section on Tail-call Optimization discusses the difference in ordering, but I don''t think that the effect on args[1] follows. Regards, Robert
>> If the code above were compiled into a tail call, the >> foo:return probe would fire before the bar:entry probe. >> The value of arg1 or args[1] will not contain the value >> returned to the caller of foo(). > > Many thanks for the clarification. May I suggest that > the fbt chapter in the DTrace wiki be updated to reflect > this? The section on Tail-call Optimization discusses the > difference in ordering, but I don''t think that the effect > on args[1] follows.Sounds good. If you -- or anyone else -- has suggestions for how to improve the documentation here, please follow the normal procedure of posting to the list so the community can approve before updating the wiki. Thanks, and apologies for the confusing behavior. Adam -- Adam Leventhal, Fishworks http://blogs.sun.com/ahl