MATLAB: How to copy input port dimensions to output port dimensions in a Level 2 s-function on initialisation

level-2 s-functionMATLABoutportportportssimulink

I have some inputs that inherit sizes
block.InputPort(1).Dimensions       =   -1;
block.InputPort(1).SamplingMode='Sample'; 
block.InputPort(2).Dimensions       =  -1; 
block.InputPort(2).SamplingMode='Sample'; 
block.InputPort(3).Dimensions       =   1;
block.InputPort(3).SamplingMode='Sample';
I then have some outputs
block.OutputPort(1).Dimensions      =   block.InputPort(2).Dimensions
block.OutputPort(1).SamplingMode='Sample';
block.OutputPort(2).Dimensions      =   block.InputPort(2).Dimensions
block.OutputPort(2).SamplingMode='Sample';
block.OutputPort(3).Dimensions      =   block.InputPort(2).Dimensions
block.OutputPort(3).SamplingMode='Sample';
block.OutputPort(4).Dimensions      =   1;
block.OutputPort(4).SamplingMode='Sample';
The above code doesn't do what I intend i.e. to make output ports 1,2 and 3 the same dimensions as input port 2.
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