ERROR Probe

`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 11-7 Showing How to Record Errors

BEGIN
{
*(char *)NULL;
}

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

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

# .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 reporting the error. dtrace has its own enabling of the ERROR probe to allow it to report errors. Use the ERROR probe to customize error handling.

The arguments to the ERROR probe are as follows:

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

The following table describes the arg4 fault types and the value and its meaning for the arg5.

Table 11-2 ERROR Probe Fault Types and Values

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
`DTRACEFLT_BADSTACK`	Invalid user process stack	Address of invalid stack pointer

If the error actions taken in the ERROR probe cause an error, the error is dropped and the ERROR probe is not recursively invoked.