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S-Function Examples
Note The mdlUpdate and mdlTerminate functions
both use the macro UNUSED_ARG. This macro is defined
inmatlabroot/simulink/include/simstruc_types.h and is used to indicate
that one of the input arguments to these callbacks is required even though it
is not used in these callbacks. The macro UNUSED_ARG accepts only a single
input argument so must be called once for each callback input argument that
is not used in the callback.
Example of a Variable-Step S-Function
The example S-function matlabroot/simulink/src/vsfunc.c
uses a variable-step sample time. This S-function is used in the Simulink model
matlabroot/toolbox/simulink/simdemos/simfeatures/sfcndemo_vsfunc.mdl.
Variable step-size functions require a call to mdlGetTimeOfNextVarHit, which
is an S-function routine that calculates the time of the next sample hit.
S-functions that use the variable-step sample time can only be used with
variable-step solvers. vsfunc is a discrete S-function that delays its first
input by an amount of time determined by the second input.
The output of vsfunc is simply the input u delayed by a variable amount
of time. mdlOutputs sets the output y equal to state x. mdlUpdate
sets the state vector x equal to u, the input vector. This example calls
mdlGetTimeOfNextVarHit, an S-function routine that calculates and sets
the time of the next hit, that is, the time when vsfunc is next called. In
mdlGetTimeOfNextVarHit, the macro ssGetInputPortRealSignalPtrs is used
to get a pointer to the input u. Then this call is made.
ssSetTNext(S, ssGetT(S)(*u[1]));
The macro ssGetT gets the simulation time t. The second input to the block,
(*u[1]), is added to t, and the macro ssSetTNext sets the time of the next hit
equal to t+(*u[1]), delaying the output by the amount of time set in (*u[1]).
matlabroot/simulink/src/vsfunc.c
The beginning of each S-function must include #define statements for
the S-function’s name and level, along with a #include statement for the
simstruc.h header. Following these statements, the S-function can include
or define any other necessary headers, data, etc. . In the vsfunc.c example
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