What "thread safe" really means...In Practical terms
please bear with my newbie questions..
I was trying to convert PDF to PNG using ghostscript, with ASP.NET and C#. However, I also read that ghostscript is not thread safe. So my questions are:
What exactly does "ghostscript is not thread safe" mean in practical terms? What impact does it have if I use it in a live ASP.NET(aspx) web application with many concurrent users accessing it at the same time?
I also read from another site that the major feature of ghostscript ver. 8.63 is multithreaded rendering. Does this mean our thread safe issue is now resolved? Is ghostscript thread safe now?
I am also evaluating PDF2Image from PDFTron, which is supposed to be thread safe. But the per CPU license doesn't come cheap. Is it worth paying the extra money for "thread safe" vs "not safe"?
Answer
A precise technical definition that everyone agrees on is difficult to come up with.
Informally, "thread safe" simply means "is reasonably well-behaved when called from multiple threads". The object will not crash or produce crazy results when called from multiple threads.
The question you actually need to get answered if you intend to do multi-threaded programming involving a particular object is "what is the threading model expected by the object?"
There are a bunch of different threading models. For example, the "free threaded" model is "do whatever you want from any thread; the object will deal with it." That's the easiest model for you to deal with, and the hardest for the object provider to provide.
On the other end of the spectrum is the "single threaded" model -- all instances of all objects must be accessed from a single thread, period.
And then there's a bunch of stuff in the middle. The "apartment threaded" model is "you can create two instances on two different threads, but whatever thread you use to create an instance is the thread you must always use to call methods on that instance".
The "rental threaded" model is "you can call one instance on two different threads, but you are responsible for ensuring that no two threads are ever doing so at the same time".
And so on. Find out what the threading model your object expects before you attempt to write threading code against it.