Matlab S-Function: Why does ssGetInputPortSignal() return NULL?

There are several different reasons why ssGetInputPortSignal() returns NULL or nullptr?

• You might be passing bad parameters to ssGetInputPortSignal(). A typical use would be

real_T *input = ssGetInputPortSignal(S, 0);

• The input port might not be connected. In this case, ssGetInputPortSignal() will return NULL or nullptr. You can check if the port is connected using ssGetInputPortConnected():

if (!ssGetInputPortConnected(S, 0)) {
    mexPrintf("Input port 0 is not connected\n");
}

• You might be calling ssGetInputPortSignal() in the wrong place. For example, if you call it in the mdlInitializeSizes() or mdlOutputs() functions, it will return NULL or nullptr. You should call it in the mdlStart() or mdlUpdate() functions instead.