11.1.3 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 fires, as shown in the following example.

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
  1      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 16
^C

The output shows that the ERROR probe fired and that dtrace reported 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 shown in the following table:

Argument	Description
`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 the action or -1 if not applicable
`arg4`	The fault type
`arg5`	Value particular to the fault type

The following table describes the various fault types that can be specified in arg4 and the values that arg5 can take for each fault type:

`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 actions taken in the ERROR probe itself cause an error, that error is silently dropped. The ERROR probe is not recursively invoked.