ERROR Probe

The ERROR probe fires when a run-time error occurs in executing a clause for a DTrace probe. For example, if a clause attempts to dereference a NULL pointer, the ERROR probe will fire, as shown in the following example.

Example 17-1 error.d: Record Errors

BEGIN
{
    *(char *)NULL;
}

ERROR
{
    printf("Hit an error!");
}

When you run this program, you will see output like the following example:

# dtrace -s ./error.d
dtrace: script './error.d' matched 2 probes
CPU     ID                    FUNCTION:NAME
  2      3                           :ERROR Hit an error!
dtrace: error on enabled probe ID 1 (ID 1: dtrace:::BEGIN): invalid address
(0x0) in action #1 at DIF offset 12
dtrace: 1 error on CPU 2

The output shows that the ERROR probe fired, and also illustrates dtrace(1M) reporting the error. dtrace has its own enabling of the ERROR probe to allow it to report errors. Using the ERROR probe, you can create your own custom error handling.

The arguments to the ERROR probe are as follows:

arg1	The enabled probe identifier (EPID) of the probe that caused the error
arg2	The index of the action that caused the fault
arg3	The DIF offset into that action or -1 if not applicable
arg4	The fault type
arg5	Value particular to the fault type

The table below describes the various fault types and the value that arg5 will have for each:

arg4 Value	Description	arg5 Meaning
DTRACEFLT_UNKNOWN	Unknown fault type	None
DTRACEFLT_BADADDR	Access to unmapped or invalid address	Address accessed
DTRACEFLT_BADALIGN	Unaligned memory access	Address accessed
DTRACEFLT_ILLOP	Illegal or invalid operation	None
DTRACEFLT_DIVZERO	Integer divide by zero	None
DTRACEFLT_NOSCRATCH	Insufficient scratch space to satisfy scratch allocation	None
DTRACEFLT_KPRIV	Attempt to access a kernel address or property without sufficient privileges	Address accessed or 0 if not applicable
DTRACEFLT_UPRIV	Attempt to access a user address or property without sufficient privileges	Address accessed or 0 if not applicable
DTRACEFLT_TUPOFLOW	DTrace internal parameter stack overflow	None

If the actions taken in the ERROR probe itself cause an error, that error is silently dropped — the ERROR probe will not be recursively invoked.