If you trace every call to the write()
system
call, it causes a cascade of output because each call causes the
dtrace command to call
write()
as it displays the output, and so on.
This feedback loop is a good example of how the
dtrace command can interfere with the desired
data. To prevent this type of unwanted data from being traced, use
a simple predicate like the one that is shown in the following
example and save it in a file named stringof.d
:
syscall::write:entry /pid != $pid/ { printf("%s", stringof(copyin(arg1, arg2))); }
In the previous example, the $pid
macro
variable expands to the process identifier of the process that
enabled the probes. The pid
variable contains
the process identifier of the process whose thread was running on
the CPU where the probe was fired. Therefore, the predicate
/pid != $pid/
ensures that the script does not
trace any events related to itself.