Thread Safety in C

Question

imagine I write a library in C. Further, imagine this library to be used from a multi-threaded environment. How do I make it thread-safe? More specific: How do

Answer 1

You have two main options:

1) You specify which multi-threaded environment your library is thread-safe in, and use the synchronisation functions of that environment.

2) You specify that your library is not thread-safe. If your caller wants to use it in a multi-threaded environment, then it's their responsibility to make it thread-safe, by using external synchronisation if necessary to serialise all calls to your library. If your library uses handles and doesn't need any global state, this might for instance mean that if they have a handle they only use in a single thread, then they don't need any synchronisation on that handle, because it's automatically serialised.

Obviously you can take a multi-pack approach to (1), and use compile-time constants to support all the environments you know about.

You could also use a callback architecture, link-time dependency, or macros, to let your caller tell you how to synchronise. This is kind of a mixture of (1) and (2).

But there's no such thing as a standard multi-threaded environment, so it's pretty much impossible to write self-contained code that is thread-safe everywhere unless it's completely stateless (that is, the functions are all side-effect free). Even then you have to interpret "side-effect" liberally, since of course the C standard does not define which library functions are thread-safe. It's a bit like asking how to write C code which can execute in a hardware interrupt handler. "What's an interrupt?", you might very well ask, "and what things that I might do in C aren't valid in one?". The only answers are OS-specific.