simulink function block error: Function output 'y' cannot be an mxArray in this context.

i am dealing with a function block in simulink, trying to calculate the interval of a signal with variable frequency to get the real time frequency. My thought is to use tic/toc get the interval between two peaks.

So confused by this error. The diagnostic says:

Function output 'y' cannot be an mxArray in this context. Consider preinitializing the output variable with a known type. Function 'MATLAB Function1' (#312.9.10), line 1, column 10: "y" Launch diagnostic report.

Simulink cannot determine sizes and/or types of the outputs for block 'signalTimer/MATLAB Function1' due to errors in the block body, or limitations of the underlying analysis. The errors might be inaccurate. Fix the indicated errors, or explicitly specify sizes and/or types for all block outputs.

function y = timer(u)
coder.inline('never');
coder.extrinsic('toc')
coder.extrinsic('tic')

persistent x1 x2
% x1=0;
% x2=0;
if isempty(x1)
    x1 = 0;
    x2 = 0;
else
x2=x1;
end
x1=u;
if  x1<x2
    y=toc;
    y=double(y);
    tic
end
end

[Vishal Chaudhary]

"tic' must be called without an output argument before calling "toc".

Documentation for tic: https://www.mathworks.com/help/matlab/ref/tic.html

[Jiang Ping]

Thank you, but this problem can be addressed if I execute tic in the command line before run the script. Actually what I am really want to do is to measure the frequency of sqaure wave whose frequency is variable in real time .