MATLAB: Does ssGetInputPortSignal fail when called from an S-function’s mdlUpdate method in Simulink 7.4 (R2009b)

simulinkss_reusable_and_globalss_reusable_and_local

When I execute the following code in my mdlUpdate method, I get a NULL pointer which indicates that ssGetInputPortSignal failed:

const void *ip = ssGetInputPortSignal(S, 0);

Best Answer

In Simulink 7.4 (R2009b), ssGetInputPortSignal will fail in the mdlUpdate method when the input port is set to REUSABLE. For example, inserting the following line in mdlInitializeSizes will result in the behavior described:

ssSetInputPortOptimOpts(S, 0, SS_REUSABLE_AND_LOCAL);

The input port should not be set to REUSABLE if it is required in the mdlUpdate method. As mdlOutputs is called before mdlUpdate, the InputPortSignal memory location would have already been reused by the time it reached mdlUpdate resulting in ssGetInputPortSignal returning a NULL pointer. The process view in the following documentation page illustrates the order in which various functions within an S-function are called:

 web([docroot '/toolbox/simulink/sfg/f8-37326.html'])

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MATLAB: Does ssGetOutputPortRealSignal return a valid pointer in mdlOutputs but a null pointer in mdlUpdate in Simulink 6.5 (R2006b)

This bug has been fixed in Release 2007b (R2007b). For previous product releases, read below for any possible workarounds:

Documentation on Configuring output ports of a block that are reusable in S function is missing from Simulink 6.5 (R2006b). This situation occurred because of the way the output port was configured. The output ports are explicitly set to be reusable. In this case, the output port memory will be accessed only during the outputs for that block. This is to enable 'Signal Storage Reuse'. The signal storage reuse option allows us to conserve memory being used around the model to pass signals, but the result is that the output port signals are not always available.

Instead of setting the output port the following way:

 ssSetOutputPortOptimOpts(S, O_OUT1, SS_REUSABLE_AND_GLOBAL); /* enables use w/ merge blocks*/

Here is what it has to be to access the output during the update function:

 ssSetOutputPortOptimOpts(S, O_OUT1, SS_NOT_REUSABLE_AND_GLOBAL);

MATLAB: Is there a different syntax in the documentation for the S-function callback method mdlSetDefaultPortDimensionInfo in Simulink 6.0 (R14) and the example listed in the reference sfun_matadd.c

This is an error within the documentation for Simulink within the S-function callback method mdlSetDefaultPortDimensionInfo. The documentation should read as follows:

Syntax

void mdlSetDefaultPortDimensionInfo(SimStruct *S)

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MATLAB: Does ssGetOutputPortRealSignal return a valid pointer in mdlOutputs but a null pointer in mdlUpdate in Simulink 6.5 (R2006b)

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