An important difference is that you have all the operating system overhead of calling a second program and transfering data to it. If your external program has a big enough task to do and the data you have to exchange between your script and the c program is not too big, that works well. But if you want to speedup some small calculation that is performed a thousand times in your script you will likely make it slower

In reply to Re: What is the difference: Calling a (c based) program OR inline c code by jethro
in thread What is the difference: Calling a (c based) program OR inline c code by baurel

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