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.