CARLsim Documentation:
Class SpikeMonitor

To retrieve outputs, a spike-monitoring callback mechanism is used. This mechanism allows the user to calculate basic statistics, store spike trains, or perform more complicated output monitoring. Spike monitors are registered for a group and are called automatically by the simulator every second. Similar to an address event representation (AER), the spike monitor indicates which neurons spiked by using the neuron ID within a group (0-indexed) and the time of the spike. Only one spike monitor is allowed per group.
For more information see Section Interacting with the simulation: Spike monitoring in the Tutorial.
In order to create a custom spike monitor one needs to create a new class that derives from the SpikeMonitor class:

class MyMonitor: public SpikeMonitor { ...

One does then need to define an update method.

^SpikeMonitor::update

CARLsim ≥ 2.0

virtual void update(CpuSNN* s, int grpId, unsigned int* Nids,
                    unsigned int* timeCnts)

Input arguments:

CpuSNN* s: pointer to a CpuSNN object
int grpId: neuron group ID for which the spike generator is called

Output arguments:

unsigned int* Nids: array of neuron IDs for which spikes are monitored
unsigned int* timeCnts: 1000-element strong array (for each millisecond in a second) that contains the ID of neurons that spiked