Mex only on file change

For large Simulink projects with lots of s-function, performing a mex of all files can be time consuming. Leaving it up to the user might result in simulation runs with older mex files because the user might forget to re-mex the updated file. You can automate the process as follows:

• Create a hash list file that consists of file names used by s-functions and corresponding hashes.
• Before simulation start, in InitFcn, 
• Calculate hashes of files used by s-functions and compare them with the values saved in hash list file. Mark files whose hashes are different as "changed".
• If an existing binary mex file date is earlier than any of the dates of the source files used in its build, mark that binary as "changed".
• Mex files marked as "changed".

For hash calculation you can use [status, cmdout] = system(['certutil -hashfile', fileName, ' SHA256']) and parse cmdout via str=splitlines(cmdout); hash=str[2]; to get hash of file.