When the block is opened.

Generally, use this parameter with Subsystem blocks.

The callback executes when you double-click the block or when you use open_system with the block as an argument. The OpenFcn parameter overrides the normal behavior associated with opening a block, which is to display the block dialog box or to open the subsystem. Examples of tasks that you can use OpenFcn for include defining variables for a block, making a call to MATLAB to produce a plot of simulated data, or generating a graphical user interface.

After you add an OpenFcn callback to a block, double-clicking the block does not open the block dialog box. Also, the block parameters do not appear in the Property Inspector when the block is selected. To set the block parameters, select Block Parameters from the block context menu.