Hello all,
I'm currently doing undergraduate research using dtrace at the University of
Wisconsin-Madison, and I had a question concerning DTrace on Leopard. If the
non-Solaris-oriented nature of the question makes this inappropriate for this
list, then I apologize.
I have been using the syscall provider to obtain info about IO, and in nearly
all cases it works correctly. For one particular syscall (open_nocancel) (and
I realize this is almost definitely OS dependent), I get a garbage string for
arg0, which baffles me, as arg0 should be the pathname for the open call.
My code looks like this:
syscall::open_nocancel:entry
{
this->now = timestamp - timeZero;
printf("at %d nsecs %s called %s ", this->now, execname, probefunc);
self->path = arg0;
}
syscall::open_nocancel:return
{
printf("with filename:%s and file descriptor %d\n",
copyinstr(self->path), arg1);
fileNames[arg0] = copyinstr(self->argsPtr->path);
}
I've considered the option that arg0 is not what I think it is, but when I ran
this piece of C code:
int fd = syscall(SYS_open_nocancel, "foo.txt", O_RDWR| O_CREAT | O_TRUNC);
It behaves exactly like a call to open, leading me to believe (perhaps
erroneously) that the arguments should be the same.
Essentially I was wondering two things that would help me understand this.
First, I was wondering how the syscall provider obtains the arguments that
correspond to arg0, arg1, ... argN, as I have been unable to locate this in the
documentation, and also if it could be the case that a memory location
referenced by a syscall argument could be manipulated between the time at which
the entry and the return probes fire. Again, these are probably more OS
implementation related questions than DTrace questions and may be inappropriate
for this mailing list. Thank you very much for your time and patience,
Michael Vaughn
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