Making C Modules

This section covers how to write a C Basilisk Module. A sample C Basilisk module is found in C Module: cModuleTemplate.

Simulation Time

Basilisk passes the current simulation time to a C module as the unsigned integer callTime in nanoseconds. Convert an absolute timestamp to seconds with an explicit cast and NANO2SEC:

#include "architecture/utilities/macroDefinitions.h"

double absoluteTimeSec = (double) callTime * NANO2SEC;  /* [s] */

This conversion remains finite during simulations longer than \(2^{53}\) nanoseconds, approximately 104 days, although a double can no longer preserve every individual nanosecond at that scale. Do not use nanoToSec(callTime) for an absolute timestamp because nanoToSec() deliberately returns NAN above that exact-integer limit.

For a relative time, subtract the integer timestamps before converting. The diffNanoToSec() helper handles either timestamp order and preserves the precision of a bounded interval:

double elapsedTimeSec = diffNanoToSec(callTime, previousTimeNanos);  /* [s] */

Store previousTimeNanos as a uint64_t and update it only after the elapsed-time calculation. diffNanoToSec() returns NAN if the interval itself is greater than \(2^{53}\) nanoseconds. Rebase the stored reference timestamp when exact nanosecond resolution is required across a long-running simulation.

Unused Lifecycle Parameters

Basilisk lifecycle functions have framework-defined parameter lists. Keep an unused parameter in the function signature and cast it to void in the function body to document that it is intentionally unused and suppress compiler warnings:

void Reset_myModule(MyModuleConfig *configData, uint64_t callTime, int64_t moduleID)
{
    (void) configData;
    (void) callTime;
    (void) moduleID;
}

Remove the cast if the implementation later uses the parameter.