switch flag,

%Initialization，flag=0时，具体初始化函数参照35行 case 0 [sys,x0,str,ts] = mdlInitializeSizes;

%Calculate outputs case 3 sys = mdlOutputs(t,x,u);

%Unused flags case {1,4,2,9} sys = []; % do nothing

otherwise error('Simulink:blocks:unhandledFlag', num2str(flag));end%%=============================================================================% mdlInitializeSizes% Return the sizes, initial conditions, and sample times for the S-function.%=============================================================================%初始化函数是必不可少的，其中的结构体形式是固定不变的function [sys,x0,str,ts] = mdlInitializeSizes

sizes = simsizes;%用于设置模块参数的结构体用simsizes生成sizes.NumCOntStates= 0;%模块连续状态变量的个数sizes.NumDiscStates = 0;%模块离散状态变量的个数sizes.NumOutputs = 1;%模块输出变量的个数sizes.NumInputs = 1;%模块输入变量的个数sizes.DirFeedthrough = 1;%模块是否存在直通反馈，存在为1。直通的意思是输入能直接控制输出sizes.NumSampleTimes = 1;%模块的采样时间个数，至少是一个

sys = simsizes(sizes); %设置完后赋给sys输出str = []; % str是保留参数，一般在初始化置空x0 = []; % x0是状态变量的初始值，显然初始值置空ts = [-1 0]; % sample time: [period, offset]

%start mdlOutputs%当运行到mdlOutputs，就会输出20*u。function sys = mdlOutputs(t,x,u)sys=20*u;% end mdlOutputs